Q&A: What is a Platonic Year?

The Platonic Year in Astronomy and Astrology

Italian version

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The rotation plane of the Earth, which is defined by the equator, is tilted at an angle of 23° 27′ relative to the plane of the ecliptic. The ecliptic is the plane formed by Earth’s orbit around the Sun, which appears as the Sun’s path throughout the year when observed from Earth. This tilt results in the celestial equator, the projection of Earth’s equator into space, intersecting with the ecliptic at two points, creating the axis of the equinoxes. When Earth reaches these two intersection points in its orbit, known as the vernal (or Aries) point and the autumnal (or Libra) point, day and night are of equal duration. These events mark the beginning of spring and autumn, respectively.

The Earth is not a perfect sphere, and the gravitational influence of other planetary bodies affects its shape, particularly the equatorial bulge. This fact results in a slow, conical movement of the Earth’s orbital axis, known as precession. As a consequence of this precessional movement, the vernal point, or gamma point (spring equinox), shifts relative to fixed reference stars at a rate of approximately 1° of ecliptic longitude every 72 years. That means that the Earth’s initial orientation is roughly restored every 25,920 years, or according to astronomical calculations, every 25,772 years.

The period known as the Platonic Year, or Great Year, is named after the Athenian philosopher who first brought it to the attention of the classical world, although he did not base his ideas on precessional motion.