Great year

The term Great Year has a variety of related meanings. It is defined by NASA as "The period of one complete cycle of the equinoxes around the ecliptic, about 25,800 years ... also known as [a] Platonic Year." One complete cycle of the equinoxes here means one complete cycle of axial precession. Although Plato uses the term "perfect year" to describe the return of the celestial bodies (planets) and the diurnal rotation of the fixed stars (circle of the Same) to their original positions, there is no evidence he had any knowledge of axial precession. The cycle which Plato describes is one of planetary and astral conjunction, which can be postulated without any awareness of axial precession. In fact, Hipparchus is the first Greek credited with discovering axial precession roughly two hundred years after Plato's death (see below). Cicero followed Plato in defining the Great Year as a combination of solar, lunar and planetary cycles. (61 XX) Nicholas Campion writes of "periods of History, analogous to the solar year, known as 'Great Years'".

Plato's description of the perfect year is found in his dialogue Timaeus

And so people are all but ignorant of the fact that time really is the wanderings of these bodies, bewilderingly numerous as they are and astonishingly variegated. It is none the less possible, however, to discern that the perfect number of time brings to completion the perfect year at that moment when the relative speeds of all eight periods have been completed together and, measured by the circle of the Same that moves uniformly, have achieved their consummation."

The 'Great Year' as defined by the historian Josephus is 600 years in length. (Book 1, Chapter 3, Paragraph 9)

God afforded them a longer time of life on account of their virtue, and the good use they made of it in astronomical and geometrical discoveries, which would not have afforded the time of foretelling [the periods of the stars] unless they had lived six hundred years; for the great year is completed in that interval.

In De Natura Deorum, Cicero wrote

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