The Platonic Year in Astronomy and Astrology
The rotation plane of the Earth – that is, the maximum circle delimited by the equator – is inclined to the plane of the ecliptic with an angle of 23 ° 27 ′ (remember that the ecliptic is the plane circumscribed by the Earth’s orbit around the Sun, which, seen from the Earth, becomes the apparent annual path of the Sun). This inclination causes the plane of the celestial equator (which is the projection of the Earth’s equator) and the ecliptic plane to cross at two points to form the axis of the equinoxes. When the Earth crosses these two points in the course of its orbit (the vernal or Aries point and the Libra point), the days and nights are the same lengths, marking the beginning of the spring and autumn season, respectively.
Since the Earth is not a perfect sphere, the gravitational influences of the other planetary bodies cause an attraction on the equatorial bulge, generating a slow conical or spinning movement of the Earth’s orbital axis. This precession movement causes the vernal point or gamma point (spring equinox) to move relative to the fixed reference stars at a rate of about 1° of ecliptic longitude every 72 years. In this way, the initial orientation is proposed again approximately every 25,920 years (or, according to astronomical calculations, every 25,772 years).
This period is called the Platonic Year or Great Year in honour of the Athenian philosopher who first brought it to the attention of the classical world (although he did not establish his assumptions on precessional motion). The Platonic year is the basis of considerations on the so-called eras of humanity: approximately every 2160 years (25.920 divided by 12), there is a phase shift of a zodiac sign to the constellation of reference; this implies, according to astrological thought, a reorientation in the evolution of humanity.