Great year

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 25,765 years, at the current rate. Astrologers consider it important, while modern day astronomers account for precession to determine a star's position but care little about a complete precession cycle. The impact of the precession is raised in discussions of the origins of myths by scholars such as Robert Graves, in his comprehensive work The Greek Myths, who sought those origins from fragments of earlier myths, contained in those from the earliest historical records and prehistoric symbols discovered through archaeology.

Usually in literature one finds the duration of the precession 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 243.8 microradians (50.3 arcseconds) per year which would mean 25765 years for one cycle to complete, but speeds of 243.6 microradians (50.25 arcseconds) and 244.0 microradians (50.34 arcseconds), which would lead to the same rounded value 243.8 microradians would result in 25791 and 25744 years, respectively.

The precessional speed is slightly increasing each year, and therefore the cycle period is decreasing. Numerical simulations of the solar system over a period of millions of years give a period of 257 centuries. [A.L. Berger; Obliquity & precession for the last 5 million years; "Astronomy & astrophysics 51" (1976), 127] but no one is certain of the exact precession rate over long periods of time. Near the turn of the 20th century astronomer Simon Newcomb invented a "constant" to account for the increasing annual precession rate. Over the last 100 years this constant has been found to have underestimated the actual acceleration in the rate.

Most astrologers use a precession rate rounded to 50 arc seconds per year to derive a Great Year period of 25,920 years, the period required for the equinox to move through all twelve of the classic zodiacal signs. Some, such as Boris Cristoff prefer to round the age of one sign of the zodiac to 2100 years, which equates to a Great Year duration of 25,200 years. The Sanskrit scholar Swami Sri Yukteswar puts the length of a Great Year at 24,000 years, composed of one ascending age of 12,000 years and one descending age of 12,000 years. In India the Great Year is called a Yuga cycle.

The Greeks broke the ascending and descending portions of the Great Year into four ages each known as the Iron, Bronze, Silver and Golden Ages. The Indian Yuga cycle also breaks each ascending and descending arc into four periods; the Kali, Dwapara, Treta and Satya yugas.

In the history of astronomy, a great year may 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 there are over 200 myths or folk stories from over thirty ancient cultures that refer to a Great Year tied to the movement of the equinox or the motion of the heavens.

The book, The Great Year, written by Nicholas Campion describes some of the ancient and modern mythology of the Great Year concept. The documentary film, The Great Year, written by Walter Cruttenden and narrated by James Earl Jones describes some of the archaeological and astronomical evidence for the Great Year.

References

ee also

*Discovery of precession
*Astrological age
*Yuga