Observing the Sky: Precession of the Equinoxes

Long ago, people began identifying themselves by what zodiac sign they were "born under." Even today you can check the newspaper for your horoscope, a prediction supposedly based on the movements of heavenly bodies. But many of the zodiac signs with which we identify are wrong. For example, if you look up the horoscope for a person born on January 8th, you will find that person is a Capricorn, which should mean the Sun was in the constellation Capricornus on January 8th. But if you look on a celestial sphere or use a computer program such as Starry Night™, you'll find that the Sun is actually passing through Sagittarius on January 8th. You'll also find that many other birthdates seem to be misplaced. What's going on? Many, many years ago when people first started practicing astrology, the Sun could be found in Capricornus on January 8th. But since then, our view from Earth has changed (and with it the Sun's apparent location) because in addition to its rotation and revolution, the Earth wobbles.

The Earth wobbles like a spinning top that has slowed down and is about to fall over. The wobble is caused by the planet's odd shape. Instead of being perfectly round, the Earth is an oblate spheroid, which is a globe that's slightly flattened at the poles and bulging at the equator. The gravitational pulls of the Sun and Moon tug at this bulge and cause the Earth to wobble very slowly.