Great year

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A Great year (also known as a Platonic year or Equinoctial cycle) is the time required for one complete cycle of the precession of the equinoxes: about 25700 years. Although astrologers consider it very important, modern day astronomers care little about it.

Usually in literature one finds the duration given as 26000 years, being a rounding of the supposedly more accurate value of 25800 years. In reality the exact duration cannot be given, as the speed of the general precession is a value changing over time. This speed is currently 50.3" per year which would mean 25765 years for one cycle to complete. But speeds of 50.25" and 50.34" which would lead to the same rounded value of 50.3" would result in 25791 and 25744 years respectively. As such the rounded value of 25800 is understandable.

But the precessional speed is currently increasing and as such the period is decreasing. Numerical simulations of the solar system over a period of millions of years give a figure of 257 centuries.*A.L. Berger; Obliquity & precession for the last 5 million years; Astronomy & astrophysics 51 (1976), 127

Astrologers do not agree: they just decree that the speed is 50" per year per definition, and then find a period of 25920 years.

The great year is also a prominent feature of Mayan astronomy and cosmology. Cycles lasting about 25800 years supposedly marked the time between successive rebirths of the planet (see Maya calendar).

In the history of astronomy, a great year can refer to any real or imagined cycle with astronomical or astrological significance. The Greeks sometimes called the period of time required for the naked eye planets to realign a great year. It was an important concept in ancient Stoicism.

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