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Precession of the equinoxes: where does this phenomenon come from?

September 23, 2022

The precession of the equinoxes is one of the many movements that the Earth performs, demonstrating that our planet and the solar system are in constant motion. This phenomenon corresponds to the slow change of direction of the Earth's axis of rotation. Why does it happen?

Theequinox is the exact time of the year when the Sun crosses the equatorial plane of the Earth, i.e. is perpendicular to the earth's axis. Day and night have exactly the same duration. One year has two equinoxes per year: the first between 20 and 22 March, the second between 21 and 23 September. It is at this time of the year that the great tides.

Let us use the example of the router to understand this phenomenon. When a spinning top is launched, its axis of rotation becomes unbalanced under the action of gravity. Instead of rotating around its vertical axis, it rotates with an inclined axis, and then describes a cone On the ground. This is what happens during the precession of equinoxes. The Earth's tilt axis is 23.5 degrees, a rather small value, explaining the fact that this phenomenon is very slow. It takes place in approximately 25,670 years. In a year, its movement is less than a degree, which makes it almost imperceptible.

The discovery of this phenomenon is attributed to theastronomer Greek Hipparchus in -130 BC. AD, although other peoples such as the Aztecs or the Egyptians have left traces of work on this subject. It is moreover Hipparque who created the term "precession", initially used to speak of a shift towards the front. This term is also used for objects like satellites, gyroscopes, or the Foucault pendulum. Hipparchus would have arrived at this discovery by noticing a shift in position of the stars in the sky.

Why are the tides behind this phenomenon? What are the consequences ?

The Moon exerts a force of attraction on ocean water and causes tides. On the day of the equinox, the Sun is perfectly perpendicular to theEcuador and comes to add its force to that of the Moon. Their forces of attraction add up and cause stronger tides. These tidal forces are trying to bring the excess of mass present at the level of the Equator towards the Earth's orbital plane. Thus, they generate a displacement of the axis of rotation of the Earth, this is called the precession of the equinoxes.

This phenomenon of rotation of the terrestrial axis is at the origin of a change of position of the stars, which would no longer be fixed in the celestial sphere as evidenced by previous research. From the point of view of seasons, the changes are practically nil, but from an astronomical point of view, they impact the work of experts. Indeed, nowadays, the equinoxes occur when the Sun is positioned above the constellation of Pisces. On the other hand, in Antiquity, it would have been positioned on the constellation of Aries.

The precession of the equinoxes also generates a modification of the time of rotation of the Earth on itself. Thus, the length of the seasons is 20 minutes shorter than the time necessary for the Earth to occupy the same position. Every 2000 years, this represents a lag of 1 month. It is this discrepancy which is at the origin of the leap years. Indeed, if we add a day to the calendar every 4 years, it is to make up for this shift in rotation of the Earth. Without it, our current seasons would advance every year in time.

This will also interest you

Galaxy: where is the Earth in the Milky Way? The Gaia satellite has mapped more than a billion stars. Thanks to this data, this ESA video transports us to our residential area in the galaxy, to discover some 600,000 stars around us. The most brilliant and known from the ground, such as Sirius, Betelgeuse, Véga, Aldébaran…, have their names marked. The journey ends with a dive towards the Sun (Sol), a small star more than 26,000 light years from the center of the Milky Way.

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