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, 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, from his studying in Egypt for 13 years. 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."
In De Natura Deorum, Cicero wrote
On the diverse motions of the planets the mathematicians have based what they call the Great Year," which is completed when the sun, moon and five planets having all finished their courses have returned to the same positions relative to one another. The length of this period is hotly debated, but it must necessarily be a fixed and definite time." 
"The difficulty with the term "great year" lies in its ambiguity. Almost any period can be found sometime or somewhere honored with this name." 
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 15 Exeligmos cycles. (p. 96)  (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). 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. However, like most Greek scholars and servants of antiquity, Hipparchus conducted his studies in Egypt, just as Plato had, specifically at the library of Alexandria and therefore actually "rediscovered" the Precession which was actually recorded and common scientific knowledge in Egyptian systems.
The Ptolemies had systematically collected all they could of the educational and learned papyrus and housed them in the library. This collection comprised over 700,000 volumes and contained manuscripts from all over the world. It was the collection of scientific manuscripts that were the most impressive and were the impetus for the knowledge that Plato, Pythagoras, Hipparchus and others carried back to Greece and other parts, claiming or being credited with them, though the knowledge existed long before.
Some time around the middle of the second century BC, the astronomer Hipparchus discovered that the fixed stars as a whole gradually shifted their position in relation to the annually determined locations of the Sun at the equinoxes and solstices... Otto Neugebauer argued that Hipparchus in fact believed that this [36,000 years] was the maximum figure and that he also computed the true rate of one complete precession cycle at just under 26,000 years...
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.
More accurate, though, is the contention that Greek Scholars such as Plato and Pythagoras are credited with the introduction of knowledge that, though far advanced from anything Greece had, they did not fully understand. Plato wants to put forth knowledge and deal with astronomy, but does not want to compromise himself and since they are not his discoveries, he deals in vague abstracts or slightly skewed information. Such as in the Timaeus which is so hugely inspired by the Heliopolitan cosmogony that he actually reproduces sentences from the Egyptian text without citing where they came from. Pythagoras goes even further, studying for 22 years in Egypt, absorbing all the information and knowledge that he can from the metempsychosis or transmigration of souls down to the "Pythagorean theorem" which are simple restatements of Egyptian concepts and scientific knowledge such that Herodotus refers to Pythagoras as nothing more than a vulgar plagiarist of his Egyptian teachers.
With the true nature of the confusion and errata identified, we can turn back to the accurate information and true source of the knowledge of the Great Year, Egypt. It was the astronomer Biot who was the first in modern times to recognize or acknowledge that the Egyptians had discovered the Precession aka the Great Year. This acknowledgement was further corroborated by major authorities such as Norman Lockyer, a renowned astronomer and discoverer of helium, who was able to provide further evidence by demonstrating that all of the important Egyptian temples were precisely oriented to the major northern and southern stars and star groups and therefore the Egyptians had to have known of the Precession. Gerald Massey and R.A. Schwaller de Lubicz confirmed Biot and Lockyer's findings, and then 20th Century scientific authorities, Giorgio de Santillana and Livio Stechinni further corroborated these findings.
Probably the clearest and best evidence of the Egyptian knowledge of the Great Year is the zodiac of the Temple of Hathor at Denderah, built in 100 B.C.E. during the Hellenistic domination. Because of the date of its construction, Egyptologists originally believed that the Egyptian zodiac was derived from the Greek. However, it was discovered that the present stonework was constructed over the foundation of a previous temple that dated to 1600 B.C.E. proving that the rebuilding of the temple with its original zodiac establishes the Egyptian as preceding the Greek. The zodiac, which was originally inscribed in a second floor chapel within the temple, is held in the Louvre and contains the most solid empirical information:
The mythological figures representing the constellations are entwined in two circles—one around the north pole and one around the pole of the ecliptic. Where these two circles intersect marks the point of the equinox, or due east. The zodiac thus becomes a calendar going back to remote antiquity.
A line due east, which runs between the end of the Ram and the beginning of Pisces, indicates the time when the temple was rebuilt, about 100 B.C. An earlier line runs right through the Ram, indicating a date about 1600 B.C. at the heigh of Amonian domination, during the Twelfth Dynasty.
What this makes clear, is that Egyptian knowledge of the zodiac and the Precession must predate the Greek rather than the other way around. In fact, Lockyer believed the 1600 construction ACTUALLY represents the first re-building of the temple, and that it was originally constructed in 3200 B.C. Nile Valley astronomers were obviously firmly aware of the Precession at the start of the historical period and had used this knowledge to create a calendar that measured millennia versus days, months, and years...a calendar which we know as the Great Year.
