# Precession and nutation

The previous page is about the two Earth movements more obvious, translation and rotation. This explains two other less noticeable, but equally important: precession and nutation.

The spring and autumn equinoxes are not fixed, because the plane of the equator rotates in relation to the plane of the ecliptic. This turn is completed once every 25,868 years.

The movement of the equinoxes in the ecliptic is called precession of the equinoxes. To establish the actual position of the stars at a given time, a precession correction must be applied to the celestial cards.

For its part, the nutation it's a slight swinging that the Earth experiences because of the gravitational attraction of the Moon. But let's go in parts:

## Precession

The Earth is not a perfect sphere, but is an irregularly shaped ellipsoid, crushed by the poles and deformed by the gravitational attraction of the Sun, the Moon and, to a lesser extent, the planets. This causes a kind of very slow balancing on the planet during its translational movement called "precession of the equinoxes", which is done in the opposite direction to that of rotation, that is to say in a retrograde sense (clockwise).

Under the influence of these attractions, the axis describes a double cone 47º opening, whose vertex is in the center of the Earth.

Due to the precession of the equinoxes, the position of the celestial pole changes over the centuries. Currently the Polar star does not match exactly with the Celestial North Pole. Others solar system planets They also have this behavior.

## Nutation

There is another movement that overlaps with precession. It is called nutation and consists of a small sway of the Earth's axis. As the Earth is not spherical, the attraction of the Moon over the equatorial bulge of our planet causes the movement of nutation.

To get an idea of ​​this movement, let's imagine that, while the axis of rotation describes the conical movement of precession, it travels in turn a small ellipse or loop in a period of 18.6 years.

In a complete round of precession (25,767 years) the Earth performs more than 1,300 nutation loops. The Earth nutation movement was discovered by British astronomer James Bradley.

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