Why is the Moon smooth near the equator and bumpy near the poles?

Why is the Moon smooth near the equator and bumpy near the poles?

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It looks as if it was polished by applying emery to its spinning equator.

My initial guess was that it was simply an illusion caused by light direction, but the terrain map shows the same phenomena.

Google Moon photo and terrain maps (source)

North pole mosaic assembled from photos by NASA's Lunar Reconnaissance Orbiter (source):

South pole shot by NASA's Clementine (source):

There are a few factors that create this effect:

First in the visual light images, the view around the equator is seen back lit, with almost no shadow, the few around the pole is seen with low lighting, and low lighting brings out features of the terrain. The Polar mosaics have an unreal quality, since they are made of images all taken when the sun is locally at its maximum above the horizon, so the light appears to be coming from all directions at once.

The first mosaic and the topographic map have a projection that is "conformal" this means that circular craters appear circular on the map, but it does introduce very large distortions of scale this, combined with shading which has been added to the map, makes regular-sized craters near the poles appear huge. The shading scales with the map, so craters near the pole have much more dramatic shading than those near the equator, that is an artistic effect, not real.

Finally on one face of the moon, that which faces us, and is in the middle of the pictures, there are large "maria". These are relatively flat lava plains. There are no maria at the poles. Most are on the near side of the moon: see Why are most lunar maria on the visible side?

So this is a combination of low angle of light at the poles, the way that google has projected and shaded their map and the distribution of lava plains on the moon. The poles are not more bumpy than the equatorial region in general. They are more bumpy that the lunar maria, but the apparent effect you describe is an illusion.

Ten Things You Don't Know About the Earth

Look up, look down, look out, look around. — Yes, "It Can Happen"

Good advice from the 70s progressive band. Look around you. Unless you’re one of the Apollo astronauts, you’ve lived your entire life within a few hundred kilometers of the surface of the Earth. There’s a whole planet beneath your feet, 6.6 sextillion tons of it, one trillion cubic kilometers of it. But how well do you know it?

Below are ten facts about the Earth — the second in my series of Ten Things You Don’t Know (the first was on the Milky Way). Some things I already knew (and probably you do, too), some I had ideas about and had to do some research to check, and others I totally made up. Wait! No! Kidding. They’re all real. But how many of them do you know? Be honest.

Why is the Moon smooth near the equator and bumpy near the poles? - Astronomy

A Brief Note On The "Super" Moon of March 19, 2011

A comparison of the apparent size of perigee and apogee full moons (Image Credit JPL/USGS/NASA)

(Image Credit Galileo, JPL, NASA)

Lunar Sample Disk 137 (click on image for detailed discussion).
One of a number of disks used by NASA to promote public understanding of the Moon.
Samples of moon rocks and moon soil are embedded in a clear plastic disk for convenient viewing.

Above, an Earth-based image of the full moon (Consolidated Lunar Atlas, USNO, Lunar and Planetary Institute)
Note the lack of shadows, and the presence of bright rays, in contrast to the mosaic images below

A lunar nearside mosaic (Image Credit GSFC / Arizona State Univ. / Lunar Reconnaissance Orbiter NASA)

Image Credit as for the unlabeled image

Closer view of the area near Tycho (rayed crater near bottom).
The large crater near top left is Copernicus, which is shown in more detail below.
(Steve Mandel, Hidden Valley Observatory, apod010809)

A third-quarter moon unlike any ever seen from Earth. The western "limb" of the Moon, photographed by the Galileo spacecraft as it flew by the Earth. (Galileo Project, JPL, NASA, apod990326)

A labeled view of a portion of the same Lunar Orbiter image of Mare Orientale.
(NASA, Lunar Orbiter 4, Sky and Telescope)

The Alpine Valley, near Plato (Image Credit & © Jim Misti, Misti Mountain Observatory used by permission)

The waning crescent moon as photographed with the ESO 2.2 meter wide field camera.
(WFI Team, ESO, MPI-A, OAC, apod990129)

A close-up of Mare Humorum. The large crater at the top left is Gassendi.
(WFI Team, ESO, MPI-A, OAC, apod990212)

Ariadeus Rille, photographed by Apollo 10 astronauts.
Linear rilles are created by tectonic deformations of unknown nature and origin.
(Apollo 10, NASA, apod)

Ask Us

The Earth was born about 4 and a half billion years ago, at the same time the whole solar system (the Sun, Earth, and other planets) formed. An enormous cloud of gas started to get smaller and smaller as the gas particles attracted each other with gravity. Most of the gas went to the center of the solar system and formed the Sun, but several other pieces spinning about the Sun solidified into the planets, including the Earth.

Current scientific thinking puts the Earth at about 4.5 billion years old.

The primary separation of the elements takes place when the rock is molten. Differences in melting and condensation temperatures and different densities cause the similar molecules to clump. There are other smaller effects, such as like molecules forming crystals (they fit together better). There is also some separation (due to differences in condensation temperature) in the proto-solar-system gas cloud. This is why there are different types of meteoroids.

Dr. Eric Christian
(September 2001)

The mass of Earth is 5.98 x 10 24 kg, or 1.32 x 10 25 lb. I was able to look up the mass of Earth at the Welcome to the Planets link and then converted the kilograms to pounds.

There is an AU (Astronomical Unit), which is defined as the average distance from the Earth to the Sun, but I have never heard of AO or AOS as a unit of measurement. AOS is used as an abreviation for "Acquisition of Signal" for spacecraft telemetry, however.

Dr. Eric Christian
(October 2000)

The Earth (and other planets, and stars) is spherical because the spherical shape is the lowest energy state that a group of matter can be in. Small asteroids and moons can be non-spherical, but after they reach a certain size (when the force of their gravity can "break" the rock from which they are made), all the bumps are pulled down, and they become more spherical. There is a maximum size that mountains can attain that gets smaller as the planetoid gets more massive. So Mars can have larger mountains (Olympus Mons for example) than the Earth can, because it weighs less. As long as the maximum mountain size is small compared to the radius of the planetoid (true for objects considerably smaller than the Moon), the body will be spherical.

The Earth is not a perfect sphere, so the distance to the center of the Earth varies from 6378 km (3963 miles) at the equator to 6357 km (3950 miles) at the poles.

The inner core may be hotter, but it is at a much higher pressure. At a fixed temperature (say 5000 degrees), high pressure can make a liquid into a solid. So, even though the temperature increases as you move into the inner core, the pressure increases faster and wins.

Dr. Eric Christian
(March 2001)

It's a combination of radioactivity (the radioactive materials in the Earth generate heat) and the residual heat from the formation of the Earth. When all of the matter that created the Earth fell together, it picked up kinetic energy falling in. When it stopped at the proto-Earth, the kinetic energy was turned into heat. The Earth hasn't cooled yet. The Moon, being much smaller, has had time to cool and probably has a solid core.

I'm an astrophysicist, not a geologist. I got my answer from long ago geology courses and research on the web. It's a standard part of geological theory that radioactivity is part of the heating process. See, for example, this Scientific American article.

Why is there not an abundance of fission products observed in materials emitted from either volcanoes or sea-borne vents? Or why are there not more fission products as one penetrates the Earth's crust into deep mines?

There IS increased radioactivity seen in volcanoes (see, for example this news article).

I don't know about undersea vents, but I wouldn't be surprised if there was increased activity coming from them as well. Your question implies that you think there should be lots and lots of radioactive gas, but you don't need that much radioactivity in the mantle and core.

As for deep mines, local composition is far more important for how much radioactivity there is than depth.

If you want more information, I suggest you contact a real geologist or go to your local college/university library to do some research.

Dr. Eric Christian
(September 2007)

Earth's core temperature is about 6,000° C. By coincidence, this is about the same as the Sun's surface temperature (but much cooler than the Sun's core temperature, which is about 15,600,000° C). The Earth's core is cooling, but at a very slow rate. Over the past three billion years it has probably cooled by a few hundred degrees. Currently, the Earth's core temperature is not changing much because, through radioactive decay (nuclear fission - the breakup of the nuclei of heavy elements, like uranium), it is generating about as much heat as it is losing.

To answer the second part of this question, some definitions are in order. A star is a self-luminous body that shines by generating energy internally through nuclear fusion (the combining of nuclei of light elements like hydrogen and helium). The Sun is a star. A planet shines by reflected light from the Sun. The solar system has nine "major" planets (of which Earth is one) and innumerable "minor" planets (asteroids and comets of various kinds).

Star masses range from about 0.04 times, to 150 times, the mass of the Sun. The mass of the Earth is 0.000003 times that of the Sun (and the mass of Jupiter, the largest planet in the solar system, is 0.001 times that of the Sun).

Although stars lose mass as they evolve, none lose enough to wind up anywhere near the mass of even the most massive planet. So, the bottom line is: Stars do not evolve into planets.

For a great animated simulation showing the evolution of stars with masses between 0.1 and 120 times that of the Sun see the stellar evolution simulation created by Terry Herter for his Astronomy 101/103 course at Cornell University. You will need a JAVA enabled browser to view this simulation.

Dr. Ed Tedesco
(January 2005)

As you go further inside the Earth, the force you feel due to gravity lessens, assuming the Earth is has a uniform density all the way throughout. Less force means you weigh less.

The reason is that the mass attracting you is inside a sphere, and is given by M = (4/3) * pi * (radius) 3 * density

The force you feel is given by F = G * M * (your mass) / (radius) 2

This means the net force is F = G * (4/3) * pi * radius * density * (your mass)

(pi=3.14159 and G = Newton's gravitational constant)

So as you go further inside the Earth, the radius is decreasing , so the force you feel is decreasing. The mass above you oddly enough doesn't contribute at all to any net force on your body.

In reality, of course, the Earth is not of uniform density, and there is a slight increase in force as you go down from the surface, before it begins to decrease again. Still, you weigh less standing on the Earth's core.

As far as what happens above sea level - you must realize that what happens outside the Earth is different from what happens inside the Earth. Inside, as you go deeper and deeper, the mass attracting you is less and less (as stated). Above sea level (the surface of the Earth, specifically) as you go further and further away, the mass remains constant (obviously), but the distance gets larger and larger, which makes the force (given by F = G * M(Earth) * M(you) / r 2 ) smaller.

Notice that the formula that applies inside the Earth is different from the one that applies outside.

Dr. Louis Barbier
(October 2003)

First, the most important thing to know is that gravity exists absolutely everywhere in the universe. Every bit of matter exerts a force on every other bit of matter. This means that you are attracted, and attract, everything in the universe! The force exerted depends on the distance of the object and the mass. The Earth exerts the most force on you because it is close (right here!) and very massive.

Forces add like vectors, so their direction is very important. If you could be at the exact center, the forces that each bit of Earth matter exerted on you would cancel out (up cancelling down, east cancelling west, etc.). This only occurs for a single point, though, and you would still feel a gravitational force on the rest of your body.

Remember, gravity is universal and exists everywhere. This is the fundamental law of physics.

Angela Richard
(March 2003)

Gravitation is attractive, never repulsive, so, in the absence of other forces there, matter will always tend to move to the center of the sphere.

Dr. Eric Christian
(July 2002)

The major probe of the Earth's interior is seismic waves from earthquakes. Scientists can use an array of seismometers to track the speed of the waves as a function of depth, and from this can infer density, temperature, etc. The sharp discontinuities at the mantle-crust and mantle-core interfaces are especially noticible.

These values are directly calculable and have been proven by measurements repeatedly.

Radius at poles = 6,356,800 meters
Radius at equator = 6,378,400 meters
omega (angular velocity) = .00007292115 s -1

If you plug these numbers in you find:
gravitational acceleration at poles = 9.8322 m s -2
no centrifugal acceleration at poles
Total acceleration at poles = 9.8332 m s -2

gravitational acceleration at equator = 9.7805 m s -2
centrifugal acceleration at equator = -.0002 m s -2
Total acceleration at equator = 9.7803 m s -2

So it is obvious that the oblateness of the Earth is 250 times more important than the centrifugal acceleration.

"Up" and "down" are different depending upon where you are. They come from gravity, which is the force that holds everyone and everything onto the Earth. "Down" points in the direction of gravity, which is toward the center of the Earth, and "up" is in the opposite direction. If you look at a globe, no matter where a person is standing, "up" is the direction away from the center of the Earth and points to the sky.

Dr. Eric Christian
(May 2011)

We do not stay on the Earth because it is spinning, but because of the force of gravity.

I am not aware of any planets that do not spin.

Dr. Louis Barbier
(November 2001)

You already have half the answer! The speed of the Earth's rotation can be thought of in two ways - the angular speed (which you've calculated) and the linear speed (of a point on the surface).

Express the angular speed (traditionally referred to by the Greek letter omega) in radians/sec:

omega = 360 degrees / 24 hours = 2 * pi radians / (24 * 60 * 60) second
(pi = 3.14159. )
= 7.272 x 10 -5 radians/sec = 0.00007272 radians/second

Now to convert the angular speed to the linear speed of a point on the Earth's surface, multiply omega by the radius of the Earth, R.

To put it in familiar units, let's express R in miles: R = 3822 miles (roughly). So the linear velocity on the surface V is:

V = omega * R = 0.00007272 * 3822 = 0.278 miles/second
V = about 1000 miles/hour (Surprisingly fast!)

The Earth's spin is slowing down by about 1.5 - 2 milliseconds per century, and that angular momentum is moving into the Moon's orbit, which is getting larger. The reason for this, and the reason a figure skater can only spin for so long, is friction. In the case of the skater, it's air resistance and friction with the ice. In the case of the Earth, it's the friction due to tides moving around the Earth.

Dr. Eric Christian
(June 2000)

It takes nearly four minutes for a star at the celestial equator to move one degree (about a finger's width held out at arm's length). Stars nearer to the poles will move even less (Polaris doesn't appear to move at all in the northern hemisphere). So in five seconds you won't get much of a streak. With longer exposures you could maybe do it, but you'll need to accurately measure the length of the track and know how far the star is from the pole (its celestial latitude) to know the total length of the circle it would make if you could photograph it for a full day.

An easier way is to measure transit times. Measure the time that a star passes the same point (goes behind the roof of a building or crosses an overhead wire) on successive days and divide the time interval by the 360 degrees the star has traveled. If you miss a day or more because of clouds, it still works if you divide by 360 times the number of days. You have to keep your eyes in exactly the same spot for this to work, so a mounted telescope (that doesn't move) or just a cardboard tube will help.

Dr. Eric Christian
(April 2000)

The Earth's orbit varies from 0.983 AU out to 1.067 AU. We are actually closer to the Sun in the winter than in the summer. The orbit crosses the 1 AU point two times a year - in spring and fall.

Dr. Louis Barbier
(November 2002)

Your question provides the opportunity to elaborate on the connection between Kepler's Laws of planetary motion and Newton's Law of Gravitation.

The simple answer to your question is that there is nothing special at the other focal point. Here's the explanation.

In the late 16 th century, Tycho Brahe made the most accurate observations of the positions of the planets at that time, with his state-of-the-art observatory on the Danish island of Hven. In his attempt to model and explain Brahe's observations, Johnannes Kepler devised his laws of planetary motion. The first two are relevant here.

Kepler's first law states that all planets move on ellipses, with the Sun in one of its two foci, as you mentioned, and this still holds. Looking at this statement as a purely mathematical description, it is not obvious whether the Sun should be in one or the other focus, or whether there is any special significance to the second focus. That is where a physical argument and/or an observation would have to come in, which would then define the correct initial condition to determine the correct focus.

Brahe's observations also provided the handle to connect Kepler's laws with Newton's Law of Gravitation. The Sun is the source of the gravitation that pulls radially on the planets in our solar system and keeps them in orbit. It also slows them down on their way out to the furthest part of their orbits, and speeds them up on their way back in. If, in fact, there were another equal source of gravitation located in the other focus of the ellipse, there would be no reason for a planet to move faster near the one focus and slower near the other focus, as was observed.

Let us explore this with an illustrative fictitious experiment, diagrammed in the figure below.

Shown are three cases with different initial speed (arrows)
and the respective elliptical orbits in color. In all cases,
the satellite is closest to the planet and Focus 1 when it is moving
with its fastest speed.

We will launch a satelllite from a high tower on a planet that does not rotate (to make things simpler). This figure is obviously not to scale, but to orient you, in the diagram, the tower comes out of the planet on the top here, and the satellite launch goes straight to the right from its top.

When the satellite is launched from the tower at its slowest speed, its orbit looks like the red (innermost) ellipse. It is so slow that after launch it starts to fall toward the planet - and thus speeds up and falls into this type of orbit. As explained, the satellite is fastest where it is closest to the planet: on the side of the orbit opposite the launch tower. We'll call the planet's center Focus 1 (black "plus sign"). Focus 2, for this orbit with the slowest speed, is at the red plus sign (which would not actually be on the tower if the diagram were to scale).

Launching the satellite at a higher speed pushes the far side of the orbit further and further out. In the black (middle) orbit example, the speed is such that the ellipse turns into a circle. The planet is now in the center of the circle, and a circle is a special case of an ellipse. Because it's a circle, the locations of both foci are the same, at the black plus sign.

