“In the Greek world the first distinct mention of the Great Year was made by Plato, who argued in his Timaeus that time is produced by the celestial bodies: the moon determines the month, the sun the year; but the times of the planets and of the sphere of the fixed stars are so great that it can hardly be known whether they are times at all.
In any case it is clear that the perfect number of time fulfills the perfect year at the moment at which the sun, the moon, the planets, and the fixed stars have all completed their courses and have again reached their starting point. (Plato, Timaeus, 39c, 39d).
By this is meant that the Great Year is completed when the celestial bodies have reached the same positions in relation to each other as they had at the beginning of that period. The identical conception is found in Cicero, qualified by the statement that the actual duration of such a period is a matter of controversy (Cicero, De natura deorum, II, 51-2).
But in his Hortensius, the book which was later to make such a strong impression on the young Augustine, Cicero equated the Great Year with 12,954 ordinary years, as we know from Tacitus and Servius (Tacitus, Dialogus de oratoribus, 16, 7. Maurus Servius Honoratus, Commentary on the Aenid of Virgil, I, 296. The same number is given by Solinus in connection with the phoenix, Solini Polyhistor, cap. xxxvi).
In addition to these opinions about the Great Year there is another according to which the sun, the moon, and the five planets all return at the end of the Great Year to one and the same sign of the Zodiac, the one under which they were when it began. According to Censorinus, Aristotle himself had put forward this same view, and preferentially indicated this period as “the Greatest Year.” This year, like the ordinary solar year, was thought to have a summer and winter too, the summer culminating in a world conflagration and the world in a world flood. (Censorinus, De die natali, 18, II. ).
How much of this really goes back to Aristotle cannot be said with certainty. (V. Rose, Aristotelis fragmenta, Lipsiae, 1886, 39, frg. 25). According to Seneca, Berossus, the Babylonian priest of Bel who wrote in the third century BC, propagated the same doctrine in a more detailed form: when the sun, the moon, and the planets came to lie in a straight line under the sign of Cancer, the world would burst into flames; and if they reached that position under Capricorn, the world would be inundated. (Seneca, Naturales Quaestiones, III, 29, I).
These rather improbable theories were especially favored among astrologers, since Greek astronomy had already reached a point of development at which the doctrines of Berossus could not be accepted. (J. Bidez, Bérose et la grande année, in Melanges Paul Fredericq, Brussels, 1904, 9-19.)
These texts treating the views of Aristotle and Berossus say that world catastrophes corresponding to the summer and winter of the solar year can occur in the course of the Great Year. The period between two world catastrophes could also be seen as a Great Year, but only in the derivative sense. The true Great Year, which might with Aristotle be called the Greatest Year, coincided with a complete cosmic revolution, whether interpreted in the sense of Plato and Cicero or in that of Aristotle and Berossus.
The Great Year of the Classical world arose from the purely mythical conception of a cosmic periodicity ultimately traceable to Babylonia.” (B.L. van der Waerden, Das gross Jahr und die ewige Wiederkehr, in Hermes, 80, 1952, 135-43.)”
R. van den Broek, The Myth of the Phoenix: According to Classical and Early Christian Traditions, Brill Archive, 1972, pp. 72-6.