Precession of the Equinoxes
Precession is a change in the orientation of the rotational axis of a rotating body, such as the earth in its rotation around the sun.
The earth rotates on an axis, which can be visualised as a line through the North and South Poles, much like the center of a spinning top. This axis shifts in space - very slowly, though, during millenia. On a time-frame of about 26,000 years, the earth's rotating axis seemingly describes a small circle or cone, which has been nicknamed a "wobble" in the earth's rotation. An analogy would be a gyroscope or toy top, that does not maintain an identical axis from year to year as it spins around.
From a very long-term perspective on earth, this "wobble" means that if one wished to fix the spring equinox at a point when the sun is at zero degrees in the sign of Aries, as astrologer Claudius Ptolemy did over 1800 years ago; over the centuries, the equinox point would appear to be slipping backward into the previous sign of Pisces, and headed on its way to Aquarius. The stars appear fixed in space from an earth-bound perspective, but the equinox and solstice points would appear to be moving in a retrograde direction at the rate of about one degree per 72 years.
In most western astrology, the tropical zodiac fixes the signs at 30-degree sectors, which today hardly overlap with the constellations for which they were once named, due to precession. The frame of reference is the spring equinox point as 0 degrees Aries, regardless of its seeming retrograde motion, as the date of equal hours of daylight and darkness. In the sidereal zodiac favoured by Hindu astrologers and some western astrologers, the signs more closely overlap with their constellations. The difference between the two systems, in terms of the sign placement of a given planet, is about 23.5 degrees (the so-called ayanamsha).
Some critics of astrology argue that its practitioners are ignorant of precession, and that a person born with her sun at, say, 15 degrees of Capricorn, would actually have her sun in Sagittarius. Astrologers are well aware of precession, but most western astrologers favour the tropical zodiac, which forever fixes the vernal equinox of 0 degrees Aries in relation to a solar, not constellational calendar.
- Wikipedia: Axial precession
- Precession and Zodiac (Dieter Koch, 2002; Astrodienst)
- The Tropical and Sidereal Zodiaks & The Cycle of Earth's Precessional Cross (Nick Fiorenza, 2015)
Notes and References
- The exact rate of precession is 50.1″ per annum, and it therefore requires 25,868 years for the equinoxes to describe a complete circle on the ecliptic