The slipping zodiac

Over the last few weeks the media have given lots of coverage about the slippage of the zodiac, and that whatever sign you think you are, you are probably really the sign before that. If you are an Aries, you might now be a Pisces, and so on. Actually, this slippage issue has been there for more than 2,000 years, which makes its sudden discovery in the last few weeks surprising.

Over the last few weeks the media have given lots of coverage about the slippage of the zodiac, and that whatever sign you think you are, you are probably really the sign before that. If you are an Aries, you might now be a Pisces, and so on. Actually, this slippage issue has been there for more than 2,000 years, which makes its sudden discovery in the last few weeks surprising.

It might be hard to accept it these days, but astronomy and astrology are siblings, born when we started to systematically observe the sky. The first observations were measurements of positions and motions; how the moon, sun and planets seem to move. One of these discoveries was that those bodies always move in one strip of sky, with a particular set of constellations forming the backdrop: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricornus and Pisces. The path followed by the sun, moon and planets is called the ecliptic, and the set of background constellations the zodiac.

The explanation for this is simple. All the planets, including the Earth, orbit the sun in the same plane, like marbles rolling in concentric circles around a flat plate, with the sun sitting in the middle. The complicated to and fro movements of the planets along the ecliptic come from our observing from the surface of one of the planets as it moves around the sun in its own orbit. However, all the planets would seem to us to be moving along a strip of sky containing the constellations in line with the edge of the plate.

At any time the sun and other bodies will lie in front of particular background constellations. For example, the sun may be in front of Taurus, Mars in front of Sagittarius and so on. Eventually, astronomy and astrology went their separate ways. Astronomy became a science, applying the process of observation, theory, tests, more observations and so on. Astrology became something different.

By convention, the first sign of the zodiac and the beginning point of the ecliptic lie at the point in the sky where the ecliptic crosses the equator, heading north — the spring equinox. Back when the astrological system was set up, the constellation behind this point was Aries, and the point was named the First Point of Aries. However, since then, things have changed. The Earth spins like a top, once a day, with its axis of rotation tilted at an angle of 23 degrees. Thanks to the shape of the Earth and the gravitational tugs from the moon and sun, the top wobbles. Each wobble takes 26,000 years, and is known as precession. During that time the spring equinox point and the beginning of the zodiac slips backwards, sign by sign. Between 4500 and 2000 BC the first sign was Taurus. Then, from 2000 BC to 100 BC it was Aries. Since then, until 2700 AD it will be Pisces. In short, Pisces has been the first point in the zodiac for most of the duration of Western civilization.

If the zodiac is the chain of constellations we see the planets, moon and sun move against, then we have left out one constellation, Ophiuchus, which lies between Scorpius and Sagittarius.

Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory in Penticton.