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In the history of astronomy, a great year may refer to any real or imagined cycle with astronomical or astrological significance. The most common 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, presently about 25,765 years. The Greeks sometimes called the period of time required for the naked eye planets to realign, a great year; this was an important concept in ancient Stoicism.^{citation needed}

Astronomical value

The duration of the precession is often given as 25,920 or 26,000 years. In reality the exact duration cannot be given, as the rate of precession is changing over time. This speed is currently 243.8 microradians (50.3 arcseconds) per year which would give 25,765 years for one cycle to complete.

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.¹ 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.

Early cultures and mythology

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. 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.

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.^{citation needed}

Significance in astrology

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.

Footnotes

1. ^ A.L. Berger; Obliquity & precession for the last 5 million years; Astronomy & astrophysics 51 (1976), 127

References

Nicholas Campion, The Great Year

See also

• Discovery of precession
• Astrological age
• Yuga