Platonic Year
Platonic Year :
A unit of time used in astronomy. The earth's axis of rotation is not fixed in space; the attraction of the moon causes it to slowly trace out a circle in the sky. This motion, called precession, changes the orientation of the sky as seen from the earth's surface: the poles appear to shift their locations and the sun's point of crossing the equator slowly rotates through the constellations of the Zodiac. The Platonic year is the length of time required for one complete precessional rotation: about 25 800 years. The unit is named for the ancient Greek philosopher Plato (ca. 428-348 BCE). It is sometimes called the great year
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