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In the history of astronomy, a Great Year often refers to one complete precession of the equinox but may also refer to any real or imagined cycle with astronomical or astrological significance. The most common Great Year (sometimes confused with the Platonic year) 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}

Confusion of Precession and Platonic Year

The Great Year, based on the cycle of precession, is often called Platonic Year based on a confusion with a concept defined by the philosopher Plato, who in his Timaeus defined the "perfect year" as the return of the celestial bodies (planets) and the fixed stars (circle of the Same) to their original positions:

The Platonic year, based on the revolution of the planets and estimated by Macrobius as lasting 15,000 solar years, has no connection to the "precessional period of 36,000 years"², caused by the slow gyration of the Earth's axis and discovered by the Greek astronomor Hipparchus:

The confusion originates with the astronomoer 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."³

Astronomical value of the precession cycle

The duration of the precession cycle, the time it takes for the equinox to precess 360 degrees relative to the fixed stars, 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.⁴ 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.^{citation needed}

Early cultures and mythology

The Greeks broke the Great Year into four ages known as the Golden, Silver, Bronze, and Iron Ages. The Indian Yuga cycle is also broken into four periods; the Satya, Treta, Dvapara, and Kali yugas, which each last millions of years.

According to Giorgio de Santillana and Hertha von Dechend, in their book Hamlet's Mill, 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,^{citation needed} which equates to a Great Year duration of 25,200 years.

Footnotes

See also

• Discovery of precession
• Astrological age
• Yuga