Astrology: Precession of the Equinoxes
Precession refers to a gravity-induced change in an astronomical body’s rotational axis or orbital path. It is often slow, happening over several millennia, but continuous. For Earth, the precession is caused by the gravitational pull of the sun, the moon and to a lesser extent, other celestial bodies.
The precession of the Earth is estimated to happen over a period of 26,000 years. This movement accounts for the astrological ages. It is also called precession of the equinoxes, because the equinoxes are said to move westward along the ecliptic. The slight change is said to be relative to the fixed stars and opposite the motion of the sun.
The precession of the equinoxes also accounts for the difference between sidereal astrology and tropical astrology. Sidereal astrology is patterned after the actual constellations upon which the zodiac signs are based. Tropical astrology uses the tropics as the main point of reference. The precession of the equinoxes is said to result in a shift between the two systems by about one degree every 70 years.
This concept is often used in astrology as it accounts for differences between traditions. Even horoscopes based on sidereal and tropical astrology result in different start and end dates for each signs of zodiac.
The discovery of the procession of the equinoxes is credited to Hipprachus. He is said to have discovered the astronomical phenomenon in 130 BC. It is attributed to him by Ptolemy. At that time of discovery, they noted that the sun was in a slightly different position at the time of Spring Equinox each year, as measured against fixed star sign. This observed behavior is called a progression.
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