Precession of the Equinoxes

Precession is a slow "wobble" in the direction of the Earth's rotational axis, taking about 25,800 years to complete one cycle.

Causes of Precession

The Earth is not a perfect sphere - it bulges at the equator. The gravitational pull of the Moon and Sun on this bulging mass causes the Earth's axis to wobble slowly, like a spinning top.

Effects of Precession

• Celestial Pole Motion - The north celestial pole slowly traces a circle among the stars. Polaris hasn't always been the North Star. About 5,000 years ago, Thuban in Draco was the pole star. In 12,000 years, Vega will be nearest the pole.
• Equinox Shift - The vernal and autumnal equinoxes slowly shift westward along the ecliptic. The Sun reaches the vernal equinox about 20 minutes earlier each year.
• Zodiac Shift - The constellations of the zodiac are no longer aligned with the astrological signs they were named after 2,000 years ago.

Precession Epoch

Because of precession, an object's right ascension and declination change over time. Star atlases and ephemerides often use a fixed epoch, such as 2000.0 (J2000), for reporting coordinates. SkySafari can compute coordinates for any epoch.