To further understand the Egyptian origin, we must clarify from the concepts that have adopted this term, what the Great Year is. Finch describes:
What exactly is the Great Year? It is determined by the Precession of the Equinoxes, itself a function of the 23 1/2 degree tilt of the earth's axis. This tilt give us two north poles, the magnetic north pole defined by the earth's tilted northern axis and the vertical north pole, sometimes referred to as True North or the north pole of the ecliptic. Parenthetically we might note that two-thirds of this axial tilt is caused by the gravitational pull of the moon and the other third by that of the sun. This 23 1/2 degrees of tilt gives the earth a wobbling motion, like a spinning top, as it rotates and revolves around the sun. As a result of this wobble, the magnetic north pole describes a slow, retrograde circle around the north pole of the ecliptic. Over the slow course of time, this means that the position of the equinoxes against the background of stars gradually shifts in a counter-clockwise movement and the pole-star itself is displaced for another. It takes between 25,860 and 25, 920 years for the earth's axis to complete this circle. This is the Great Year and the apparent retrograde movement of the equinoxes relative to the circle of constellations represents the Precession. Once this was discovered after untold centuries of painstaking observation by pre-historic Kamite astronomer priests, the heavenly circle was divided into 12 arcs, each dominated by a constellation associated with a mythic type. Since most of these astro-mythic types were animals, the term "zodiac" from "zoion" meaning "animal" was given to this celestial circle by the Greeks. Each of the 12 arcs of the Precessional circle represents a "month" of 2155-2160 years in the Great Year of nearly 26,000 years. Each "month" constitutes an "age" and each age is dominated thematically by its own astro-mythic typology. Since the great zodiacal clock moves continuously and the equinoxes shift from age to age, the elements of astro-mythology change pari passu
What this helps put into context is the symbolism of the times influenced by the astronomical phenomenon. Such as worship going from a golden calf to a lamb of god to a fisher of men: Taurus to Aries to Pisces—in retrograde. It even is remarked in song such as the Fifth Dimension's famous line about the dawning of the Age of Aquarius. More importantly, though, the Great Year provides the evidence that Egyptian civilization predates all other civilizations and that based on the chronology of Manetho which starts at 36,766 B.C. and charts 36,525 years until the time of his writing his history in 241 B.C. It is also virtually identical to the start of the previous Great Year in the Age of Leo at 36,768 B.C., the most recent one commenced around 10858 B.C., and this other evidence that the Egyptian dynastic period actually begins between 4400-4200 B.C. and that remote Afro-Kamitic ancestors who would have passed this knowledge onto the Egyptians, had discovered and been studying this phenomenon for as much as 39,000 years ago according to Massey.
Having clarified the Great Year, we can return to the "confusion" and accusations of "fraud" around Ptolemy's observations and a period of 36,000 years as a measurement of the Great Year to show that this confusion is nothing more than scholars not fully understanding the work of the Egyptians and choosing the simplest explanation versus seeking out true answers. Due to their advance knowledge, the Egyptians devised three calendars: solar, lunar and stellar, all based on the movements of astronomical bodies. They had tracked the lunar cycle which led to the lunar calendar of 354 days which still determines the Muslim calendar. They had measured their solar calendar at 365 days, but being practical, used 360 days for the conventional year, adding 5 epagomenal days to complete it. The Egyptians knew the earth was round and took an elliptical path around the sun and therefore struck the balance because of the 360 degrees in a geometric circle and the subsequent divisions from this number.
Subsequent to these calendars, two other precise measurements solidified the final piece: the solar year measured from solstice to solstice at 365.24219 days and the year measurement based on the heliacal rising of Sirius at 365.25636 days also known as the Sothic or sidereal calendar. The phenomenon this was based on was the annual appearance of Sirius at the latitude of Thebes in Upper Egypt just before dawn at the summer solstice. Within 20 days after this rising, the Nile flooding would occur. Sirius became known as the herald of the sun, announcer of the flood and harbinger of the New Year. They rounded off this "tropical year" at the mean between the two measurements settling on 365 1/4 days. This meant that the 365 day civil calendar used by the Egyptians diverged in retrograde, recoinciding with the true year every 1460 years, which became known as the Sothic cycle.
Once this is taken into account as well as the fact that Manetho's chronology spans 36,525 it suggests a secondary "Great Year" measured as the chronology which is the equivalent of 25 cycles the Sothic year of 1461 years. This, some scholars believe, that the Egyptians were operating under two different synchronized Great Years overlaid on the history of Manetho. This may have been lost over time in or discarded in favor of the standard Precession.
Other ideas still emerged for concept and debate.
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.
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
Some people called it the Yuga cycle, others called it the Grand cycle and others the Perfect Year...But the most common name found in use from ancient Europe to ancient China, was simply the Great Year".
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Boris Cristoff proved that duration in his book "El destino de la humanidad" (Barcelona, editorial Martínez Roca, 1981; colección "Fontana Fantástica").
Callatay G. de, Annus Platonicus, a study of worldcycles in Greek Latin and Arabian sources, Publication de l'Institut Orientaliste de Louvain #47, Louvain, 1996
Nicholas Campion, "The Great Year" (book)
Walter Cruttenden, "The Great Year" (documentary film)-->