Finally, in the green (outermost) orbit case, the launch speed is fast enough for the satellite to continue to increase its distance from the planet. It slows down as it reaches the maximum distance possible with this speed - on the opposite side from the launch point. Again Focus 1 (in the planet center) is close to the point in the orbit where the satellite is at its fastest speed, and Focus 2 for the green orbit (green plus sign) is on the side where the speed is slowest.

Kepler's second law, derived solely from observations at the time of its discovery, states that on a planet's orbit around the Sun, the planet-Sun line sweeps out equal areas during equal time periods (fractions of a planet's orbital period). This quantifies the speed of an orbiting planet based on its distance from the Sun, and is in accordance with actual observations. A planet reaches its fastest speed at perihelion, its closest location to the Sun. This observational fact also provides a basis for the location of the Sun as one of the two foci. Again, it has to be the one on the side where the planet moves fastest.

This web page at the University of Kentucky may be helpful.

Dr. Eberhard Moebius
(October 2007)

Of course, this can't happen, but if it did, everything not attached would go flying off to the east, parallel to the surface of the Earth. The speed would depend upon your latitude. Only the people at the poles would be safe. You wouldn't go flying off into space because the 1000 mph maximum (at the equator) isn't enough to overcome gravity, which would still be present. If you survived, the resulting six month day and six month night would probably take care of you pretty quick.

Dr. Eric Christian
(August 2000)

Yes, your logic is correct. If the Earth stopped rotating on its axis everything on Earth, away from the poles, would appear to weigh more due to the absence of centrifugal force.

Mathematically, F = mg, where g is the acceleration due to gravity at the Earth's surface (9.80 kg/m 2 ), m is your mass, and F is the force (in this case, weight) resulting from mass m in gravitational field g. If you are very far from any massive body then g

0 and so you are weightless (F = m x 0 = 0).

However, for most places on a rotating Earth, we feel a centrifugal force that slightly decreases our weight. This is because for circular motion, the force (F c ) due to acceleration (a) is given by F c = ma = m(v 2 /R) Cos(Theta), where v is the speed of the object (you, in this case), R is the radius of the circle it is moving in, and Theta is the angle between the rotation axis and your position on the Earth's surface. For the case of the Earth, R is its radius and Theta = 0 deg at the equator and 90 deg at the poles. So, since Cos (0) = 1 and Cos (90) = 0, the force on you, in a direction away from the rotation axis, is m(v 2 /R) at the equator and zero at either pole.

Numerically, this means that if your mass is 70 kg, the force of gravity acting on you is F = mg = 70 x 9.8 = 686 (kg/m) 2 (a unit called a "Newton"), or 154 pounds.

If you are standing on the equator, the centrifugal force acting on you is F c = m(v 2 /R) Cos(Theta) = 70(463 2 / 6371000) Cos (0) = 2.4 Newtons = 0.54 pounds.

So, you would weigh about a half-pound less at the equator than you would at the North or South Pole. (For the purists, the explanation above assumes the Earth is a sphere of uniform density and neglects relativistic forces.)

Dr. Ed Tedesco
(January 2006)

The mass of all the people on Earth is miniscule compared to the mass of the Earth, so neither of these actions would have any effect on Earth's motion.

Dr. Louis Barbier
(March 2002)

Check out the Windows to the Universe, the Starchild Web site, the [email protected] site, or the Bad Astronomy site, which talk about the Earth, seasons, etc. Check Imagine the Universe! for information on the "equation of time" -- the asymmetrical change in the amount of daylight between sunrise and sunset through the year.

Dr. Louis Barbier and Beth Barbier

I know that as we approach the summer solstice, we gain hours of daylight. Do we gain the same amount of daylight each day as we approach the longest day of the year?

No, the day/night dividing line moves in a curve, not in a linear fashion. It's due to the interaction of several three-dimensional vectors, and so there are sines and cosines involved (and actually the product of sines and cosines). For a simple proof, near the poles, the gain of daylight goes to zero before the summer solstice is reached, because the day is already 24 hours long.

Dr. Eric Christian
(November 2000)

You can balance an egg any day of the year. The equinox is not special. See The Straight Dope web site, for example. This is a common misconception.

Dr. Eric Christian
(September 2000)

The thing that you've missed is that a "day" is not the length of time that the Earth takes to rotate through 360 degrees. Instead, the day is defined as the time it takes for the Sun to move from zenith to zenith. Because the Earth has travelled almost a degree through its orbit, it actually has rotated almost 361 degrees in 24 hours. Those extra degrees add up over a half year to keep the day synchonized. But the constellations do shift, so that what you see during the summer is overhead during the day in the winter and vice versa. The astronomical term for the time it takes the Earth to rotate 360 degrees is "sidereal day", which is 23 hours 56 minutes 4.09 seconds long.

Dr. Eric Christian
(September 2000)

The reason is that the axis of the Earth's rotation is tilted relative to the plane of the Earth's orbit around the Sun. So the circle on the Earth where the Sun is directly overhead moves north and south over the year, from the equator to the Tropic of Cancer, back to the equator, then to the Tropic of Capricorn and back to the equator. This causes sunrise and sunset to move north and south over the year as well. This effect also causes the seasons and the shortening and lengthening of the day.

For more information on Sun and Moon rise and set times, Moon phases, eclipses, seasons, positions of solar system objects, common astronomical phenomena, calendars and time, and related topics, check out the Astronomical Applications Department of the U.S. Naval Observatory.

The Sun gives off all colors of light, but blue light is bounced around the atmosphere a lot more than red light is (it's called scattering). The sky is blue because of the blue light bouncing around "lights up" other parts of the sky.

Dr. Eric Christian and Beth Barbier
(April 2000)

The Sun is always a little redder because of the scattering, but at sunrise and sunset the light has to pass through more atmosphere and loses much more blue light, so appears much redder.

Beth Barbier
(September 2003)

Hail can be formed by rain rising and freezing, but snow is directly formed in its solid, crystalline form.

Dr. Eric Christian
(November 2001)

Both the Earth moving (spinning) and gravity affect the wind. But the primary cause of the wind is temperature differences, not the moving of the Earth or gravity.

Dr. Eric Christian
(March 2003)

Though not our area of expertise, this is an area of current scientific study. Check out the European Science Agency's (ESA) article from September 29, 2000, Climate change: New impressions from space.

There would be an increase in temperature if the Earth to Sun distance became much smaller, but 1 meter is insignificant. For example, the Earth is actually closer to the Sun during the northern hemisphere winter.

Angela Richard
(February 2003)

The Earth's orbit changes on several time scales, each of which affect the intensity of radiation we observe at Earth.

The first change is the winter and summer solstice, where the Sun-Earth distance varies between 91,400,000 miles and 94,400,000 miles.

Now on longer time scales, the actual shape of the Earth's orbit changes every 100,000 years, vacillating between more circular and more elliptical. In this case, when the Earth is closest to the Sun it actually receives 20-30% more sunlight.

In addition, the Earth wobbles on its axis every 26,000 years, changing the time at which winter and summer occur.

Finally, the tilt of the Earth varies every 40,000 years by about 2 arc degrees, which affects the temperature difference between winter and summer.

These changes are termed the "Milankovitch theory" after the geophysicist who first proposed it and are believed to operate together to produce dramatic temperature variations on Earth. You might want to read more about the Milankovitch theory and solar radiation variations at the U.S. Naval Observatory Astronomical Applications Department and the Encyclopedia of the Atmospheric Environment.

Dr. Georgia de Nolfo
(June 2003)

A sidereal day is the length of time it takes the Earth to rotate 360 degrees. Since the Earth is revolving around the Sun, it actually has to rotate almost one degree (360 degrees/365.25 days) further until the Sun is in the same place in the sky, which is the definition of a day that everyone is used to. So a sidereal day is a little shorter, but the stars return to the same positions every sidereal day, so sidereal time is used sometimes in astronomy.

If a sidereal day is 23 hours and 56 minutes, what happens to the remaining 4 minutes?

There is a good explanation of sidereal time on the Imagine the Universe website.

I assume you want to know what would happen if the axis of the Earth moved (it is already turning on an axis that is vertical at the poles). The position of the equator is determined by the spin axis of the Earth, so if the new axis were at the right point (0 degrees latitude, 90 degrees longitude) then the old Prime Meridian would become the new equator (or at least half of it, the equator is a circle, the prime meridan is a semi-circle).

But the Prime Meridan is just a reference line picked by man, and could have been put anywhere. It goes through Greenwich, England, but the French were pushing hard for a French origin. If the axis moved, the new Prime Meridan could be put anywhere. Luckily, conservation of angular momentum insures that this drastic change in the Earth's axis is not going to happen.

The statement that it is the Sun's and Moon's gravity that causes tides is not incorrect, just simplified. More accurately, it is the gradient of the gravity that causes it. What happens is that the water on the side closest to the Moon is closer to the Moon than the center of gravity of the Earth and so the Moon exerts more pull on that water and it bulges out (high tide). On the other side of the Earth, the Moon is pulling more on the center of mass of the Earth than on the water and so the Moon pulls the Earth out from under the water a little bit, causing another bulge or high tide. That is why there are two high tides per day. Hope this helps.

Your question is well beyond our area of expertise or interest, and we cannot answer it. I did find a bit of information on a NOAA website that may get you started, at the Center for Operational Oceanographic Products and Services FAQ.

Contributed by Konstantin Parchevsky, HEPL, Stanford University (May 2007):

The Moon's gravity causes the appearance of two "bumps" of water on the Earth -- one on the Moon's side and one on the opposite side. Theoretically, in the ocean, neglecting the viscosity of water, the highest spot of the "bump" should be just below the Moon.

In reality, near the coast there is a time lag between the moment when the Moon passes the meridian (the so-called culmination) and the moment of the highest tide. This time lag is called the "lunitidal interval". It depends on the viscosity of water and the shape of the coastline and the seabed, and it has to be taken from observations. If you want your watch to predict the tides correctly, you have to input the lunitidal interval. For some cities, you can take it from the table at the end of the watch's manual.

Unfortunately, there were no data for Half Moon Bay, CA (near me). So I had to calculate the lunitidal interval by myself. This is a good time to remember a saying: "If you are stuck, read the manual!" Fortunately, there was enough information in the watch manual to do this: "When setting the lunitidal interval for this watch, use the time difference between the Moon's transit over the meridian until high tide."

That's it! Just find the time difference between the moments of culmination of the Moon and the next high tide! First you have to calculate culmination of the Moon. Google gave me a lot of links for the phrase "Moon calculator". The second link is exactly what we are looking for. Just enter the date (such as 9 May 2007) and your position (Half Moon Bay, CA) and press 'Enter'. What we need here is the Moon transit (6:59 a.m.). Then we have to find the time of the next high tide in Half Moon Bay.

This time, Google "tides Half Moon Bay". The first link [similar sites can be found -- ed.] gives us a graphic representation of the tides and the moment of the closest high tide (6:14 p.m.). Time difference is 11h 15m. This is the sought-after lunitidal interval. And it works! Now my watch predicts the tides correctly.

For the record, I am an astrophysicist (solar physicist). I work at Stanford University and perform numerical simulations of propagation of the acoustic waves in the Sun.

You might want to check out the Views of the Solar System page on meteoroids and meteorites.

Your question about an asteroid impacting Earth is well covered by our sister site, Imagine the Universe!, in Is Earth in danger of being hit by an asteroid?.

I think this will answer your questions well: Caveat Impactor

(Don't believe everything you hear on Fox News!)

Yes, but unless the meteorite is really enormous (big enough to destroy life on Earth), the Earth's gravity changes by an amount that is too small to measure. The Earth is picking up several tons per day of dust and other stuff, not including the very uncommon meteorite.

Dr. Eric Christian
(November 2000)

The cusps are due to the interaction of the Earth's magnetic field and the magnetic field imbedded in the solar wind. They move around as the Earth moves around the Sun and as the tilt of the Earth's axis moves. They also move in position, as well as distance from the Earth, as the magnetic field of the Sun changes. So they don't point at any fixed direction.

Dr. Eric Christian
(November 2000)

Your question has a tremendous amount of latitude in it, both literally and figuratively. The magnitude of the Earth's magnetic field within the magnetosphere varies greatly with location within the magnetosphere. It is not a simple question to answer.

The magnetosphere is formed when the flow of the solar wind impacts the Earth's magnetic field (a dipole field), compressing it, causing magnetic reconnection, causing complex currents to flow, etc. The dipole field changes greatly under the influence of the solar wind.

Try to get a copy of "Physics of the Magnetopause" published by the AGU. That will help.

The magnetic field in the solar wind near Earth is about 5 nT, or 5 x 10(-5) Gauss. The magnetic field on the surface of the Earth is about 0.5 Gauss - big difference.

If you were to imagine a spacecraft passing from the solar wind and into the magnetosphere along the Earth-Sun line (at the sub- solar point) you would see the following:

The 5 nT field changes by a factor of 4 as you pass through the Earth's bow shock. This is a compression wave (discontinuity) that results from a supersonic flow striking the Earth's magnetic field and being stopped).

The magnetic field then climbs by another factor of 4 as the spacecraft passes through the magnetosheath, a turbulent region of flow behind the shock and still upstream of the magnetosphere.

The spacecraft then encounters a region where the flow stops moving toward Earth and flows exclusively around the obstacle that is the Earth's magnetosphere. This is the magnetopause. Behind the magnetopause the magnetic field is about 40 nT. So the compression of the flow has picked up almost a factor of 10 in intensity of the magnetic field (and the plasma density with it).

From this point on, it depends greatly on where you go. If you go Earthward, it is to leading order a dipole field (somewhat squashed, but the scaling is like that). If you go behind the planet, all kinds of things are happening and location is everything.

I really can't go into much more detail than that without a more specific question, and to be honest I mostly work outside the magnetosphere, so my knowledge is limited. But basically if you go from the magnetopause to the Earth's surface, you see a magnetic field that grows ever stronger as you decend within the dipole.

Dr. Charles Smith
(October 2003)

The Sun shifts it's magnetic field every 11 years, and it has already happened for this solar cycle. The Earth's magnetic field flip is much more erratic and has happened approximately 25 times in the last 5 million years. It's been about 740,000 years since the last flip, however, so we're long overdue. There is evidence that we may be heading towards a reversal (the dipole magnetic field is weakening and the higher order terms are increasing), but we can't predict when it would happen. Depending upon how quickly the field reversal happens, it could cause problems for things like electric power lines and oil pipelines, and if the field goes to near zero, it might cause a higher background radiation at the ground, but there is no evidence that previous reversals have had any major biological effect. The forces due to the interaction of the solar and terrestrial magnetic fields are only very small perturbations.
(October 2000)

How do the magnetic poles reverse? Is it for the same reason that the Sun's poles reverse?

Both the Sun and the Earth are electromagnets, not permanent magnets (despite the Earth's core being made up of iron and nickel). So it is electric currents moving through the plasma of the Sun and the molten rock of the Earth's interior that generate the magnetic fields. These currents have instabilities that build up until the field reverses to relieve stress. The fact that the magnetic pole moves is tied in with the same instabilities, but the flip probably happens pretty quickly on Earth (less than

1000 years is all they can tell).

As far as I know, there is no significant change of Earth's gravitation with time, because this is only a function of the Earth's size and mass, both of which are not changing significantly over time. There is only one regular change of the gravitational acceleration over time at a fixed location, and that has to do with the reason for the tides. The gravitational forces of the Moon and the Sun at a specific location change with their relative position in the sky. The effect is of the order of a few 100,000ths of the gravity at the Earth's surface. However, I am not the instant expert on precision gravity related questions.

  1. The natural magnetic field of the Earth changes over time due to changes in the strength and configuration of its core, which are driven by the hot liquid interior of the Earth.
  2. The magnetic field changes because of gigantic currents that flow around the Earth in the magnetosphere. Depending on the space weather conditions, the currents in the magnetosphere change over time, and so does the Earth's magnetic field.

Neither type of change is just a change in the strength of the field. The direction of the magnetic field also changes. Therefore, the knowledge of magnetic field strength over time at only one location would not be sufficient to track changes in the Earth's magnetic field.

A detailed introduction into the Earth's magnetic field and related changes, with links to databases, may be found at the National Geophysical Data Center maintained by NOAA. On this page, General Information provides a very good introduction. Magnetic Declination On-Line allows computation of the magnetic declination (the direction a compass needle will point) for any location on Earth and for any date from 1900 to today (and even into the near future). The World Magnetic Model provides maps with magnetic parameters for specified locations or the entire globe.

Dr. Eberhard Moebius
(January 2003)

Earth's atmosphere reaches over 560 kilometers (348 miles) from the surface of the Earth. You can read more about it at the NASA homepage.

The question of whether light/photons reach the Earth's surface depends on several factors. Recall that light can get both absorbed and scattered by molecules in the atmosphere. The fate of a given photon as it enters the atmosphere depends on the chemical nature of our atmosphere. The Earth's atmosphere is primarily nitrogen and oxygen.

