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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."<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> One complete cycle of the equinoxes here means one complete cycle of axial precession; this precession was known to Plato, who defined the "perfect year" as the return of the celestial bodies (planets) and the diurnal rotation of the fixed stars (circle of the Same) to their original positions. Cicero followed Plato in defining the Great Year as a combination of solar, lunar and planetary cycles <ref name="archive.org">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> (61 XX) Nicholas Campion writes of "periods of History, analogous to the solar year, known as 'Great Years' " <ref name="Campion">Nicholas Campion, "The Great Year: Astrology, Millenarianism and History in the Western Tradition" (Arkana/Penguin Books, 1994), p. 6.</ref>

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."<ref>Plato, Timaeus 39d, in John M. Cooper (ed.), "Plato: Complete Works" (Hackett Publishing Company, 1997), p. 1243</ref>

In De Natura Deorum, Cicero wrote

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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

"The difficulty with the term "great year" lies in its ambiguity. Almost any period can be found sometime or somewhere honored with this name." <ref>Neugebauer O., (1975)A History of Ancient mathematical astronomy, Birkhäuser, p.618</ref>

Macrobius in his commentary on Cicero's Somnium Scipiones states that 'the philosophers' reckon the Great Year as 15,000 years.<ref name="books.google.co.uk">{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> (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.<ref name="books.google.co.uk"/> (p. 96) <ref>http://www.dioi.org/vols/wb1.pdf</ref> (p. 6)

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).<ref>William Harris Stahl, "Macrobius: Commentary on the Dream of Scipio" (Columbia University Press, 1952), p. 21</ref> and two centuries after Plato, Hipparchus is credited with discovering the period of equinox precession,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.

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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."<ref name="Campion">Nicholas Campion, "The Great Year: Astrology, Millenarianism and History in the Western Tradition" (Arkana/Penguin Books, 1994), p. 246–247.</ref></blockquote> 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.<ref name="newton">R.R.Newton, "The Authenticity of Ptolemy's star data" ([1])</ref>

Josephus refers to a 'Great Year' (Ancient Greek: μέγας ἐνιαυτός{{#invoke:Category handler|main}}) of 600 years.<ref name="sacred-texts.com">http://www.sacred-texts.com/jud/josephus/ant-1.htm</ref> (ch.4)

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It has been suggested that he obtained this value from Berossos who reckoned time in intervals of 60, 600 and 3600 years.<ref>Josephus, Jewish Antiquities, Loeb, p.1, note a,</ref>

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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> (letter 17)

Walter Cruttenden wrote of the Great Year

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Great Year sections
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