precession of the equinoxes

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Astrology as it is known today was developed between the fourth and first centuries B.C.E. in the Mediterranean Basin. At the beginning of the year, marked by the spring equinox, the sun rose in the constellation Aries. After several centuries of observations, around 125 B.C.E., a Greek astrologer named Hipparchus discovered that very gradually the sun was moving in relation to the zodiac; that is, the precession of the equinoxes. There is some evidence that the phenomenon had been discovered earlier, but since Hipparchus, Western astrologers in general were aware of it.

The precession is caused in part by the slant of the Earth. It spins on an axis slanted at 23 degrees relative to its orbit around the Sun. That slant immediately accounts for the seasons. As the Earth moves around the Sun, where the axis is pointed toward the Sun, summer occurs, and where it is inclined away from the Sun, there is winter. However, as the Earth spins on its axis, because it is not a perfect sphere, it also wobbles slightly. It is this wobble that causes it to move slightly backward each year. That movement is hardly noticeable, being only one degree every 71 years.

Most Western astrologers use what is termed the tropical zodiac. The beginning of the year is marked by positioning 0° Aries at the point where the sun is located on the spring equinox. However, that point changes slightly every year, hence the zodiac moves slightly every year. Some astrologers use what is termed a sidereal equinox, in which the sun's true alignment with the constellations is retained. In the sidereal zodiac, the traditional relationship of the zodiac with the seasons of the year is lost.

This movement is slight from year to year but over the centuries makes a real difference. It takes approximately 2,150 years for the spring equinox to move from one sign to another and approximately 27,000 years for the wobble to make that point to return to its previously held position. The movement of the Sun's position at 0° Aries within one sign over a 2,100-year period defines an astrological age. Astrologers believe that different historical periods are ruled by different signs. In our own day we are believed to be experiencing the transition of the sun from the sign Pisces to Aquarius. The sign of Pisces the fish is often associated with Christianity, of which the fish has been a popular symbol. The contemporary revival of astrology has seen the twentieth century as the beginning of the Aquarian Age and has projected a hope that it will bring broad characteristic changes.

Sources:

Bach, Eleanor. Astrology from A to Z: An Illustrated Source Book. New York: Philosophical Library, 1990.

Campion, Nicolas. "The Age of Aquarius: A Modern Myth." In The Astrology of the Macrocosm. Edited by Joan Evbers. St. Paul, Minn.: Llewellyn Publications, 1990.

Filbey, John, and Peter Filbey. The Astrologer's Companion. Wellingsborough, Northampton, UK: Aquarian Press, 1986.