The basic idea to keep in mind is that the atoms and molecules have discrete energy levels which are known to be quantized (from quantum mechanics). Photons also have quantized energy. Because atoms or molecules require discrete energy boosts in order to be excited (i.e. cause an electron to transition from a low energy level to a higher one), the arriving photon, in order to get absorbed, must carry at least the difference in energy between the low and high energy states of the atom. If the photon is of even greater energy, it may also get absorbed by kicking out the electron altogether (with the electron departing with some extra kinetic energy).

So, photons will get absorbed depending on the chemical/atomic structure of the molecules in the atmosphere and on the energy of the incident photons. For instance, there are two compounds responsible for absorption in our atmosphere in particular: oxygen (O 2 ) and water vapor (H 2 O). Molecular oxygen and ozone are the strongest absorbers in the ultraviolet and water vapor, while methane, nitrous oxide, ozone, and carbon dioxide absorb in the infrared. X-rays can be absorbed in the atmosphere by individual nitrogen or oxygen atoms since they generally have enough energy to kick out an electron (and the chances of seeing an individual atom in a thick atmosphere such as Earth's are pretty high!).

For more on how radiation interacts with matter, see the Interaction of Radiation with Matter at HyperPhysics.

Dr. Georgia de Nolfo
(December 2004)

I'm sure that this number (5 ft/year) is WAY too large. That would put the Earth 1000 miles larger a million years ago. 5 feet per million years is probably more like it. The effect on the atmosphere would be small, because it is the total amount of mass of the Earth that provides the gravity that holds the atmosphere in place, and that is only increasing (about 1 ton per hour from micrometeorites). Over the billions of years that the Earth has been around, it has lost its atmospheric hydrogen and some of its helium just by random diffusion.

The National Space Science Data Center (NSSDC) at NASA GSFC has a page on the Moon with the distance from Earth listed as 384,467 km. It would be standard practice to give center-to-center distances.

What is the distance between the surface of the Earth and the surface of the Moon?

With the center-to-center distance above and the diameters of the Earth and the Moon, you could figure out the distance from surface to surface. From The Nine Planets, the diameter of the Earth is 12,756 km, and the diameter of the Moon is 3,476 km. Just subtract half of the diameters (= the radius), from the center-to-center distance:

  • Radius of Earth: 6,378 km
  • Radius of the Moon: 1,738 km
  • Surface-to-surface distance: 376,351 km

The mountains on the Moon and on Earth, for the most part, formed in completely different ways. Because the Moon is so much smaller, it cooled most of the way through, and so is mostly rocky now, although it was molten at one time. Most lunar mountains are caused by impact craters. On the Earth, the upper rocky crust floats on the molten mantle, and mountains are caused by plate tectonics, when enormous plates of the crust bump up against each other. The deepest craters on the Moon are about the same size as Mt. Everest, however.

Dr. Eric Christian
(October 2000)

You are asking two related questions, so let me tackle them in sequence. First, you asked why the Moon is covered with impact craters from meteors and asteroids, while the Earth seems to be much luckier in that respect. You answered part of that question yourself: the Earth has an atmosphere, which protects its surface from meteors up to a moderate size. These meteors evaporate upon entry into the atmosphere and can be seen as "shooting stars." Larger meteors, though, reach the ground. There are quite a few famous meteor craters on Earth, for example, the Meteor Crater near Flagstaff, Arizona, or the Noerdlinger Ries, an almost perfectly circular valley in Germany. Admittedly, these are fewer craters than are found on the Moon, and most of the Earth's craters are much younger. In fact, the Earth was pelted with many impacts at the same time when the Moon was hit so badly, up to about 3.5 billion years ago.

Why don't we find these craters anymore on Earth? The answer to this part of the question is that the Earth is geologically active and has a severe climate, which leads to hefty erosion by water, ice and wind over the long time scale of billions of years. In other words, the Earth's surface has been completely reshaped several times over by the ongoing plate tectonics, which moves the continents around and builds up mountain ranges. On the other hand, all these formations, including impact craters, are subject to severe weathering already on the scale of hundreds of millions of years.

Let me come back to your second question: Why does the Earth have an atmosphere and not the Moon? The answer lies in the fact that the Moon features so much less mass than the Earth it has only about 1/80 the mass of the Earth. As a consequence the strength of gravity on the Moon's surface is about 1/6 that on Earth, and the escape speed, i.e. the minimum speed that an object must have to escape the gravitational grip of the Moon, is much less (about 1/5) than that for Earth. (For the return trip from the Moon the Apollo lander had to achieve only a much lower speed than the Saturn rocket when leaving the Earth.) Therefore, atoms and molecules of a gas are much more likely to escape the Moon than the Earth. Being at the same distance from the Sun, the average temperatures on Earth and the Moon are very similar (not the extremes because of the differences in the atmosphere). Therefore, the average energy of the atoms and molecules in a gas, or their average speed, is the same on Earth and on the Moon. As a consequence the Moon has lost all of its gases in the distant past. To play with the idea of keeping or losing an atmosphere you may want to try out an in-class activity from my general astronomy class. You will find it at
and the solutions are available at

Dr. Eberhard Moebius
(April 2003)

How big are those craters on the Moon? Aren't they gigantic? How could we lose or ignore such huge structures on Earth?

There are huge structures on Earth, too, like in the Hudson Bay, but most of them have been wiped out by geology. 70% of the Earth's surface is ocean floor. And the ocean floor is being completely renewed on a time scale of 300 - 500 million years. It is as if we push old sheet metal into a melting oven and out comes completely new sheet metal on the other end.

Dr. Eberhard Moebius
(April 2003)

Because the Moon is in orbit around the Sun (as well as in orbit around the Earth). Another way of looking at it is that the center of gravity of the Earth-Moon system is revolving around the Sun, and the Earth and the Moon are revolving around their common center of gravity. The Sun is pulling on the Moon, but the Earth is "falling" right along with it, so they stay together.

You might want to look at the Sky and Telescope eclipse page. It addresses this question.

It's possible that you did see a "moon dog". Check out the Imagine the Universe! site's answer on this subject.

Beth Barbier
(February 2001)

And why can't we assume that the boundaries of atoms are set by heavenly motion? Is it possible that what we call "the strong nuclear force" is nothing more than the result of small entities racing in acute paths, keeping pace with larger bodies?

Although the Earth's position in space IS determined by the sum of various motions (Earth around Sun, Sun around Galaxy, Galaxy in supercluster, etc.), there is NO evidence that this has anything to do with atoms. Why should the boundaries of atoms be set by heavenly motion?

The theory of General Relativity fits the motions of the Earth extremely well, and Quantum Chromodynamics explains ALL observations of the strong nuclear force. Your hypothesis would have the strong force be different at different places and velocities. The evidence (stars, etc. seem to "burn" the same throughout the Universe) is against you.

The average radius of the Earth's orbit around the Sun is 93 million miles, so the distance is

584,000,000 miles / 8,764 hours = 66,660 mph

It is impossible for there to be another planet on the opposite side of the Sun from the Earth, in the same orbit. We would have seen the gravitational effect of this planet on other planets, and, because the Earth's orbit is an ellipse, not a circle, we probably would have seen the planet as well.

The equation for gravitation acceleration is G x M / r 2 , where G is the gravitational constant (6.67 x 10 -11 m 3 kg -1 sec -2 ).

The mass of the Sun is 2 x 10 30 kg, and the distance is 1.5 x 10 11 meters.

The Moon is 7.3 x 10 22 kg, and the distance is 3.8 x 10 8 m.

You then find that the acceleration from the Sun is .0059 m/s 2 (meters per second squared) and from the Moon is .000034 m/s 2 or 176 times smaller.

As Isaac Newton figured out, the force of gravity is universal for every object that has mass. And as you pointed out correctly, the Sun's gravity is much more powerful than Earth's, and that is because of its much larger mass.

However, the effects of gravity on other objects also scale inversely with the square of their distance from each other. (By the way, distance counts between the mass centers of the objects under consideration.) If an object is 10 times as far away, gravity drops by a factor of 100.

Therefore, the motion of objects in the vicinity of the Earth's surface is almost completely determined by Earth's gravity. The Earth itself, of course, is held in orbit by the Sun's gravity. We have to and we do consider the Sun.s gravity (or better, its variation with distance across the Earth) when it comes to the tides. The tides are mainly caused by the Moon, but the Sun produces half the effect of the Moon. Therefore, we have very strong tides at new moon and full moon (the Sun pulls on the water in the same direction or opposite to the Moon, and their effects add up), but very weak tides at half moon (the Sun pulls at right angle relative to the Moon, and their effects partially cancel). For additional explanations concerning the tides see our answer about the tides.

The Earth keeps objects within its own gravitational force up to a distance of 1.5 million km (almost 1 million miles). Beyond that, objects enter the region where the Sun dominates. This distance is about 1% the distance of the Earth from the Sun, which demonstrates how much stronger the Sun is.

Dr. Eberhard Moebius
(August 2004)

1 millisecond per 50 years. I also understand that this is basically due to the moon's pull on the tides. I also understand that the moon is moving away from the earth at a rate of

3.8 cm/year. This would place the moon

100,000 miles closer (or twice as close) to the Earth 4.5 billion years ago. When the moon was closer, shouldn't it have had a greater pull? Thus, shouldn't the moon moving further away make the earth's rotation increase?

Also, if I were to backtrack the 1 millisecond per 50 years that the earth's rotation is slowing down (if this rate were constant), the earth couldn't be 4.5 billion years old. Obviously the rate isn't constant, but I don't understand why.

There is a pretty complicated tidal interaction between the earth and the moon, but what it boils down to is that angular momentum (which is conserved in a system) is being transferred from the earth's spinning to the moon's orbiting. This causes the day to get longer and the moon to orbit farther away. The transfer happens in that direction because the earth is spinning faster than the moon revolves around the earth, and the earth's spinning pulls the tides so that the one facing the moon actually precedes the time when the moon is overhead. That speeds up the moon (which gets a larger orbit) and slows down the earth.

Your problem with the age of the earth comes about because you're thinking of things upside down. In the definition of angular momentum, which is what is really changing, time (the length of the day) is in the denominator. So 4.5 billion years ago, the day wasn't less than zero (which is what you get if you take the current rate of change of the day), but the angular momentum was more than twice what it is now (which is what you get if you convert the rate of change of the day to a rate of change of the earth's angular momentum). So the day was less than 12 hours long (not zero).

There are added complications in the fact that the moon was closer then, but that's the gist of it.

Dr. Eric Christian
(July 2010)

Gravitational force, F, is defined as:

F = G * M1 * M2/(r*r)
G is the gravitational constant
M1 and M2 are the masses (of the Earth and the Moon in this case)
r is the distance between them

This depends upon the product of the two masses and so is exactly the same for both.

What is really happening is that both the Earth and the Moon are orbiting the center of mass of the two bodies. Because the Earth is much heavier than the Moon, that center of mass is actually inside the surface of the Earth (it's about 3000 miles from the center).

Dr. Eric Christian
(November 2007)

This is beyond our area of expertise, but you can find answers to some questions on this page, and you can find a lot more information on the Moon at The Nine Planets site.

Well, I'm no expert on motions of the Earth, but the Moon does add drag to the Earth's rotation in the form of tides, both oceanic and internal. This added drag tends to stabilize the rotation. It is also gradually slowing down the rotation of the Earth, which gradually lengthens Earth days.

Dr. Eric Christian and Beth Barbier

Because of the rotation of the Earth, there will be a moonrise and moonset about once a day. Because of the orbit of the Moon around the Earth, the time of moonrise and moonset will move relative to sunrise and sunset. At the time of the new moon (when solar eclipses can happen), moonrise and sunrise are at about the same time and place, as are moonset and sunset. Fourteen days later at the full moon the moon rises at sunset and sets at sunrise. Things aren't exact because the orbit of the Moon is slightly tilted relative to the Sun.

Dr. Eric Christian
(March 2001)

When we look at the Moon, we are seeing sunlight that is reflected off the Moon's surface and bounced toward the Earth. When the Moon is on the side Earth away from the Sun, the light is reflected back up to Earth's night side, which is in shadow. Then we see the Moon at night. It looks bright because we see it against a dark sky there aren't any other strong sources of light from that direction except for the distant stars. It also looks generally full and round because light from the whole face of the Moon can get reflected back to Earth.

The Moon moves around the Earth roughly once every 28 days. When it is on the side closer to the Sun and at least a little to one side of the Earth-Sun line, light can still reflect from the Moon and reach the Earth. In this case, the light will be reflected onto the sunward side of the Earth and we will see the Moon in the daytime. Of course only light striking the "edge" of the Moon will get reflected toward the Earth so we see only a crescent shape. The Moon seems to look dimmer in the daytime because your eye has to pick out the light from it against the wash of light that comes directly from the Sun and is spread around by the atmosphere.

Another way to think about it is to remember that the Moon is a sphere, like the Earth, and the sunward half of that sphere is always lit by the Sun. Whatever part of that lit half of the Moon can be observed from Earth is what we see. If the Moon is in a position to be observed from the sunward side of Earth, we see it in the daytime. If the Moon is on the side of Earth away from the Sun, we will see it in the night. The Moon seems to change shape during the month because we are able to see only part of its sunlit side depending on how the Moon is lined up with the Earth and Sun.

There are two special points in the Moon's orbit around the Earth when it is very nearly lined up with the Earth-Sun line. If it is on the side away from the Sun, we see the entire lit half of the Moon, we call that a full moon. Sometimes the Moon actually crosses directly into the Earth's shadow, that's called a lunar eclipse. If the Moon is lined up on the sunward side of the Earth, we can see only unlit side. This is called a new moon. Once in a while the Moon's shadow falls on the Earth. This is a solar eclipse.

Dr. Jeff George
(March 2003)

Half the Moon is in direct sunlight nearly all the time. That is why we can see the Moon. The phases are because we see only the sunlit part, and only during a full moon does the sunlit half fully face the Earth. The only times the Moon doesn't get direct sunlight is during lunar eclipses, when the Earth's shadow blocks the Sun.

The temperature changes drastically depending upon whether that portion of the Moon has sunlight (day) or not (night).

Dr. Eric Christian
(September 2007)

The basic reason is because the Moon is so far away, but it also has to do with how your eyes and mind work together (it is what is called an optical illusion). If you look at two things (say a building and a tree) that are different distances from you and then walk sideways, you will see that the two objects shift, but that the closer one shifts to the side more than the one that is farther. The Moon is so far away that it does not appear to shift at all. The human mind interprets this in a funny way. It thinks that the Moon is closer than it really is, but that it is moving sideways at the same speed as you are. So you think that the Moon is following you.

Dr. Eric Christian
(September 2002)

In order for it to maintain its location, the Moon would have to be moved about four times farther away from the Earth, to the L1 libration point. Currently two spacecraft, ACE and SOHO are located near the L1 point, and always stay between the Earth and the Sun. The problem with the Moon at L1 is that, being farther away, the Moon would appear much smaller and not block the entire Sun (although maybe that's not really a problem - the Earth would get colder, but would still get some Sun).

The classic "Earthrise" photo was taken by Apollo 8, which was in orbit around the moon. You are correct, except for regions near the edge of the earthward side of the moon (there is some tilt and wobble). Most of the moon has the Earth in the same place in the sky.

The Earth and the Moon revolve around the center of mass (CM) of the Earth/Moon system, which in turn is revolving around the CM of the solar system, which in turn is revolving around the CM of the Galaxy.

The Earth is most of the mass of the Earth/Moon system, so the CM is closer to the Earth, and the Sun is most of the mass of the solar system, so the CM of the solar system is actually inside the Sun. But from far away, the paths of the Earth and Moon WOULD look like braided strings around the Sun.

A separate motion of the Moon is its rotation (spinning). Since it spins with the same period as its revolution around the Earth/Moon CM, one face always points towards the Earth (not the Sun, all parts of the Moon get day and night).

I can find no news on new intense lunar quakes, so I find this story very hard to beleive. There is no way the Moon, with it's solid core, could shake itself apart. There just isn't enough energy. There are moonquakes, but they are small. They can be caused by tidal forces (with the Earth), temperature fluctuations between day and night, contraction of the Moon's core (by gradual cooling), or meteorite impact. But I fully expect the Moon to be there in one piece 6 months or 6 million years from now.

Dr. Eric Christian
(May 2000)

First, let me state for the record, that planetary objects and satellites do not, as a rule, blow up. If the Moon did, you could expect large chunks of it to hit the Earth at pretty high speeds. Very messy. After a while, the survivors might see a very pretty ring of debris about the Earth.

Dr. Eric Christian
(September 2000)

Your question is beyond our area of expertise, but you might want to check out the Astronomy Society of the Pacific web page on this subject.

The Moon is never lit only on the top or bottom. You can read more about this on Wikipedia.

