Plato observed, just as we observe today, that the Moon, the planets, and the stars appear to orbit around the Earth. In his famous work Timaeus, he posited the "perfect year," in which the rotation of all astronomical objects lines up, so that they achieve a complete orbit around the Earth in unison. He suggested in later work that such a unified orbit would take place about every 36,000 years. While such a "perfect" rotation does not, to our current understanding, ever occur, the precession of the Earth's spin causes the direction of the Earth's axis of rotation to drift among the stars, completing one full rotation about every 30,000 years. Since the rate of this precession is comparable to Plato's "perfect year," it has been dubbed the Platonic Year.
Plato's fascinating work on the properties of time and the universe, Timaeus, is available here. The Platonic Year has been referenced in literature spanning hundreds of years, including Spenser's Faerie Queene and Yeats' poem "Nineteen Hundred and Nineteen," which is replete with interesting astronomical metaphors.
This page was last updated January 28, 2019.