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, at the current rate. 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 arcseconds per year which would mean 25765 years for one cycle to complete. But speeds of 50.25 arcseconds and 50.34 arcseconds which would lead to the same rounded value of 50.3 arcseconds 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.<ref>*A.L. Berger; Obliquity & precession for the last 5 million years; Astronomy & astrophysics 51 (1976), 127</ref>

Astrologers do not agree but most use a precession rate rounded to 50" per year, to derive a Great Year period of 25920 years. Some, like Boris Cristoff, the Uruguayan astrologer and author of the book in Spanish "El destino de la Humanidad", prefers to round the age of one sign of the zodiac to 2100 years which gives equates to a Great Year duration of 25200 years. The Sanskit scholar Swami Sri Yukteswar puts the length of a Great Year at a period of 24,000 years, comprised of one ascending age of 12,000 years and one descending age of 12,000 years. Some scholars believe this measurement serves as the original basis for our current system of daily time; one 24 hour day with one 12 hour period of increasing light (AM) and one 12 hour period of increasing darkness (PM).

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.

According to Giorgio de Santillana, the former professsor of history at MIT, there are over 200 myths or folkstories from over 30 ancient cultures that refer to a Great Year tied to the motion of the heavens. Most of these ancient cultures believed that during the course of one Great Year civilization will rise for about 12,000 years, culminating in a Golden Age, then fall for 12,000 years, culminating in a Dark Age, before rising again. Thus a Great Year is thought to be a cyclical measurement of time with periods of waxing and waning light and darkness similar to the earth's daily and yearly periods of time.