Dr. Louis Barbier
(April 2008)

  1. Take three students and make them the Sun, Earth, and Moon. Stand the Sun up in front of the classroom facing the Earth and the Moon.
  2. The Earth should face the Sun -- and then the Earth's front is lit by the Sun (day), and its back is dark (night).
  3. Put the Moon behind the Earth, also facing the Sun. Again, the Moon's front is sunlit, and its back is dark. Now only people on the night side of the Earth can see the Moon, but all they see is the front (sunlit) part of the Moon. This is a full moon, and everyone on Earth sees it (when they're on the correct side of the Earth).
  4. If you move the Moon to the Earth's right side, still facing the Sun, everyone on the right side of the Earth sees half of the Moon's front (sunlit) and half of their back (dark). This is the third quarter moon. And again, everyone sees the same Moon.
  5. Stand the Moon directly in front of the Earth, and all you can see is the Moon's back, which is the new moon. Solar eclipses can only happen during a new moon.
  6. If you put the Moon ahead and to the left of the Earth, people on the Earth can see mostly back, but a little bit of the front. This is the waxing crescent.

So the phase of the Moon is set by the position of the Moon relative to the Earth and the Sun, not by where you stand on the Earth.

Dr. Eric Christian
(February 2010)

One good activity involves having students play the parts of the Earth and the Moon. There's a description on this Astronomical Society of the Pacific site.

Beth Barbier
(January 2005)

The Moon rotates on it's axis and revolves around the Earth in 27 1/3 days. The time from one full moon to the next is 29 1/2 days. The reason they are not the same is that during the 27 1/3 days that it take the Moon to go around the Earth, the Earth has moved relative to the Sun, and it takes another 2 days or so before the Moon is directly on the other side of the Sun (which is what gives a full moon). It is similar to the reason why the Earth rotates on its axis once every 23 hours 56 minutes and 4.1 seconds, but its 24 hours from noon to noon.

Dr. Eric Christian
(September 2001)

One good activity involves having students play the parts of the Earth and the Moon. There's a description on this Astronomical Society of the Pacific site: The Moon: It's Just a Phase It's Going Through.

Beth Barbier
(January 2005)

Most of the light from the Moon is direct reflection of sunlight. A lunar eclipse happens because the Earth's shadow crosses the Moon, preventing the Sun from directly illuminating the Moon. You can see some "earthlight", reflection of sunlight that hits the Earth, then hits the Moon and bounces back to the Earth. When there is a crescent moon, you can frequently (if the sky is dark) still see the "unlit" portion of the Moon. That light (much dimmer than normal moonlight) is from the Earth.

Dr. Eric Christian
(November 2000)

Shadows are caused by sunlight, and since the Sun shines on the Moon, there must be shadows. In fact you can determine the height of objects on the Moon by the length of their shadows (just like on Earth).

Dr. Louis Barbier
(April 2001)

I'm not certain what you're referring to here. If you mean that pictures on the Moon show a black sky, you should know that there isn't an atmosphere on the Moon to reflect sunlight, so its sky always looks black. The surface of the Moon, however, does reflect sunlight, so the surface facing the Sun is well-lit, and that side is experiencing daytime. (Think about the definition of day and night on Earth.)

Water was found to exist as very small, very spread out ice crystals in craters on the north and south poles of the Moon. Craters are the result of ancient meteorites that hit the Moon's surface. The amount of ice is very uncertain, but estimates are 10 - 300 million tons spread over the polar regions.

The short answer is "no". Not directly at least. Meteoroids impact the Moon (the Earth, and other planets) because the trajectory of their orbits and that of the Moon intersect. Of course, gravity is responsible for the shape of the orbits themselves, so one might indirectly say gravity is responsible. But not in the way I think you are asking.

Imagine, for example, when you fire a bullet from a gun at a target. While it is true that there is a gravitational attraction between the bullet and the target, the bullet would still hit the target if there were no gravity. Its trajectory will intersect the target location.

Meteoroids are of two distinct classes. Some revolve around the Sun (like the planets do) and have orbits of small eccentricity (nearly circular) and those orbits are near the plane in which the planets orbit. The more common type is associated with debris streaming off comets as they approach the Sun. This debris ends up scattered along the (highly eccentric) orbit of the comet. As the Earth's orbit passes through these debris "clouds" we have meteor showers. Since the Moon is moving along with the Earth, it will often experience many of these meteoroids too, which will strike the Moon's surface.

Dr. Louis Barbier
(April 2003)

The first attempt to measure the amount of meteoritic dust falling onto Earth was made by Hans Pettersson in the 1950s. His measurement came up with a maximum infall rate across the entire Earth (what scientists call an upper limit) of about 15 million tons per year. His sample was contaminated by volcanic dust, etc., and the real number (measured out in space) is only about 20,000 - 40,000 tons per year.

Creationists ignore the new measurements, and the fact that Pettersson's value was an upper limit, and misinform the public. Interestingly enough, there are plenty of places on the Moon where the dust is more than 100 feet deep, but most of the dust is from meteoric impacts on the Moon itself throwing up debris. NASA was NOT expecting a deep layer of dust where Apollo landed (in the highlands). Remember, we had already landed unmanned probes on the Moon before Apollo (Surveyor).

For more information, you can check the Talk.Origins archive.

Dr. Eric Christian
(November 2000)

The mass of the Moon is less than 1/80 (0.0123) that of the Earth, and its diameter is a little more than a quarter (0.273). Gravitational acceleration is proportional to M / (R * R), so for the Moon (0.0123) / (.273 * .273) = 0.165 or about 1/6 that of the Earth.

Dr. Eric Christian
(May 2000)

The gravity on the Moon is about 1/6th that of Earth, which comes out to about 5.3 feet per second per second acceleration (32 * 0.165 = 5.28 feet/sec/sec). To get something off the surface of the Moon you'll need energy equal to the mass of the object times this acceleration times the height you want to achieve (M x G x H). A balloon gets its lift for the same reason a boat floats, it weighs less than what it's displacing. If there is no air to displace, you get no lift.

Drs. Eric Christian and Louis Barbier

The magnetic field of the Earth only extends about a quarter of the way to the Moon. So the compass wouldn't point at the Earth, and since the Moon has nearly no magnetic field, it wouldn't be much good at all.

Dr. Eric Christian
(September 2001)

Definitely, a magnet would stick to an iron bar on the Moon. In fact, it will stick to an iron bar any place in the universe, provided the temperature is not excessively high. The effects of magnetism, such as a magnet and iron sticking together, are based on the presence of a magnet and have nothing to do with the question whether the environment is within a larger magnetic field or not. If astronauts bring a permanent magnet to the Moon they will observe that it will attract any piece made of iron.

They only question may be whether the magnet keeps its magnetic quality or not. If a magnet is heated to more than about 700 degrees centigrade, it will lose its magnetism and behave like an ordinary hunk of iron. However, on the Moon it is not hot enough that this would happen. So the magnet will still work.

Now to the second question: will the magnet induce magnetism on the iron? Yes, it will. Again, any permanent magnet will induce magnetism on a piece of iron on the Moon. The only difference between the Earth and the Moon is that the Earth's magnetic field can induce magnetism on a piece of iron by itself, similar to what a refrigerator magnet can do. This can't happen on the Moon, but any magnet still can induce magnetism there.

Dr. Eberhard Moebius
(August 2003)

Nothing in the flashlight (basically just batteries, wires, switch, and bulb) requires air, so it would work fine.

Dr. Eric Christian
(April 2002)

There is no air on the Moon. NASA embedded stiff wire in the American flag so that it wouldn't just hang straight down, and adjusted the wire so that the flag appeared to be waving. They thought (probably rightly) that a flag as flat as a board wouldn't look right.

Dr. Eric Christian
(November 2000)

Parachutes use air resistance to slow down a descent. Since there is no air on the Moon, they would be completely ineffective. The parachute would fall at the same rate as a rock.

Dr. Eric Christian
(July 2002)

Actually, both arguments have some element of truth. You do need oxygen to ignite the firearm and space is for all intents and purposes a vacuum (with no available oxygen to take part in combustion). Now engineers have been able to get around this difficulty over the years with rocket fuel by mixing both a fuel and an oxidizer together in the rocket's combustion chamber. For example in solid rocket boosters (SRB) the typical mixture consists of an ammonium perchlorate (oxidizer, 69.6 percent by weight), aluminum (fuel, 16 percent), iron oxide (a catalyst, 0.4 percent), a polymer (a binder that holds the mixture together, 12.04 percent), and an epoxy curing agent (1.96 percent). You can read more about SRBs on the Kennedy Space Center site.

Now as it turns out, guns are also self-igniting systems, where the mixture of fuel and oxidizer is usually located inside the bullet itself. So indeed, guns can be fired in space, although you won't hear the typical accompanying "boom"! Of course, a firearm could indeed be used as a method for propulsion, although there may be more efficient ways to acquire propulsion in space. See this site on rocket propulsion for more details on propulsion in space.

Dr. Georgia de Nolfo
(April 2003)

The StarChild website has such a test: Problems in Space.

Beth Barbier
(November 2004)

The Moon will not leave the Earth's gravity, even though the orbit of the Moon is increasing slightly. The Earth's rotation is slowing down (due to "tidal braking"), and to conserve angular momentum the Moon is accelerating. The Moon's orbit increases by about 3 cm/year.

The Earth and the Moon eventually will be "locked" together with each only having one side constantly facing the other. (Right now the same side of the Moon faces the Earth, but all sides of the Earth see the Moon. In the future this will not be true!) Life on Earth will be quite different then, but this won't occur for billions of years yet. When it does occur, the Moon's orbit will be 50% larger than it is now, and a month will be about 50 days.

The Great Wall can barely be seen from the Shuttle, so it would not be possible to see it from the Moon with the naked eye.

The Moon was called Selene or Artemis by the Greeks and Luna by the Romans. I'm sure other cultures also had names for the Moon. But in English, Moon (from Mona and Moone in Old and Middle English) was used before anyone had any idea that the other planets had moons. So it was more a case that the specific name for the Moon was extended to mean small bodies revolving around planets elsewhere. The Moon's name is the Moon.


The usual English proper name for Earth's natural satellite is simply the Moon, with a capital M. [18] [19] The noun moon is derived from Old English mōna, which (like all its Germanic cognates) stems from Proto-Germanic *mēnōn, [20] which in turn comes from Proto-Indo-European *mēnsis "month" [21] (from earlier *mēnōt, genitive *mēneses) which may be related to the verb "measure" (of time). [22]

Occasionally, the name Luna / ˈ l uː n ə / is used in scientific writing [23] and especially in science fiction to distinguish the Earth's moon from others, while in poetry "Luna" has been used to denote personification of the Moon. [24] Cynthia / ˈ s ɪ n θ i ə / is another poetic name, though rare, for the Moon personified as a goddess, [25] while Selene / s ə ˈ l iː n iː / (literally "Moon") is the Greek goddess of the Moon.

The usual English adjective pertaining to the Moon is "lunar", derived from the Latin word for the Moon, lūna. The adjective selenian / s ə l iː n i ə n / , [26] derived from the Greek word for the Moon, σελήνη selēnē, and used to describe the Moon as a world rather than as an object in the sky, is rare, [27] while its cognate selenic was originally a rare synonym [28] but now nearly always refers to the chemical element selenium. [29] The Greek word for the Moon does however provide us with the prefix seleno-, as in selenography, the study of the physical features of the Moon, as well as the element name selenium. [30] [31]

The Greek goddess of the wilderness and the hunt, Artemis, equated with the Roman Diana, one of whose symbols was the Moon and who was often regarded as the goddess of the Moon, was also called Cynthia, from her legendary birthplace on Mount Cynthus. [32] These names – Luna, Cynthia and Selene – are reflected in technical terms for lunar orbits such as apolune, pericynthion and selenocentric.

Isotope dating of lunar samples suggests the Moon formed around 50 million years after the origin of the Solar System. [33] [34] Historically, several formation mechanisms have been proposed, [35] but none satisfactorily explained the features of the Earth–Moon system. A fission of the Moon from Earth's crust through centrifugal force [36] would require too great an initial rotation rate of Earth. [37] Gravitational capture of a pre-formed Moon [38] depends on an unfeasibly extended atmosphere of Earth to dissipate the energy of the passing Moon. [37] A co-formation of Earth and the Moon together in the primordial accretion disk does not explain the depletion of metals in the Moon. [37] None of these hypotheses can account for the high angular momentum of the Earth–Moon system. [39]

The prevailing theory is that the Earth–Moon system formed after a giant impact of a Mars-sized body (named Theia) with the proto-Earth. The impact blasted material into Earth's orbit and then the material accreted and formed the Moon [40] [41] just beyond the Earth's Roche limit of

2.56 R . [42] This theory best explains the evidence.

Giant impacts are thought to have been common in the early Solar System. Computer simulations of giant impacts have produced results that are consistent with the mass of the lunar core and the angular momentum of the Earth–Moon system. These simulations also show that most of the Moon derived from the impactor, rather than the proto-Earth. [43] However, more recent simulations suggest a larger fraction of the Moon derived from the proto-Earth. [44] [45] [46] [47] Other bodies of the inner Solar System such as Mars and Vesta have, according to meteorites from them, very different oxygen and tungsten isotopic compositions compared to Earth. However, Earth and the Moon have nearly identical isotopic compositions. The isotopic equalization of the Earth-Moon system might be explained by the post-impact mixing of the vaporized material that formed the two, [48] although this is debated. [49]

The impact released a lot of energy and then the released material re-accreted into the Earth–Moon system. This would have melted the outer shell of Earth, and thus formed a magma ocean. [50] [51] Similarly, the newly formed Moon would also have been affected and had its own lunar magma ocean its depth is estimated from about 500 km (300 miles) to 1,737 km (1,079 miles). [50]

While the giant-impact theory explains many lines of evidence, some questions are still unresolved, most of which involve the Moon's composition. [52]

In 2001, a team at the Carnegie Institute of Washington reported the most precise measurement of the isotopic signatures of lunar rocks. [53] The rocks from the Apollo program had the same isotopic signature as rocks from Earth, differing from almost all other bodies in the Solar System. This observation was unexpected, because most of the material that formed the Moon was thought to come from Theia and it was announced in 2007 that there was less than a 1% chance that Theia and Earth had identical isotopic signatures. [54] Other Apollo lunar samples had in 2012 the same titanium isotopes composition as Earth, [55] which conflicts with what is expected if the Moon formed far from Earth or is derived from Theia. These discrepancies may be explained by variations of the giant-impact theory.

The Moon is a very slightly scalene ellipsoid due to tidal stretching, with its long axis displaced 30° from facing the Earth, due to gravitational anomalies from impact basins. Its shape is more elongated than current tidal forces can account for. This 'fossil bulge' indicates that the Moon solidified when it orbited at half its current distance to the Earth, and that it is now too cold for its shape to adjust to its orbit. [56]

Internal structure

Lunar surface chemical composition [57]
Compound Formula Composition
Maria Highlands
silica SiO2 45.4% 45.5%
alumina Al2O3 14.9% 24.0%
lime CaO 11.8% 15.9%
iron(II) oxide FeO 14.1% 5.9%
magnesia MgO 9.2% 7.5%
titanium dioxide TiO2 3.9% 0.6%
sodium oxide Na2O 0.6% 0.6%
99.9% 100.0%

The Moon is a differentiated body that was initially in hydrostatic equilibrium but has since departed from this condition. [58] It has a geochemically distinct crust, mantle, and core. The Moon has a solid iron-rich inner core with a radius possibly as small as 240 kilometres (150 mi) and a fluid outer core primarily made of liquid iron with a radius of roughly 300 kilometres (190 mi). Around the core is a partially molten boundary layer with a radius of about 500 kilometres (310 mi). [59] [60] This structure is thought to have developed through the fractional crystallization of a global magma ocean shortly after the Moon's formation 4.5 billion years ago. [61]

Crystallization of this magma ocean would have created a mafic mantle from the precipitation and sinking of the minerals olivine, clinopyroxene, and orthopyroxene after about three-quarters of the magma ocean had crystallised, lower-density plagioclase minerals could form and float into a crust atop. [62] The final liquids to crystallise would have been initially sandwiched between the crust and mantle, with a high abundance of incompatible and heat-producing elements. [1] Consistent with this perspective, geochemical mapping made from orbit suggests a crust of mostly anorthosite. [14] The Moon rock samples of the flood lavas that erupted onto the surface from partial melting in the mantle confirm the mafic mantle composition, which is more iron-rich than that of Earth. [1] The crust is on average about 50 kilometres (31 mi) thick. [1]

The Moon is the second-densest satellite in the Solar System, after Io. [63] However, the inner core of the Moon is small, with a radius of about 350 kilometres (220 mi) or less, [1] around 20% of the radius of the Moon. Its composition is not well understood, but is probably metallic iron alloyed with a small amount of sulfur and nickel analyses of the Moon's time-variable rotation suggest that it is at least partly molten. [64] The pressure at the lunar core is estimated to be 5 GPa . [65]

