What is "great year"

Platonic \Pla*ton"ic\, Platonical \Pla*ton"ic*al\, a. [L. Platonicus, Gr. ?: cf. F. platonique.]

1. Of or pertaining to Plato, or his philosophy, school, or opinions.

2. Pure, passionless; nonsexual; philosophical.

   Platonic bodies, the five regular geometrical solids; namely, the tetrahedron, hexahedron or cube, octahedron, dodecahedron, and icosahedron.

   Platonic love, a pure, spiritual affection, subsisting between persons of opposite sex, unmixed with carnal desires, and regarding the mind only and its excellences; -- a species of love for which Plato was a warm advocate.

   Platonic year (Astron.), a period of time determined by the revolution of the equinoxes, or the space of time in which the stars and constellations return to their former places in respect to the equinoxes; -- called also great year. This revolution, which is caused by the precession of the equinoxes, is accomplished in about 26,000 years.
   --Barlow.

n. time required for one complete cycle of the precession of the equinoxes, about 25,800 years [syn: Platonic 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

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

In De Natura Deorum, Cicero wrote

By extension, the term "Great Year" can also be used for any concept of eternal return in the world's mythologies or philosophies. Otto Neugebauer wrote

Macrobius in his commentary on Cicero's Somnium Scipiones states that 'the philosophers' reckon the Great Year as 15,000 years. (p. 97) Censorinus wrote that Aristarchus of Samos reckoned a Great Year of 2484 years: it has been argued that this is a miscopying of 2434, which represents 45 Exeligmos cycles. (p. 96) (p. 6)

As noted above, the Platonic Year in origin would seem to have no connection with this concept (as the precession of the equinox was unknown to Europe in Plato's time). and two centuries after Plato, Hipparchus is credited with discovering the period of equinox precession, and the term "Great Year" eventually came to be applied to the period of that precession caused by the slow gyration of the Earth's axis.

It is argued that the confusion originates with the astronomer Ptolemy, who "adopted the larger, erroneous, figure, with the result that henceforth the two versions of the Great Year — the Platonic Great Year, defined by the planets, and the precessional, defined by the stars — were to be increasingly confused."

Ptolemy has been accused of committing scientific fraud by making up observations that would give the figure of 36,000 years even though the data available to him were good enough to get very near the true figure of 26,000.

Josephus refers to a 'Great Year' of 600 years. (ch.4)

It has been suggested that he obtained this value from Berossos who reckoned time in intervals of 60, 600 and 3600 years.

Isaac Newton determined the cause of precession and established the rate of precession at 1 degree per 72 years, very close to the true value, thus demonstrating the magnitude of the error in the value of 1 degree per century. (letter 17)

Walter Cruttenden wrote of the Great Year