NUTATION AND PRECESSION MOTIONS
Precession is a terrestrial motion that occurs because Earth is not a perfect homogenous sphere, so as a result, the rotation axis changes over time. To imagine this movement, watch a whistling top spin. It could be expected only to spin, but in reality, the whistling top, besides spinning, its rotation axis is in a circular motion, like a nodding kind of thing, and that motion is exactly what happens to Earth. If we were able to go through it with a gigantic pencil, which could draw a line in the sky, we would see a circle, and the process of drawing that line would take around 26,000 years.
This phenomenon (known as “axial precession”) was described by the great Greek astronomer and mathematician, Hipparcos, in the 2nd century B.C., who was based on observations that Babylonian astronomers and astrologists carried out. Hipparcos realized that the position of the stars during equinoxes was slightly different from the ones observed by astronomers in the past, and he calculated that it should move 1° per century (the value given today is 1.38° per century). In the 4th century A.D., this phenomenon was also explained by the astronomer Yu-Xi from the Jin dynasty, in China. In spite of these insights, the answer to why precession occurred remained unknown, and only because of the studies of celestial mechanics by Isaac Newton, it could be understood fully: the precession refers to the forces that the Sun and the Moon cause on Earth, and to Earth being a non-perfect spheroid.
On the other side is nutation, which is a motion overlapping precession. It happens mainly for a gravitational attraction that Earth feels towards the Moon and Sun[1] and, mainly, due to its 5° inclination of the plane in which the Moon orbits the Earth regarding the plane where the Earth orbits the Sun.
To sum up, these two motions are caused by the rotation of the Earth on its axis and its non-perfect sphere shape. In this motion, the Moon creates a small extra distortion because it is not aligned correctly with Earth’s orbit.
- ↑ Planets also help in changing and add little tiny perturbations to nutation that would be created only by the Moon and the Sun.
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