Magnetic field

The Moon has an external magnetic field of generally less than 0.2 nanoteslas, [66] or less than one hundred thousandth that of Earth. The Moon does not currently have a global dipolar magnetic field and only has crustal magnetization likely acquired early in its history when a dynamo was still operating. [67] [68] However, early in its history, 4 billion years ago, its magnetic field strength was likely close to that of Earth today. [66] This early dynamo field apparently expired by about one billion years ago, after the lunar core had completely crystallized. [66] Theoretically, some of the remnant magnetization may originate from transient magnetic fields generated during large impacts through the expansion of plasma clouds. These clouds are generated during large impacts in an ambient magnetic field. This is supported by the location of the largest crustal magnetizations situated near the antipodes of the giant impact basins. [69]

Surface geology

The topography of the Moon has been measured with laser altimetry and stereo image analysis. [70] Its most extensive topographic feature is the giant far-side South Pole–Aitken basin, some 2,240 km (1,390 mi) in diameter, the largest crater on the Moon and the second-largest confirmed impact crater in the Solar System. [71] [72] At 13 km (8.1 mi) deep, its floor is the lowest point on the surface of the Moon. [71] [73] The highest elevations of the Moon's surface are located directly to the northeast, which might have been thickened by the oblique formation impact of the South Pole–Aitken basin. [74] Other large impact basins such as Imbrium, Serenitatis, Crisium, Smythii, and Orientale possess regionally low elevations and elevated rims. [71] The far side of the lunar surface is on average about 1.9 km (1.2 mi) higher than that of the near side. [1]

The discovery of fault scarp cliffs suggest that the Moon has shrunk by about 90 metres (300 ft) within the past billion years. [75] Similar shrinkage features exist on Mercury. Mare Frigoris, a basin near the north pole long assumed to be geologically dead, has cracked and shifted. Since the Moon doesn't have tectonic plates, its tectonic activity is slow and cracks develop as it loses heat. [76]

Volcanic features

The dark and relatively featureless lunar plains, clearly seen with the naked eye, are called maria (Latin for "seas" singular mare), as they were once believed to be filled with water [77] they are now known to be vast solidified pools of ancient basaltic lava. Although similar to terrestrial basalts, lunar basalts have more iron and no minerals altered by water. [78] The majority of these lava deposits erupted or flowed into the depressions associated with impact basins. Several geologic provinces containing shield volcanoes and volcanic domes are found within the near side "maria". [79]

Almost all maria are on the near side of the Moon, and cover 31% of the surface of the near side [80] compared with 2% of the far side. [81] This is likely due to a concentration of heat-producing elements under the crust on the near side, which would have caused the underlying mantle to heat up, partially melt, rise to the surface and erupt. [62] [82] [83] Most of the Moon's mare basalts erupted during the Imbrian period, 3.0–3.5 billion years ago, although some radiometrically dated samples are as old as 4.2 billion years. [84] As of 2003, crater counting studies of the youngest eruptions appeared to suggest they formed no earlier than 1.2 billion years ago. [85]

In 2006, a study of Ina, a tiny depression in Lacus Felicitatis, found jagged, relatively dust-free features that, because of the lack of erosion by infalling debris, appeared to be only 2 million years old. [86] Moonquakes and releases of gas also indicate some continued lunar activity. [86] Evidence of recent lunar volcanism has been identified at 70 irregular mare patches, some less than 50 million years old. This raises the possibility of a much warmer lunar mantle than previously believed, at least on the near side where the deep crust is substantially warmer because of the greater concentration of radioactive elements. [87] [88] [89] [90] Evidence has been found for 2–10 million years old basaltic volcanism within the crater Lowell, [91] [92] inside the Orientale basin. Some combination of an initially hotter mantle and local enrichment of heat-producing elements in the mantle could be responsible for prolonged activities on the far side in the Orientale basin. [93] [94]

The lighter-colored regions of the Moon are called terrae, or more commonly highlands, because they are higher than most maria. They have been radiometrically dated to having formed 4.4 billion years ago, and may represent plagioclase cumulates of the lunar magma ocean. [84] [85] In contrast to Earth, no major lunar mountains are believed to have formed as a result of tectonic events. [95]

The concentration of maria on the near side likely reflects the substantially thicker crust of the highlands of the Far Side, which may have formed in a slow-velocity impact of a second moon of Earth a few tens of millions of years after the Moon's formation. [96] [97] Alternatively, it may be a consequence of asymmetrical tidal heating when the Moon was much closer to the Earth. [98]

Impact craters

A major geologic process that has affected the Moon's surface is impact cratering, [99] with craters formed when asteroids and comets collide with the lunar surface. There are estimated to be roughly 300,000 craters wider than 1 km (0.6 mi) on the Moon's near side. [100] The lunar geologic timescale is based on the most prominent impact events, including Nectaris, Imbrium, and Orientale structures characterized by multiple rings of uplifted material, between hundreds and thousands of kilometers in diameter and associated with a broad apron of ejecta deposits that form a regional stratigraphic horizon. [101] The lack of an atmosphere, weather, and recent geological processes mean that many of these craters are well-preserved. Although only a few multi-ring basins have been definitively dated, they are useful for assigning relative ages. Because impact craters accumulate at a nearly constant rate, counting the number of craters per unit area can be used to estimate the age of the surface. [101] The radiometric ages of impact-melted rocks collected during the Apollo missions cluster between 3.8 and 4.1 billion years old: this has been used to propose a Late Heavy Bombardment period of increased impacts. [102]

Blanketed on top of the Moon's crust is a highly comminuted (broken into ever smaller particles) and impact gardened surface layer called regolith, formed by impact processes. The finer regolith, the lunar soil of silicon dioxide glass, has a texture resembling snow and a scent resembling spent gunpowder. [103] The regolith of older surfaces is generally thicker than for younger surfaces: it varies in thickness from 10–20 km (6.2–12.4 mi) in the highlands and 3–5 km (1.9–3.1 mi) in the maria. [104] Beneath the finely comminuted regolith layer is the megaregolith, a layer of highly fractured bedrock many kilometers thick. [105]

High-resolution images from the Lunar Reconnaissance Orbiter in the 2010s show a contemporary crater-production rate significantly higher than was previously estimated. A secondary cratering process caused by distal ejecta is thought to churn the top two centimeters of regolith on a timescale of 81,000 years. [106] [107] This rate is 100 times faster than the rate computed from models based solely on direct micrometeorite impacts. [108]

Gravitational field

The gravitational field of the Moon has been measured through tracking the Doppler shift of radio signals emitted by orbiting spacecraft. The main lunar gravity features are mascons, large positive gravitational anomalies associated with some of the giant impact basins, partly caused by the dense mare basaltic lava flows that fill those basins. [109] [110] The anomalies greatly influence the orbit of spacecraft about the Moon. There are some puzzles: lava flows by themselves cannot explain all of the gravitational signature, and some mascons exist that are not linked to mare volcanism. [111]

Lunar swirls

Lunar swirls are enigmatic features found across the Moon's surface. They are characterized by a high albedo, appear optically immature (i.e. the optical characteristics of a relatively young regolith), and have often a sinuous shape. Their shape is often accentuated by low albedo regions that wind between the bright swirls. They are located in places with enhanced surface magnetic fields and many are located at the antipodal point of major impacts. Well known swirls include the Reiner Gamma feature and Mare Ingenii. They are hypothesized to be areas that have been partially shielded from the solar wind, resulting in slower space weathering. [112]

Presence of water

Liquid water cannot persist on the lunar surface. When exposed to solar radiation, water quickly decomposes through a process known as photodissociation and is lost to space. However, since the 1960s, scientists have hypothesized that water ice may be deposited by impacting comets or possibly produced by the reaction of oxygen-rich lunar rocks, and hydrogen from solar wind, leaving traces of water which could possibly persist in cold, permanently shadowed craters at either pole on the Moon. [113] [114] Computer simulations suggest that up to 14,000 km 2 (5,400 sq mi) of the surface may be in permanent shadow. [115] The presence of usable quantities of water on the Moon is an important factor in rendering lunar habitation as a cost-effective plan the alternative of transporting water from Earth would be prohibitively expensive. [116]

In years since, signatures of water have been found to exist on the lunar surface. [117] In 1994, the bistatic radar experiment located on the Clementine spacecraft, indicated the existence of small, frozen pockets of water close to the surface. However, later radar observations by Arecibo, suggest these findings may rather be rocks ejected from young impact craters. [118] In 1998, the neutron spectrometer on the Lunar Prospector spacecraft showed that high concentrations of hydrogen are present in the first meter of depth in the regolith near the polar regions. [119] Volcanic lava beads, brought back to Earth aboard Apollo 15, showed small amounts of water in their interior. [120]

The 2008 Chandrayaan-1 spacecraft has since confirmed the existence of surface water ice, using the on-board Moon Mineralogy Mapper. The spectrometer observed absorption lines common to hydroxyl, in reflected sunlight, providing evidence of large quantities of water ice, on the lunar surface. The spacecraft showed that concentrations may possibly be as high as 1,000 ppm. [121] Using the mapper's reflectance spectra, indirect lighting of areas in shadow confirmed water ice within 20° latitude of both poles in 2018. [122] In 2009, LCROSS sent a 2,300 kg (5,100 lb) impactor into a permanently shadowed polar crater, and detected at least 100 kg (220 lb) of water in a plume of ejected material. [123] [124] Another examination of the LCROSS data showed the amount of detected water to be closer to 155 ± 12 kg (342 ± 26 lb). [125]

In May 2011, 615–1410 ppm water in melt inclusions in lunar sample 74220 was reported, [126] the famous high-titanium "orange glass soil" of volcanic origin collected during the Apollo 17 mission in 1972. The inclusions were formed during explosive eruptions on the Moon approximately 3.7 billion years ago. This concentration is comparable with that of magma in Earth's upper mantle. Although of considerable selenological interest, this announcement affords little comfort to would-be lunar colonists – the sample originated many kilometers below the surface, and the inclusions are so difficult to access that it took 39 years to find them with a state-of-the-art ion microprobe instrument.

Analysis of the findings of the Moon Mineralogy Mapper (M3) revealed in August 2018 for the first time "definitive evidence" for water-ice on the lunar surface. [127] [128] The data revealed the distinct reflective signatures of water-ice, as opposed to dust and other reflective substances. [129] The ice deposits were found on the North and South poles, although it is more abundant in the South, where water is trapped in permanently shadowed craters and crevices, allowing it to persist as ice on the surface since they are shielded from the sun. [127] [129]

In October 2020, astronomers reported detecting molecular water on the sunlit surface of the Moon by several independent spacecraft, including the Stratospheric Observatory for Infrared Astronomy (SOFIA). [130] [131] [132] [133]

Surface conditions

The surface of the Moon is an extreme environment with temperatures that range from 140 °C down to −171 °C , an atmospheric pressure of 10 −10 Pa, and high levels of ionizing radiation from the Sun and cosmic rays. The exposed surfaces of spacecraft are considered unlikely to harbor bacterial spoors after just one lunar orbit. [134] The surface gravity of the Moon is approximately 1.625 m/s 2 , about 16.6% that on Earth's surface or 0.166 ɡ . [4]


The Moon has an atmosphere so tenuous as to be nearly vacuum, with a total mass of less than 10 tonnes (9.8 long tons 11 short tons). [137] The surface pressure of this small mass is around 3 × 10 −15 atm (0.3 nPa) it varies with the lunar day. Its sources include outgassing and sputtering, a product of the bombardment of lunar soil by solar wind ions. [14] [138] Elements that have been detected include sodium and potassium, produced by sputtering (also found in the atmospheres of Mercury and Io) helium-4 and neon [139] from the solar wind and argon-40, radon-222, and polonium-210, outgassed after their creation by radioactive decay within the crust and mantle. [140] [141] The absence of such neutral species (atoms or molecules) as oxygen, nitrogen, carbon, hydrogen and magnesium, which are present in the regolith, is not understood. [140] Water vapor has been detected by Chandrayaan-1 and found to vary with latitude, with a maximum at

60–70 degrees it is possibly generated from the sublimation of water ice in the regolith. [142] These gases either return into the regolith because of the Moon's gravity or are lost to space, either through solar radiation pressure or, if they are ionized, by being swept away by the solar wind's magnetic field. [140]

Studies of Moon magma samples retrieved by the Apollo missions demonstrate that the Moon had once possessed a relatively thick atmosphere for a period of 70 million years between 3 and 4 billion years ago. This atmosphere, sourced from gases ejected from lunar volcanic eruptions, was twice the thickness of that of present-day Mars. The ancient lunar atmosphere was eventually stripped away by solar winds and dissipated into space. [143]

A permanent Moon dust cloud exists around the Moon, generated by small particles from comets. Estimates are 5 tons of comet particles strike the Moon's surface every 24 hours, resulting in the ejection of dust particles. The dust stays above the Moon approximately 10 minutes, taking 5 minutes to rise, and 5 minutes to fall. On average, 120 kilograms of dust are present above the Moon, rising up to 100 kilometers above the surface. Dust counts made by LADEE's Lunar Dust EXperiment (LDEX) found particle counts peaked during the Geminid, Quadrantid, Northern Taurid, and Omicron Centaurid meteor showers, when the Earth, and Moon pass through comet debris. The lunar dust cloud is asymmetric, being more dense near the boundary between the Moon's dayside and nightside. [144] [145]

Lunar distance

Scale model of the Earth–Moon system: Sizes and distances are to scale.


Because of tidal locking, the rotation of the Moon around its own axis is synchronous to its orbital period around the Earth. The Moon makes a complete orbit around Earth with respect to the fixed stars about once every 27.3 days, [g] its sidereal period. However, because Earth is moving in its orbit around the Sun at the same time, it takes slightly longer for the Moon to show the same phase to Earth, which is about 29.5 days [h] its synodic period. [80] [146]

Unlike most satellites of other planets, the Moon orbits closer to the ecliptic plane than to the planet's equatorial plane. The Moon's orbit is subtly perturbed by the Sun and Earth in many small, complex and interacting ways. For example, the plane of the Moon's orbit gradually rotates once every 18.61 years, [147] which affects other aspects of lunar motion. These follow-on effects are mathematically described by Cassini's laws. [148]

The Moon's axial tilt with respect to the ecliptic is only 1.5427°, [8] [149] much less than the 23.44° of Earth. Because of this, the Moon's solar illumination varies much less with season, and topographical details play a crucial role in seasonal effects. [150] From images taken by Clementine in 1994, it appears that four mountainous regions on the rim of the crater Peary at the Moon's north pole may remain illuminated for the entire lunar day, creating peaks of eternal light. No such regions exist at the south pole. Similarly, there are places that remain in permanent shadow at the bottoms of many polar craters, [115] and these "craters of eternal darkness" are extremely cold: Lunar Reconnaissance Orbiter measured the lowest summer temperatures in craters at the southern pole at 35 K (−238 °C −397 °F) [151] and just 26 K (−247 °C −413 °F) close to the winter solstice in the north polar crater Hermite. This is the coldest temperature in the Solar System ever measured by a spacecraft, colder even than the surface of Pluto. [150] Average temperatures of the Moon's surface are reported, but temperatures of different areas will vary greatly depending upon whether they are in sunlight or shadow. [152]

Relative size

The Moon is an exceptionally large natural satellite relative to Earth: Its diameter is more than a quarter and its mass is 1/81 of Earth's. [80] It is the largest moon in the Solar System relative to the size of its planet, [i] though Charon is larger relative to the dwarf planet Pluto, at 1/9 Pluto's mass. [j] [153] The Earth and the Moon's barycentre, their common center of mass, is located 1,700 km (1,100 mi) (about a quarter of Earth's radius) beneath the Earth's surface.

The Earth revolves around the Earth-Moon barycentre once a sidereal month, with 1/81 the speed of the Moon, or about 12.5 metres (41 ft) per second. This motion is superimposed on the much larger revolution of the Earth around the Sun at a speed of about 30 kilometres (19 mi) per second.

The surface area of the Moon is slightly less than the areas of North and South America combined.

Appearance from Earth

The synchronous rotation of the Moon as it orbits the Earth results in it always keeping nearly the same face turned towards the planet. However, because of the effect of libration, about 59% of the Moon's surface can actually be seen from Earth. The side of the Moon that faces Earth is called the near side, and the opposite the far side. The far side is often inaccurately called the "dark side", but it is in fact illuminated as often as the near side: once every 29.5 Earth days. During new moon, the near side is dark. [154]

The Moon originally rotated at a faster rate, but early in its history its rotation slowed and became tidally locked in this orientation as a result of frictional effects associated with tidal deformations caused by Earth. [155] With time, the energy of rotation of the Moon on its axis was dissipated as heat, until there was no rotation of the Moon relative to Earth. In 2016, planetary scientists using data collected on the 1998-99 NASA Lunar Prospector mission, found two hydrogen-rich areas (most likely former water ice) on opposite sides of the Moon. It is speculated that these patches were the poles of the Moon billions of years ago before it was tidally locked to Earth. [156]

The Moon has an exceptionally low albedo, giving it a reflectance that is slightly brighter than that of worn asphalt. Despite this, it is the brightest object in the sky after the Sun. [80] [k] This is due partly to the brightness enhancement of the opposition surge the Moon at quarter phase is only one-tenth as bright, rather than half as bright, as at full moon. [157] Additionally, color constancy in the visual system recalibrates the relations between the colors of an object and its surroundings, and because the surrounding sky is comparatively dark, the sunlit Moon is perceived as a bright object. The edges of the full moon seem as bright as the center, without limb darkening, because of the reflective properties of lunar soil, which retroreflects light more towards the Sun than in other directions. The Moon does appear larger when close to the horizon, but this is a purely psychological effect, known as the Moon illusion, first described in the 7th century BC. [158] The full Moon's angular diameter is about 0.52° (on average) in the sky, roughly the same apparent size as the Sun (see § Eclipses).

