Precession is a phenomenon that occurs when massive bodies move, due to angular momentum being affected by other masses in space-time. In the words of John Archibald Wheeler, “mass tells space-time how to curve, space-time tells mass how to move”.
Precession of Earth’s rotational axis
The most familiar example is the precession of a gyroscope; its rotational axis appears to describe a circle under the influence of Earth’s gravity. Exactly the same applies to the rotational axis of the Earth under the influence of the Sun's (and to a lesser extent, the Moon's) gravity:
As most people are aware, Earth’s rotational axis is inclined ~23.5° to the plane of the ecliptic, which accounts for the seasons. Currently, the Earth’s rotational axis points almost exactly at Polaris, which is therefore called the ‘pole star’. However, the precession of Earth’s axis has a period of ~26,000 years, so that in around 13,000 years time, Earth’s axis will point at Vega, which will then be the ‘pole star’. Then, in about 26,000 years time, Polaris will again be the ‘pole star’. This is an example of rotational axis precession.
The precession of Earth’s rotational axis also accounts for the phenomenon of precession of the equinoxes. The First Point of Aries is one of the two points where the plane of the ecliptic intersects the celestial equator (Davidson, 2020). These are called vernal equinoxes. The first point of Aries was recognized in antiquity in the constellation Aries, but due to precession of Earth’s axial rotation is today located in the constellation of Pisces. Exactly 180° around the celestial equator is the first point of Libra, which today lies in the constellation Virgo.
Let’s put that precession cycle into context. The period of precession of Earth’s rotational axis is:
Human civilisations are known to have started ~6,000 years ago. The number of precession cycles during that time is not yet one quarter:
Modern Homo sapiens are believed to have emerged ~200,000 years ago. The number of precession cycles during that time is almost eight:
Earth formed ~4.5 Bn years ago. The number of precession cycles during that time is more than 170,000:
Precession of planetary orbits
As was discovered by Kepler, a planet follows an elliptical path as it orbits the Sun. The point at which the planet makes its closest approach is known as periastron. For many years, it could not be explained by Newtonian theory that the periastron of Mercury does not always occur at the same place in the Mercury’s orbit. This is because the orbit itself is subject to precession, so that over a period of time periastron occurs at a point further around the orbit. This was established by careful observation in the nineteenth century.
Since Mercury is the planet orbiting closest to the Sun, the precession of Mercury’s orbit is higher than any of the other planets.
How orbital precession works is illustrated in the diagram below.
PLEASE NOTE that a) this diagram is looking at the solar system from ABOVE; b) the diagram is emphatically NOT TO SCALE ; c) also, the orbital eccentricities are GREATLY exaggerated; and d) the angular precession angle is GREATLY exaggerated.
Newtonian gravitational theory predicts that the magnitude of the orbital precession of Mercury should be slightly more than half what is actually observed. Although many explanations were produced to account for the observations, none were considered conclusive. Einstein’s General relativity (GR), published in 1917, predicted the rate of orbital precession to be 43 arc-seconds per century. This matched the observations exactly.
In turn, let’s put that into context. How long does it take Mercury’s orbit to precess a full 360 degrees? Based on angular measure (Helps, 2020), the answer is approximately 3 million years:
Or, looked at another way: Mercury is estimated to have formed 4.5Bn years ago. That would imply that Mercury’s orbit has completed
precessions since Mercury’s formation.
This accurate prediction of 43 arc-seconds per century was the first major observational proof that General Relativity is a valid theory. Note that we say a “valid” theory rather than a “true” theory. A scientific theory cannot be proved to be true; it can be showed to accurately account for observations. A scientific theory can only ever be “proved” to be untrue. Later, GR was also able to exactly predict the much smaller orbital precession of Venus (8.6 arc-seconds per century).
The second observational evidence pointing to the validity of GR was that gravity of a large mass would “bend” light rays passing close by it - recall John Archibald Wheeler’s ‘mass tells space-time how to curve’ above. This was verified by an expedition lead by Sir Arthur Eddington to observe a total solar eclipse in 1921. But that’s another story.
John Archibald Wheeler: https://phy.princeton.edu/department/history/faculty-history/john-wheeler
Mathematics of precession: https://en.wikipedia.org/wiki/Precession
Angular size: Helps, L; WMA Blog, May 2020
Celestial equator and plane of the ecliptic: Davidson, B; WMA Blog, May 2020