The Moon's highest altitude at culmination varies by its phase and time of year. The full moon is highest in the sky during winter (for each hemisphere). The orientation of the Moon's crescent also depends on the latitude of the viewing location an observer in the tropics can see a smile-shaped crescent Moon. [159] The Moon is visible for two weeks every 27.3 days at the North and South Poles. Zooplankton in the Arctic use moonlight when the Sun is below the horizon for months on end. [160]

The distance between the Moon and Earth varies from around 356,400 km (221,500 mi) to 406,700 km (252,700 mi) at perigee (closest) and apogee (farthest), respectively. On 14 November 2016, it was closer to Earth when at full phase than it has been since 1948, 14% closer than its farthest position in apogee. [161] Reported as a "supermoon", this closest point coincided within an hour of a full moon, and it was 30% more luminous than when at its greatest distance because its angular diameter is 14% greater and 1.14 2 ≈ 1.30 approx 1.30> . [162] [163] [164] At lower levels, the human perception of reduced brightness as a percentage is provided by the following formula: [165] [166]

When the actual reduction is 1.00 / 1.30, or about 0.770, the perceived reduction is about 0.877, or 1.00 / 1.14. This gives a maximum perceived increase of 14% between apogee and perigee moons of the same phase. [167]

There has been historical controversy over whether features on the Moon's surface change over time. Today, many of these claims are thought to be illusory, resulting from observation under different lighting conditions, poor astronomical seeing, or inadequate drawings. However, outgassing does occasionally occur and could be responsible for a minor percentage of the reported lunar transient phenomena. Recently, it has been suggested that a roughly 3 km (1.9 mi) diameter region of the lunar surface was modified by a gas release event about a million years ago. [168] [169]

The Moon's appearance, like the Sun's, can be affected by Earth's atmosphere. Common optical effects are the 22° halo ring, formed when the Moon's light is refracted through the ice crystals of high cirrostratus clouds, and smaller coronal rings when the Moon is seen through thin clouds. [170]

The illuminated area of the visible sphere (degree of illumination) is given by ( 1 − cos ⁡ e ) / 2 = sin 2 ⁡ ( e / 2 ) (e/2)> , where e is the elongation (i.e., the angle between Moon, the observer on Earth, and the Sun).


Eclipses only occur when the Sun, Earth, and Moon are all in a straight line (termed "syzygy"). Solar eclipses occur at new moon, when the Moon is between the Sun and Earth. In contrast, lunar eclipses occur at full moon, when Earth is between the Sun and Moon. The apparent size of the Moon is roughly the same as that of the Sun, with both being viewed at close to one-half a degree wide. The Sun is much larger than the Moon but it is the vastly greater distance that gives it the same apparent size as the much closer and much smaller Moon from the perspective of Earth. The variations in apparent size, due to the non-circular orbits, are nearly the same as well, though occurring in different cycles. This makes possible both total (with the Moon appearing larger than the Sun) and annular (with the Moon appearing smaller than the Sun) solar eclipses. [172] In a total eclipse, the Moon completely covers the disc of the Sun and the solar corona becomes visible to the naked eye. Because the distance between the Moon and Earth is very slowly increasing over time, [173] the angular diameter of the Moon is decreasing. Also, as it evolves toward becoming a red giant, the size of the Sun, and its apparent diameter in the sky, are slowly increasing. [l] The combination of these two changes means that hundreds of millions of years ago, the Moon would always completely cover the Sun on solar eclipses, and no annular eclipses were possible. Likewise, hundreds of millions of years in the future, the Moon will no longer cover the Sun completely, and total solar eclipses will not occur. [174]

Because the Moon's orbit around Earth is inclined by about 5.145° (5° 9') to the orbit of Earth around the Sun, eclipses do not occur at every full and new moon. For an eclipse to occur, the Moon must be near the intersection of the two orbital planes. [175] The periodicity and recurrence of eclipses of the Sun by the Moon, and of the Moon by Earth, is described by the saros, which has a period of approximately 18 years. [176]

Because the Moon continuously blocks the view of a half-degree-wide circular area of the sky, [m] [177] the related phenomenon of occultation occurs when a bright star or planet passes behind the Moon and is occulted: hidden from view. In this way, a solar eclipse is an occultation of the Sun. Because the Moon is comparatively close to Earth, occultations of individual stars are not visible everywhere on the planet, nor at the same time. Because of the precession of the lunar orbit, each year different stars are occulted. [178]

Tidal effects

The gravitational attraction that masses have for one another decreases inversely with the square of the distance of those masses from each other. As a result, the slightly greater attraction that the Moon has for the side of Earth closest to the Moon, as compared to the part of the Earth opposite the Moon, results in tidal forces. Tidal forces affect both the Earth's crust and oceans.

The most obvious effect of tidal forces is to cause two bulges in the Earth's oceans, one on the side facing the Moon and the other on the side opposite. This results in elevated sea levels called ocean tides. [173] As the Earth rotates on its axis, one of the ocean bulges (high tide) is held in place "under" the Moon, while another such tide is opposite. As a result, there are two high tides, and two low tides in about 24 hours. [173] Since the Moon is orbiting the Earth in the same direction of the Earth's rotation, the high tides occur about every 12 hours and 25 minutes the 25 minutes is due to the Moon's time to orbit the Earth. The Sun has the same tidal effect on the Earth, but its forces of attraction are only 40% that of the Moon's the Sun's and Moon's interplay is responsible for spring and neap tides. [173] If the Earth were a water world (one with no continents) it would produce a tide of only one meter, and that tide would be very predictable, but the ocean tides are greatly modified by other effects: the frictional coupling of water to Earth's rotation through the ocean floors, the inertia of water's movement, ocean basins that grow shallower near land, the sloshing of water between different ocean basins. [179] As a result, the timing of the tides at most points on the Earth is a product of observations that are explained, incidentally, by theory.

While gravitation causes acceleration and movement of the Earth's fluid oceans, gravitational coupling between the Moon and Earth's solid body is mostly elastic and plastic. The result is a further tidal effect of the Moon on the Earth that causes a bulge of the solid portion of the Earth nearest the Moon. Delays in the tidal peaks of both ocean and solid-body tides cause torque in opposition to the Earth's rotation. This "drains" angular momentum and rotational kinetic energy from Earth's rotation, slowing the Earth's rotation. [173] [180] That angular momentum, lost from the Earth, is transferred to the Moon in a process (confusingly known as tidal acceleration), which lifts the Moon into a higher orbit and results in its lower orbital speed about the Earth. Thus the distance between Earth and Moon is increasing, and the Earth's rotation is slowing in reaction. [180] Measurements from laser reflectors left during the Apollo missions (lunar ranging experiments) have found that the Moon's distance increases by 38 mm (1.5 in) per year (roughly the rate at which human fingernails grow). [181] [182] [183] Atomic clocks also show that Earth's day lengthens by about 17 microseconds every year, [184] [185] [186] slowly increasing the rate at which UTC is adjusted by leap seconds. This tidal drag would continue until the rotation of Earth and the orbital period of the Moon matched, creating mutual tidal locking between the two and suspending the Moon over one meridian (this is currently the case with Pluto and its moon Charon). However, the Sun will become a red giant engulfing the Earth-Moon system long before this occurrence. [187] [188]

In a like manner, the lunar surface experiences tides of around 10 cm (4 in) amplitude over 27 days, with three components: a fixed one due to Earth, because they are in synchronous rotation, a variable tide due to orbital eccentricity and inclination, and a small varying component from the Sun. [180] The Earth-induced variable component arises from changing distance and libration, a result of the Moon's orbital eccentricity and inclination (if the Moon's orbit were perfectly circular and un-inclined, there would only be solar tides). [180] Libration also changes the angle from which the Moon is seen, allowing a total of about 59% of its surface to be seen from Earth over time. [80] The cumulative effects of stress built up by these tidal forces produces moonquakes. Moonquakes are much less common and weaker than are earthquakes, although moonquakes can last for up to an hour – significantly longer than terrestrial quakes – because of scattering of the seismic vibrations in the dry fragmented upper crust. The existence of moonquakes was an unexpected discovery from seismometers placed on the Moon by Apollo astronauts from 1969 through 1972. [189]

According to recent research, scientists suggest that the Moon's influence on the Earth may contribute to maintaining Earth's magnetic field. [190]

Before spaceflight

One of the earliest-discovered possible depictions of the Moon is a 5000-year-old rock carving Orthostat 47 at Knowth, Ireland. [191] [192]

Understanding of the Moon's cycles was an early development of astronomy: by the 5th century BC , Babylonian astronomers had recorded the 18-year Saros cycle of lunar eclipses, [193] and Indian astronomers had described the Moon's monthly elongation. [194] The Chinese astronomer Shi Shen (fl. 4th century BC) gave instructions for predicting solar and lunar eclipses. [195] ( p411 ) Later, the physical form of the Moon and the cause of moonlight became understood. The ancient Greek philosopher Anaxagoras (d. 428 BC) reasoned that the Sun and Moon were both giant spherical rocks, and that the latter reflected the light of the former. [196] [195] ( p227 ) Although the Chinese of the Han Dynasty believed the Moon to be energy equated to qi, their 'radiating influence' theory also recognized that the light of the Moon was merely a reflection of the Sun, and Jing Fang (78–37 BC) noted the sphericity of the Moon. [195] ( pp413–414 ) In the 2nd century AD, Lucian wrote the novel A True Story, in which the heroes travel to the Moon and meet its inhabitants. In 499 AD, the Indian astronomer Aryabhata mentioned in his Aryabhatiya that reflected sunlight is the cause of the shining of the Moon. [197] The astronomer and physicist Alhazen (965–1039) found that sunlight was not reflected from the Moon like a mirror, but that light was emitted from every part of the Moon's sunlit surface in all directions. [198] Shen Kuo (1031–1095) of the Song dynasty created an allegory equating the waxing and waning of the Moon to a round ball of reflective silver that, when doused with white powder and viewed from the side, would appear to be a crescent. [195] ( pp415–416 )

In Aristotle's (384–322 BC) description of the universe, the Moon marked the boundary between the spheres of the mutable elements (earth, water, air and fire), and the imperishable stars of aether, an influential philosophy that would dominate for centuries. [199] However, in the 2nd century BC , Seleucus of Seleucia correctly theorized that tides were due to the attraction of the Moon, and that their height depends on the Moon's position relative to the Sun. [200] In the same century, Aristarchus computed the size and distance of the Moon from Earth, obtaining a value of about twenty times the radius of Earth for the distance. These figures were greatly improved by Ptolemy (90–168 AD): his values of a mean distance of 59 times Earth's radius and a diameter of 0.292 Earth diameters were close to the correct values of about 60 and 0.273 respectively. [201] Archimedes (287–212 BC) designed a planetarium that could calculate the motions of the Moon and other objects in the Solar System. [202]

During the Middle Ages, before the invention of the telescope, the Moon was increasingly recognised as a sphere, though many believed that it was "perfectly smooth". [203]

In 1609, Galileo Galilei used an early telescope to make drawings of the Moon for his book Sidereus Nuncius, and deduced that it was not smooth but had mountains and craters. Thomas Harriot had made, but not published such drawings a few months earlier. Telescopic mapping of the Moon followed: later in the 17th century, the efforts of Giovanni Battista Riccioli and Francesco Maria Grimaldi led to the system of naming of lunar features in use today. The more exact 1834–1836 Mappa Selenographica of Wilhelm Beer and Johann Heinrich Mädler, and their associated 1837 book Der Mond, the first trigonometrically accurate study of lunar features, included the heights of more than a thousand mountains, and introduced the study of the Moon at accuracies possible in earthly geography. [204] Lunar craters, first noted by Galileo, were thought to be volcanic until the 1870s proposal of Richard Proctor that they were formed by collisions. [80] This view gained support in 1892 from the experimentation of geologist Grove Karl Gilbert, and from comparative studies from 1920 to the 1940s, [205] leading to the development of lunar stratigraphy, which by the 1950s was becoming a new and growing branch of astrogeology. [80]


Between the first human arrival with the robotic Soviet Luna program in 1958, to the 1970s with the last Missions of the crewed U.S. Apollo landings and last Luna mission in 1976, the Cold War-inspired Space Race between the Soviet Union and the U.S. led to an acceleration of interest in exploration of the Moon. Once launchers had the necessary capabilities, these nations sent uncrewed probes on both flyby and impact/lander missions.

Soviet missions

Spacecraft from the Soviet Union's Luna program were the first to accomplish a number of goals: following three unnamed, failed missions in 1958, [206] the first human-made object to escape Earth's gravity and pass near the Moon was Luna 1 the first human-made object to impact the lunar surface was Luna 2, and the first photographs of the normally occluded far side of the Moon were made by Luna 3, all in 1959.

The first spacecraft to perform a successful lunar soft landing was Luna 9 and the first uncrewed vehicle to orbit the Moon was Luna 10, both in 1966. [80] Rock and soil samples were brought back to Earth by three Luna sample return missions (Luna 16 in 1970, Luna 20 in 1972, and Luna 24 in 1976), which returned 0.3 kg total. [207] Two pioneering robotic rovers landed on the Moon in 1970 and 1973 as a part of Soviet Lunokhod programme.

Luna 24 was the last Soviet mission to the Moon.

United States missions

During the late 1950s at the height of the Cold War, the United States Army conducted a classified feasibility study that proposed the construction of a staffed military outpost on the Moon called Project Horizon with the potential to conduct a wide range of missions from scientific research to nuclear Earth bombardment. The study included the possibility of conducting a lunar-based nuclear test. [208] [209] The Air Force, which at the time was in competition with the Army for a leading role in the space program, developed its own similar plan called Lunex. [210] [211] [208] However, both these proposals were ultimately passed over as the space program was largely transferred from the military to the civilian agency NASA. [211]

Following President John F. Kennedy's 1961 commitment to a manned Moon landing before the end of the decade, the United States, under NASA leadership, launched a series of uncrewed probes to develop an understanding of the lunar surface in preparation for human missions: the Jet Propulsion Laboratory's Ranger program produced the first close-up pictures the Lunar Orbiter program produced maps of the entire Moon the Surveyor program landed its first spacecraft four months after Luna 9. The crewed Apollo program was developed in parallel after a series of uncrewed and crewed tests of the Apollo spacecraft in Earth orbit, and spurred on by a potential Soviet lunar human landing, in 1968 Apollo 8 made the first human mission to lunar orbit. The subsequent landing of the first humans on the Moon in 1969 is seen by many as the culmination of the Space Race. [212]

Neil Armstrong became the first person to walk on the Moon as the commander of the American mission Apollo 11 by first setting foot on the Moon at 02:56 UTC on 21 July 1969. [213] An estimated 500 million people worldwide watched the transmission by the Apollo TV camera, the largest television audience for a live broadcast at that time. [214] [215] The Apollo missions 11 to 17 (except Apollo 13, which aborted its planned lunar landing) removed 380.05 kilograms (837.87 lb) of lunar rock and soil in 2,196 separate samples. [216] The American Moon landing and return was enabled by considerable technological advances in the early 1960s, in domains such as ablation chemistry, software engineering, and atmospheric re-entry technology, and by highly competent management of the enormous technical undertaking. [217] [218]

Scientific instrument packages were installed on the lunar surface during all the Apollo landings. Long-lived instrument stations, including heat flow probes, seismometers, and magnetometers, were installed at the Apollo 12, 14, 15, 16, and 17 landing sites. Direct transmission of data to Earth concluded in late 1977 because of budgetary considerations, [219] [220] but as the stations' lunar laser ranging corner-cube retroreflector arrays are passive instruments, they are still being used. Ranging to the stations is routinely performed from Earth-based stations with an accuracy of a few centimeters, and data from this experiment are being used to place constraints on the size of the lunar core. [221]

1970s – present

In the 1970s, after the Moon race, the focus of astronautic exploration shifted, as probes like Pioneer 10 and the Voyager program were sent towards the outer solar system. Years of near lunar quietude followed, only broken by a beginning internationalization of space and the Moon through, for example, the negotiation of the Moon treaty.

Since the 1990s, many more countries have become involved in direct exploration of the Moon. In 1990, Japan became the third country to place a spacecraft into lunar orbit with its Hiten spacecraft. The spacecraft released a smaller probe, Hagoromo, in lunar orbit, but the transmitter failed, preventing further scientific use of the mission. [222] In 1994, the U.S. sent the joint Defense Department/NASA spacecraft Clementine to lunar orbit. This mission obtained the first near-global topographic map of the Moon, and the first global multispectral images of the lunar surface. [223] This was followed in 1998 by the Lunar Prospector mission, whose instruments indicated the presence of excess hydrogen at the lunar poles, which is likely to have been caused by the presence of water ice in the upper few meters of the regolith within permanently shadowed craters. [224]

The European spacecraft SMART-1, the second ion-propelled spacecraft, was in lunar orbit from 15 November 2004 until its lunar impact on 3 September 2006, and made the first detailed survey of chemical elements on the lunar surface. [225]

The ambitious Chinese Lunar Exploration Program began with Chang'e 1, which successfully orbited the Moon from 5 November 2007 until its controlled lunar impact on 1 March 2009. [226] It obtained a full image map of the Moon. Chang'e 2, beginning in October 2010, reached the Moon more quickly, mapped the Moon at a higher resolution over an eight-month period, then left lunar orbit for an extended stay at the Earth–Sun L2 Lagrangian point, before finally performing a flyby of asteroid 4179 Toutatis on 13 December 2012, and then heading off into deep space. On 14 December 2013, Chang'e 3 landed a lunar lander onto the Moon's surface, which in turn deployed a lunar rover, named Yutu (Chinese: 玉兔 literally "Jade Rabbit"). This was the first lunar soft landing since Luna 24 in 1976, and the first lunar rover mission since Lunokhod 2 in 1973. Another rover mission (Chang'e 4) was launched in 2019, becoming the first ever spacecraft to land on the Moon's far side. China intends to following this up with a sample return mission (Chang'e 5) in 2020. [227]

Between 4 October 2007 and 10 June 2009, the Japan Aerospace Exploration Agency's Kaguya (Selene) mission, a lunar orbiter fitted with a high-definition video camera, and two small radio-transmitter satellites, obtained lunar geophysics data and took the first high-definition movies from beyond Earth orbit. [228] [229] India's first lunar mission, Chandrayaan-1, orbited from 8 November 2008 until loss of contact on 27 August 2009, creating a high-resolution chemical, mineralogical and photo-geological map of the lunar surface, and confirming the presence of water molecules in lunar soil. [230] The Indian Space Research Organisation planned to launch Chandrayaan-2 in 2013, which would have included a Russian robotic lunar rover. [231] [232] However, the failure of Russia's Fobos-Grunt mission has delayed this project, and was launched on 22 July 2019. The lander Vikram attempted to land on the lunar south pole region on 6 September, but lost the signal in 2.1 km (1.3 mi). What happened after that is unknown.

The U.S. co-launched the Lunar Reconnaissance Orbiter (LRO) and the LCROSS impactor and follow-up observation orbiter on 18 June 2009 LCROSS completed its mission by making a planned and widely observed impact in the crater Cabeus on 9 October 2009, [233] whereas LRO is currently in operation, obtaining precise lunar altimetry and high-resolution imagery. In November 2011, the LRO passed over the large and bright crater Aristarchus. NASA released photos of the crater on 25 December 2011. [234]

Two NASA GRAIL spacecraft began orbiting the Moon around 1 January 2012, [235] on a mission to learn more about the Moon's internal structure. NASA's LADEE probe, designed to study the lunar exosphere, achieved orbit on 6 October 2013.


Upcoming lunar missions include Russia's Luna-Glob: an uncrewed lander with a set of seismometers, and an orbiter based on its failed Martian Fobos-Grunt mission. [236] Privately funded lunar exploration has been promoted by the Google Lunar X Prize, announced 13 September 2007, which offers US$20 million to anyone who can land a robotic rover on the Moon and meet other specified criteria. [237]

NASA began to plan to resume human missions following the call by U.S. President George W. Bush on 14 January 2004 for a human mission to the Moon by 2019 and the construction of a lunar base by 2024. [238] The Constellation program was funded and construction and testing begun on a crewed spacecraft and launch vehicle, [239] and design studies for a lunar base. [240] That program was cancelled in 2010, however, and was eventually replaced with the Donald Trump supported Artemis program, which plans to return humans to the Moon by 2025. [241] India had also expressed its hope to send people to the Moon by 2020. [242]

On 28 February 2018, SpaceX, Vodafone, Nokia and Audi announced a collaboration to install a 4G wireless communication network on the Moon, with the aim of streaming live footage on the surface to Earth. [243]

Recent reports also indicate NASA's intent to send a woman astronaut to the Moon in their planned mid-2020s mission. [244]

Planned commercial missions

In 2007, the X Prize Foundation together with Google launched the Google Lunar X Prize to encourage commercial endeavors to the Moon. A prize of $20 million was to be awarded to the first private venture to get to the Moon with a robotic lander by the end of March 2018, with additional prizes worth $10 million for further milestones. [245] [246] As of August 2016, 16 teams were reportedly participating in the competition. [247] In January 2018 the foundation announced that the prize would go unclaimed as none of the finalist teams would be able to make a launch attempt by the deadline. [248]

In August 2016, the US government granted permission to US-based start-up Moon Express to land on the Moon. [249] This marked the first time that a private enterprise was given the right to do so. The decision is regarded as a precedent helping to define regulatory standards for deep-space commercial activity in the future. Previously, private companies were restricted to operating on or around Earth. [249]

On 29 November 2018 NASA announced that nine commercial companies would compete to win a contract to send small payloads to the Moon in what is known as Commercial Lunar Payload Services. According to NASA administrator Jim Bridenstine, "We are building a domestic American capability to get back and forth to the surface of the moon.". [250]

Human impact

Beside the traces of human activity on the Moon, there have been some intended permanent installations like the Moon Museum art piece, Apollo 11 goodwill messages, six Lunar plaques, the Fallen Astronaut memorial, and other artifacts.


Longterm missions continuing to be active are some orbiters such as the 2009-launched Lunar Reconnaissance Orbiter surveilling the Moon for future missions, as well as some Landers such as the 2013-launched Chang'e 3 with its Lunar Ultraviolet Telescope still operational. [251]

There are several missions by different agencies and companies planned to establish a longterm human presence on the Moon, with the Lunar Gateway as the currently most advanced project as part of the Artemis program.

Astronomy from the Moon

For many years, the Moon has been recognized as an excellent site for telescopes. [252] It is relatively nearby astronomical seeing is not a concern certain craters near the poles are permanently dark and cold, and thus especially useful for infrared telescopes and radio telescopes on the far side would be shielded from the radio chatter of Earth. [253] The lunar soil, although it poses a problem for any moving parts of telescopes, can be mixed with carbon nanotubes and epoxies and employed in the construction of mirrors up to 50 meters in diameter. [254] A lunar zenith telescope can be made cheaply with an ionic liquid. [255]

In April 1972, the Apollo 16 mission recorded various astronomical photos and spectra in ultraviolet with the Far Ultraviolet Camera/Spectrograph. [256]

Living on the Moon

Humans have stayed for days on the Moon, such as during Apollo 17. [257] One particular challenge for astronauts' daily life during their stay on the surface is the lunar dust sticking to their suits and being carried into their quarters. Subsequently, the dust was tasted and smelled by the astronauts, calling it the "Apollo aroma". [258] This contamination poses a danger since the fine lunar dust can cause health issues. [258]

In 2019 at least one plant seed sprouted in an experiment, carried along with other small life from Earth on the Chang'e 4 lander in its Lunar Micro Ecosystem. [259]

Although Luna landers scattered pennants of the Soviet Union on the Moon, and U.S. flags were symbolically planted at their landing sites by the Apollo astronauts, no nation claims ownership of any part of the Moon's surface. [260] Russia, China, India, and the U.S. are party to the 1967 Outer Space Treaty, [261] which defines the Moon and all outer space as the "province of all mankind". [260] This treaty also restricts the use of the Moon to peaceful purposes, explicitly banning military installations and weapons of mass destruction. [262] The 1979 Moon Agreement was created to restrict the exploitation of the Moon's resources by any single nation, but as of January 2020, it has been signed and ratified by only 18 nations, [263] none of which engages in self-launched human space exploration. Although several individuals have made claims to the Moon in whole or in part, none of these are considered credible. [264] [265] [266]

In 2020, U.S. President Donald Trump signed an executive order called "Encouraging International Support for the Recovery and Use of Space Resources". The order emphasizes that "the United States does not view outer space as a 'global commons ' " and calls the Moon Agreement "a failed attempt at constraining free enterprise." [267] [268]

The Declaration of the Rights of the Moon [269] was created by a group of "lawyers, space archaeologists and concerned citizens" in 2021, drawing on precedents in the Rights of Nature movement and the concept of legal personality for non-human entities in space. [270]


In light of future development on the Moon some international and multi-space agency organizations have been created:


The contrast between the brighter highlands and the darker maria creates the patterns seen by different cultures as the Man in the Moon, the rabbit and the buffalo, among others. In many prehistoric and ancient cultures, the Moon was personified as a deity or other supernatural phenomenon, and astrological views of the Moon continue to be propagated.

In Proto-Indo-European religion, the Moon was personified as the male god *Meh1not. [271] The ancient Sumerians believed that the Moon was the god Nanna, [272] [273] who was the father of Inanna, the goddess of the planet Venus, [272] [273] and Utu, the god of the Sun. [272] [273] Nanna was later known as Sîn, [273] [272] and was particularly associated with magic and sorcery. [272] In Greco-Roman mythology, the Sun and the Moon are represented as male and female, respectively (Helios/Sol and Selene/Luna) [271] this is a development unique to the eastern Mediterranean [271] and traces of an earlier male moon god in the Greek tradition are preserved in the figure of Menelaus. [271]

In Mesopotamian iconography, the crescent was the primary symbol of Nanna-Sîn. [273] In ancient Greek art, the Moon goddess Selene was represented wearing a crescent on her headgear in an arrangement reminiscent of horns. [274] [275] The star and crescent arrangement also goes back to the Bronze Age, representing either the Sun and Moon, or the Moon and planet Venus, in combination. It came to represent the goddess Artemis or Hecate, and via the patronage of Hecate came to be used as a symbol of Byzantium.

An iconographic tradition of representing Sun and Moon with faces developed in the late medieval period.

The splitting of the Moon (Arabic: انشقاق القمر ‎) is a miracle attributed to Muhammad. [276] A song titled 'Moon Anthem' was released on the occasion of landing of India's Chandrayan-II on the Moon. [277]


The Moon's regular phases make it a convenient timepiece, and the periods of its waxing and waning form the basis of many of the oldest calendars. Tally sticks, notched bones dating as far back as 20–30,000 years ago, are believed by some to mark the phases of the Moon. [278] [279] [280] The

30-day month is an approximation of the lunar cycle. The English noun month and its cognates in other Germanic languages stem from Proto-Germanic *mǣnṓth-, which is connected to the above-mentioned Proto-Germanic *mǣnōn, indicating the usage of a lunar calendar among the Germanic peoples (Germanic calendar) prior to the adoption of a solar calendar. [281] The PIE root of moon, *méh1nōt, derives from the PIE verbal root *meh1-, "to measure", "indicat[ing] a functional conception of the Moon, i.e. marker of the month" (cf. the English words measure and menstrual), [282] [283] [284] and echoing the Moon's importance to many ancient cultures in measuring time (see Latin mensis and Ancient Greek μείς (meis) or μήν (mēn), meaning "month"). [285] [286] [287] [288] Most historical calendars are lunisolar. The 7th-century Islamic calendar is an example of a purely lunar calendar, where months are traditionally determined by the visual sighting of the hilal, or earliest crescent moon, over the horizon. [289]

Lunar effect

The lunar effect is a purported unproven correlation between specific stages of the roughly 29.5-day lunar cycle and behavior and physiological changes in living beings on Earth, including humans.

The Moon has long been particularly associated with insanity and irrationality the words lunacy and lunatic (popular shortening loony) are derived from the Latin name for the Moon, Luna. Philosophers Aristotle and Pliny the Elder argued that the full moon induced insanity in susceptible individuals, believing that the brain, which is mostly water, must be affected by the Moon and its power over the tides, but the Moon's gravity is too slight to affect any single person. [290] Even today, people who believe in a lunar effect claim that admissions to psychiatric hospitals, traffic accidents, homicides or suicides increase during a full moon, but dozens of studies invalidate these claims. [290] [291] [292] [293] [294]

1 Answer 1

Twice daily, as the earth rotates. This occurs no matter where you are.

The time between transitions varies on the observers location. Close to the equator, it is approx 12hrs between transitions, with this becoming skewed to a short and a long time between transitions, for example, 3hrs and 21hrs for an observer far north or south.

Mars orbital plane is inclined compared to earths rotational plane, therefore Mars crossing the horizon will be visible from all points (unlike the sun). The actual transition time will be quite short depending on where the viewer is located, short close to the equator (a little under 2 seconds), longer closer to the poles (I'm not sure of an upward bound, less than a few minutes I'd guess, given a perfectly smooth horizon).

The transition will be at a very slightly different point on the horizon each time, and doesn't repeat for a very long time (hundreds of years, due to mars orbit length, and earths year not being an exact multiple of its day length. You might need telescopes to measure this though).

Google doodle honors Foucault and his pendulum

It may be hard to fathom, but the idea of Earth rotating on its axis, first proposed in the 6th century, took many centuries to gain favor, and many more to be demonstrated. The Copernican theory of celestial motions was well accepted by science by the time Foucault was born. It elegantly explained the apparent "rise" and "set" of the sun, but it was difficult to "prove" by experiment.

Some folks tried to drop stones down a mineshaft to see if they deviated. Others tried something similar to the trajectory of cannonballs. But the mine shaft was too short, compared with Earth's radius, and the time traveled by the cannonballs likewise was too short to measure any difference.

The son of a publisher, Foucault showed an early aptitude for all things mechanical, and a growing aversion to all things bloody. So he gave up a medical curriculum and opted for physics.

He built his first pendulum with six feet of wire, an 11-pound ball, and a candle that "launched" the ball by burning through a string to which the ball was attached (to prevent any directional effect of pushing the bob).

His pendulum became a sensation, and he constructed several for public displays, the most famous of them at the Pantheon in Paris.

The California Academy of Sciences museum in San Francisco has a massive Foucault pendulum that swings through an arc of about 220 degrees daily.

Why not 360 degrees, you ask? The pendulum's motion is dependent on the latitude of Earth. Foucault's likewise moved 270 degrees in 24 hours. A pendulum at the North Pole would spin the full 360 degrees.

You're probably already wondering why the pendulum doesn't just slow down and stop, eventually. It does. In the early Foucault experiments, this didn't matter so much, because it swung long enough to see the floor shift in relation to the arc of the pendulum. But as the pendulum became more of a sensation, people invented ways to overcome the resistance that slows the bob.

Designers these days use electromagnets near the fastening point for the cable to overcome that force and keep things swinging. At the academy, this electromagnet turns on and off when the cable passes a beam of light.

Explain how the Foucault pendulum works.

Does your weight change between the poles and the equator?

Hi. I'm a seventh-grade student in Washington. I had this question: The earth is spinning, so there is more centrifugal force towards the equator of the earth than the north pole, because the middle of the earth is spinning faster. So if you lived on the equator wouldn't you weigh less than someone who lived on the north pole, because there is more force trying to pull you away from the earth? Thanks for your time and effort. I really appreciate it.

You are right, that because of centripetal acceleration, you will weigh a tiny amount less at the equator than at the poles. Try not to think of centripetal acceleration as a force though what's really going on is that objects which are in motion like to go in a straight line and so it takes some force to make them go round in a circle. So some of the force of gravity is being used to make you go round in a circle at the equator (instead of flying off into space), while at the pole, this is not needed. The centripetal acceleration at the equator is given by 4 times pi squared times the radius of the Earth divided by the period of rotation squared (4*pi 2 *r/T 2 ). The period of rotation is 24 hours (or 86400 seconds) and the radius of the Earth is about 6400 km. This means that the centripetal acceleration at the equator is about 0.03 m/s 2 (meters per second squared). Compare this to the acceleration due to gravity which is about 10 m/s 2 and you can see how tiny an effect this is - you would weigh about 0.3% less at the equator than at the poles!

There is an additional effect due to the oblateness of the Earth. The Earth is not exactly spherical but rather is a little bit like a "squashed" sphere, with the radius at the equator slightly larger than the radius at the poles (this shape can be explained by the effect of centripetal acceleration on the material that makes up the Earth, exactly as described above). This has the effect of slightly increasing your weight at the poles (since you are close to the center of the Earth and the gravitational force depends on distance) and slightly decreasing it at the equator.

Taking into account both of the above effects, the gravitational acceleration is 9.78 m/s 2 at the equator and 9.83 m/s 2 at the poles, so you weigh about 0.5% more at the poles than at the equator.

Thread: Speed of the Earth's Terminator

Does anyone know how to calculate the speed of the terminator as it moves across the Earth? At the moment I am working on a project in college, looking at the differences between day and night re: the ionosphere, and on my graphs I can see the sunset/sunrise. I would love to be able to take my project one step further and calculate the terminator's speed, does anyone have any ideas?

Good thinking Tog. But the terminator will actually move quicker than that. The terminator is not normally normal to our lines of longitude. Well, maybe twice a year. It feels like the maths could be quite tricky.

Well the thinking there was that it would move at about 1,000 miles per hour at the equator, but near the poles, it would move a great slower. If the circumference is only, say 25 miles instead of 25,000, then near the poles it wold move at about 1 mile per hour. Time of the year and lattitude would affect this in a big way, and the gradual nature of it as it passes by makes it seem slower, at least to me. There is also local terrain to deal with. I have high mountains to the east and a lot of nothing to the west. Sunrise and sunset look a lot different from my house.

To get an exact speed would take a lot of math and probably be valid for that minute only, but I think a general value can be figured the way I described.

or slower

For most places on earth, it's average value (over the year) is very close to one revolution per day, obviously. Tog_'s approach is even closer.

Ah I see, the 2(pi)r/t approach is the simplest and I dont need an exact answer. Thanks you for your help everyone However I do I go about calculating the circumference at my latitude, rather than taking the circumference at the equator?
Does anyone know how to calculate the speed of the terminator as it moves across the Earth? At the moment I am working on a project in college, looking at the differences between day and night re: the ionosphere, and on my graphs I can see the sunset/sunrise. I would love to be able to take my project one step further and calculate the terminator's speed, does anyone have any ideas?

Actually, its the circumference of the Earth times cosine of the latitude, but I'm sure that's what you meant

In kilometers per hour, the velocity is approximately
1669.7565 * cos(lat)
(make sure your calculator is in degrees, not radians)

1669.7565 comes from 2 * pi * r / 24, where r=6378 km (radius of the Earth) and 24 is 24 hours a day.

So for different output units, such as meters / second, you'd use Earth's radius in meters, and a day expressed in seconds (86400):
2 * pi * 6378000 / 86400 = 463.8212

463.8212 * cos(lat) = velocity in meters per second.

And as mentioned already, this is an approximation, although a very good one.

Actually, its the circumference of the Earth times cosine of the latitude, but I'm sure that's what you meant

Because I think the speed of the terminator across the ground will always be greatest at right angles to the terminator. If you define this velocity as a unit vector, then any component of this vector, such as the component along a line of latitude that is not at right angles to the terminator, will be less than unity. So the true speed of the terminator is always higher than the speed of the terminator along a line of latitude.

I don't think that's true. In fact, isn't the opposite true, all else being equal?

Imagine a terminator that is angled from the lines of longitude. When the earth turns, the ground speed of the terminator perpendicular to the terminator will be slower than the rotation speed, since it is not parallel to the rotation.

Thanks for your help everyone! Turns out when I calculate it that general way, and compare it with the result I got from my project, they differ by only 10m/s, which I can certainly live with

I always ballparked it thusly:

You don't have to calculate circumferences other than the equator.

The Earth turns 1000mph at the equator (24,902mi / 24h).
At 45 degress North (where I am), it turns at sin (45) = .707 or 707mph.

This is also a ballpark way of determining how wide a longitude is at my location.

The terminator's actually moving pretty slowly: once around in a year. The Earth rotates under it, moving its surface into darkness and then out again.
Think of it as if you were driving down the road and into the shadow of a bridge: it doesn't matter the angle at which the bridge cross the road, you still enter the shadow at the speed your car is moving.

Isn't it an imaginary line? I've never seen one go by at any speed? Our atmosphere distorts it. It would be pretty cool standing on the moon watching it slowly go by, however.
Isn't it an imaginary line? I've never seen one go by at any speed? Our atmosphere distorts it. It would be pretty cool standing on the moon watching it slowly go by, however.

That would be cool. But on the Moon I imagine it would be blurred too. As day turned to night, you'd see the ground covered in sunlight, but with long shadows. Next you would see the ground covered in darkness with the tops of rocks and hills still bathed in sunlight. And finally, total darkness. I imagine it would be difficult to identify a particluar time when the actual mathamatical terminator passed you by.

Interestingly, on the Moon, even on the equator, you could keep pace with the terminator simply by jogging.

Yes, standing on the Moon would not allow one to see the terminator.

I wonder what would produce a moving terminator observable from the ground? In a solar eclipse where the sun is directly overhead, I wonder if there is some sense of one, though our atmosphere would still smudge it, no doubt. However, in a total eclipse involving two bodies without atmospheres, it might be obvious.

Yes, definitely. If you remember to look to the side in the last seconds before totality, you can see a wall of dark air bearing down on you, the same deep blue-grey shade you see in the shadowed region below Venus's Belt at sunset.

Another perspective on the same spectacle. This shows
the degree of fuzziness of the Moon's shadow.

Astronomy Picture of the Day - August 30, 1999
Looking Back on an Eclipsed Earth
Credit: Mir 27 Crew Copyright: CNES

Here is what the Earth looks like during a solar eclipse.
The shadow of the Moon can be seen darkening part of Earth.
This shadow moves across the Earth at nearly 2000 kilometers
per hour. Only observers near the center of the dark circle
see a total solar eclipse - others see a partial eclipse
where only part of the Sun appears blocked by the Moon.
This spectacular picture of the 1999 August 11 solar eclipse
was one of the last ever taken from the Mir space station,
as Mir is being decommissioned after more than ten years of
productive use.

Yes, Grant, I see. Would a sun at zenith during eclipse accentuate the approaching shadow?

[Added: Thanks Jeff, nice image from APOD.]

Seems like it should . the nearer to overhead the sun is, the more vertical the edge of the umbra as it sweeps through the air around you, so the more abrupt the transition should be. If you had to choose a low sun, though, you'd choose one low in the south or north near local noon, I think, (in which case the tilt of the umbra will be transverse to its line of movement), rather than a morning or evening sun (in which case the umbra is sloping along the line of travel).

That is what I think, too less atmosphere, less scattering of light into dark.

Hmmm. scattering. . Perhaps there's a color question in here associated with a terminator and eclipse shadow. In the case of an eclipse, wouldn't the distant horizon become red due to the greater scattering length? If so, has this been photographed, or is everyone too busy with a corona or other cool delight?

I think I follow you. You are saying it is best to be perpedicular to the terminator's path.

I was in Hawaii in July 1991 when there was a total solar eclipse. The sky was cloudy so I couldn't see the eclipse or the encroaching wall of darkness.

But I could sense the light level changing. It was one of the weirdest things I've ever experienced. From the beginning of the eclipse, then the Moon took its first bite out of the edge of the Sun, until seconds before totality, light level dropped steadily to what appeared to be about half as bright as full sunshine. (It was probably much more, but the human eye has a logarithmic sense of brightness).

Then, in a period of about 5 seconds, it was as if someone had their hand on a dimmer switch. The light faded from what I percieved as half-brightness to 1/10 brightness. It seemed as bright as it is 15 minutes after sunset. There was a lot of open sky, but the Moon / Sun was still covered by clouds.

A few minutes later, the dimmer-switch effect returned the daylight. So even the edge of the Moon's umbra was fuzzy enough to require about 5 seconds to pass over me.

Ug, looks like the clouds got you. The good news is that when clouds get that dark normally it rains.

10 Things You Didn't Know About the Moon

As the full moon approaches, its growing brightness tends to capture our attention.

The full moon occurs when the moon is on the opposite side of Earth from the sun, so that its face is fully illuminated by the sun's light. [Photos: Our Changing Moon]

But any day of the month, the moon has some secrets up her sleeve. Here are 10 surprising and strange facts about Earth's natural satellite that may surprise you:

1) There's actually four kinds of lunar months

Our months correspond approximately to the length of time it takes our natural satellite to go through a full cycle of phases. From excavated tally sticks, researchers have deduced that people from as early as the Paleolithic period counted days in relation to the moon's phases. But there are actually four different kinds of lunar months. The durations listed here are averages.

1. Anomalistic – the length of time it takes the moon to circle the Earth, measured from one perigee (the closest point in its orbit to Earth) to the next: 27 days, 13 hours, 18 minutes, 37.4 seconds.

2. Nodical – the length of time it takes the moon to pass through one of its nodes (where it crosses the plane of the Earth's orbit) and return to it: 27 days, 5 hours, 5 minutes, 35.9 seconds.

3. Sidereal – the length of time it takes the moon to circle the Earth, using the stars as a reference point: 27 days, 7 hours, 43 minutes, 11.5 seconds.

4. Synodical – the length of time it takes the moon to circle the Earth, using the sun as the reference point (that is, the time lapse between two successive conjunctions with the sun – going from new moon to new moon): 29 days, 12 hours, 44 minutes, 2.7 seconds. It is the synodic month that is the basis of many calendars today and is used to divide the year.

2) We see slightly more than half of the moon from Earth

Most reference books will note that because the moon rotates only once during each revolution about the Earth, we never see more than half of its total surface. The truth, however, is that we actually get to see more of it over the course of its elliptical orbit: 59 percent (almost three-fifths). ['Supermoon' Full Moons Explained]

The moon's rate of rotation is uniform but its rate of revolution is not, so we're able to see just around the edge of each limb from time to time. Put another way, the two motions do not keep perfectly in step, even though they come out together at the end of the month. We call this effect libration of longitude.

So the moon "rocks" in the east and west direction, allowing us to see farther around in longitude at each edge than we otherwise could. The remaining 41 percent can never be seen from our vantage point and if anyone were on that region of the moon, they would never see the Earth.

3) It would take hundreds of thousands of moons to equal the brightness of the sun

The full moon shines with a magnitude of -12.7, but the sun is 14 magnitudes brighter, at -26.7. The ratio of brightness of the sun versus the moon amounts to a difference of 398,110 to 1. So that's how many full moons you would need to equal the brightness of the sun. But this all a moot point, because there is no way that you could fit that many full moons in the sky.

The sky is 360 degrees around (including the half we can't see, below the horizon), so there are over 41,200 square degrees in the sky. The moon measures only a half degree across, which gives it an area of only 0.2 square degrees. So you could fill up the entire sky, including the half that lies below our feet, with 206,264 full moons — and still come up short by 191,836 in the effort to match the brightness of the sun.

4) The first- or last-quarter moon is not one half as bright as a full moon

If the moon's surface were like a perfectly smooth billiard ball, its surface brightness would be the same all over. In such a case, it would indeed appear half as bright.[Phases of the Moon Explained]

But the moon has a very rough topography. Especially near and along the day/night line (known as the terminator), the lunar landscape appears riddled with innumerable shadows cast by mountains, boulders and even tiny grains of lunar dust. Also, the moon's face is splotched with dark regions. The end result is that at first quarter, the moon appears only one eleventh as bright as when it's full.

The moon is actually a little brighter at first quarter than at last quarter, since at that phase some parts of the moon reflect sunlight better than others.

5) A 95-percent illuminated moon appears half as bright as a full moon

Believe it or not, the moon is half as bright as a full moon about 2.4 days before and after a full moon. Even though about 95 percent of the moon is illuminated at this time, and to most casual observers it might still look like a "full" moon, its brightness is roughly 0.7 magnitudes less than at full phase, making it appear one-half as bright.

6) The Earth, seen from the moon, also goes through phases

However, they are opposite to the lunar phases that we see from the Earth. It's a full Earth when it's new moon for us last-quarter Earth when we're seeing a first-quarter moon a crescent Earth when we're seeing a gibbous moon, and when the Earth is at new phase we're seeing a full moon.

From any spot on the moon (except on the far side, where you cannot see the Earth), the Earth would always be in the same place in the sky.

From the moon, our Earth appears nearly four times larger than a full moon appears to us, and – depending on the state of our atmosphere – shines anywhere from 45 to 100 times brighter than a full moon. So when a full (or nearly full) Earth appears in the lunar sky, it illuminates the surrounding lunar landscape with a bluish-gray glow.

From here on the Earth, we can see that glow when the moon appears to us as a crescent sunlight illuminates but a sliver of the moon, while the rest of its outline is dimly visible by virtue of earthlight. Leonardo da Vinci was the first to figure out what that eerie glow appearing on the moon really was.

7) Eclipses are reversed when viewing from the moon

Phases aren't the only things that are seen in reverse from the moon. An eclipse of the moon for us is an eclipse of the sun from the moon. In this case, the disk of the Earth appears to block out the sun.

If it completely blocks the sun, a narrow ring of light surrounds the dark disk of the Earth our atmosphere backlighted by the sun. The ring appears to have a ruddy hue, since it's the combined light of all the sunrises and sunsets occurring at that particular moment. That's why during a total lunar eclipse, the moon takes on a ruddy or coppery glow.

When a total eclipse of the sun is taking place here on Earth, an observer on the moon can watch over the course of two or three hours as a small, distinct patch of darkness works its way slowly across the surface of the Earth. It's the moon's dark shadow, called the umbra, that falls on the Earth, but unlike in a lunar eclipse, where the moon can be completely engulfed by the Earth's shadow, the moon's shadow is less than a couple of hundred miles wide when it touches the Earth, appearing only as a dark blotch.

8) There are rules for how the moon's craters are named

The lunar craters were formed by asteroids and comets that collided with the moon. Roughly 300,000 craters wider than 1 km (0.6 miles) are thought to be on the moon's near side alone.

These are named for scholars, scientists, artists and explorers. For example, Copernicus Crater is named for Nicolaus Copernicus, a Polish astronomer who realized in the 1500s that the planets move about the sun. Archimedes Crater is named for the Greek mathematician Archimedes, who made many mathematical discoveries in the third century B.C.

The custom of applying personal names to the lunar formations began in 1645 with Michael van Langren, an engineer in Brussels who named the moon's principal features after kings and great people on the Earth. On his lunar map he named the largest lunar plain (now known as Oceanus Procellarum) after his patron, Phillip IV of Spain.

But just six years later, Giovanni Battista Riccioli of Bologna completed his own great lunar map, which removed the names bestowed by Van Langren and instead derived names chiefly from those of famous astronomers — the basis of the system which continues to this day. In 1939, the British Astronomical Association issued a catalog of officially named lunar formations, "Who's Who on the Moon," listing the names of all formations adopted by the International Astronomical Union.

Today the IAU continues to decide the names for craters on our moon, along with names for all astronomical objects. The IAU organizes the naming of each particular celestial feature around a particular theme.

The names of craters now tend to fall into two groups. Typically, moon craters have been named for deceased scientists, scholars, explorers, and artists who've become known for their contributions to their respective fields. The craters around the Apollo crater and the Mare Moscoviense are to be named after deceased American astronauts and Russian cosmonauts.

9) The moon encompasses a huge temperature range

If you survey the Internet for temperature data on the moon, you're going to run into quite a bit of confusion. There's little consistency even within a given website in which temperature scale is quoted: Celsius, Fahrenheit, even Kelvin.

We have opted to use the figures that are quoted by NASA on its Website: The temperature at the lunar equator ranges from an extremely low minus 280 degrees F (minus 173 degrees C) at night to a very high 260 degrees F (127 degrees C) in the daytime. In some deep craters near the moon's poles, the temperature is always near minus 400 degrees F (minus 240 degrees C).

During a lunar eclipse, as the moon moves into the Earth's shadow, the surface temperature can plunge about 500 degrees F (300 degrees C) in less than 90 minutes.

10) The moon has its own time zone

It is possible to tell time on the moon. In fact, back in 1970, Helbros Watches asked Kenneth L. Franklin, who for many years was the chief astronomer at New York's Hayden Planetarium, to design a watch for moon walkers that measures time in what he called "lunations," the period it takes the moon to rotate and revolve around the Earth each lunation is exactly 29.530589 Earth days.

For the moon, Franklin developed a system he called "lunar mean solar time," or Lunar Time (LT). He envisioned local lunar time zones similar to the standard time zones of Earth, but based on meridians that are 12-degrees wide (analogous to the 15-degree intervals on Earth). "They will be named unambiguously as '36-degree East Zone time,' etc., although 'Copernican time,' 'West Tranquillity time' and others may be adopted as convenient." A lunar hour was defined as a "lunour," and decilunours, centilunours and millilunours were also introduced.

Interestingly, one moon watch was sent to the president of the United States at the time, Richard M. Nixon, who sent a thank you note to Franklin. The note and another moon watch were kept in a display case at the Hayden Planetarium for several years.

Quite a few visitors would openly wonder why Nixon was presented with a wristwatch that could be used only on the moon.

What is on Dark Side of the Moon?

The other face, most of which is never visible from the Earth, is therefore called the "far side of the Moon". Over time, some parts of the far side can be seen due to libration. Only during a full Moon (as viewed from Earth) is the whole far side of the Moon dark.

Secondly, is one side of the moon permanently dark? First, the dark side isn't really any darker than the near side. Like Earth, it gets plenty of sunlight. We don't see the far side because &ldquothe moon is tidally locked to the Earth,&rdquo said John Keller, deputy project scientist for NASA's Lunar Reconnaissance Orbiter project.

Considering this, how cold is the dark side of the moon?

Does the sun shine on the dark side of the moon?

There is some Earthlight sometimes, which is technically reflected sunlight&hellip but no direct sunlight. This is because the dark side is the side facing away from the sun. 2:Is there any sunlight on the far side of the moon? Answer: half the time, yes, half the time, no.