Precession

Precession or Precession of the Equinoxes, slow wobble of Earth’s rotational axis in space, caused by the gravitational pulls of the Moon and Sun on Earth’s equatorial bulge. Earth’s axis traces out a cone in space like a spinning top whose axis is not upright. One complete cycle of precession takes about 25,868 years.

As a result of precession, the position of the celestial poles changes in a 25,868-year cycle, as well, along with the positions of all the stars as seen from Earth. For example, the particular star that serves as the North Star around which the night sky on Earth appears to revolve varies over time. The current North Star is Polaris in the constellation of the Little Dipper, but the star Vega, in the constellation Lyra, will be the North Star in the year 15,000. Polaris will be the North Star again in about 25,800 years. Astronomers need to account for these shifts in the coordinates of stars on the celestial sphere that they use to locate stars. Star positions have to be referred to a set date, such as the year 2000 (known as epoch 2000).

The effect is also known as the precession of the equinoxes, since it causes the position of the equinoxes that begin spring (vernal equinox) and autumn (autumnal equinox) to move westward or clockwise along the ecliptic by 1 degree in about 72 years. This motion means that the constellation of the zodiac in which the Sun appears at vernal (spring) equinox shifts over thousands of years. About 4,000 years ago, spring began when the Sun appeared in the constellation of Taurus. About 2,000 years ago, that vernal equinox occurred when the Sun was in Aries. Spring now begins with the Sun in Pisces, moving toward the constellation Aquarius, where spring will begin in about 300 years. The vernal equinox will be in Aries again in another 23,000 years after passing through all the constellations of the zodiac. The ancient Greek astronomer Hipparchus is credited with discovering the precession of the equinoxes in about 130 bc after studying the works of earlier astronomers.

Precession does not change the tilt angle of Earth’s axis, which is currently about 23.5°. However, another effect called nutation slowly shifts the angle of Earth’s axis between 21.5° to 24.5° over a period of 40,000 years. Precession and nutation combine with long-term variations in Earth’s orbit from a more circular to a more elliptical shape over about a 100,000-year period. Taken together, these cycles in Earth’s motions change the amount of sunlight that reaches particular parts of the planet during seasons of the year. A dramatic effect occurs when Earth’s greatest distance from the Sun coincides with a more upright tilt angle of the axis during summer in the northern hemisphere. The region near the Arctic Circle then receives very low amounts of sunlight during summer, and ice and snow do not melt. The Serbian geophysicist Milutin Milankovitch first described these combined cycles in the 1930s. Since then, a number of geological discoveries have linked the so-called Milankovitch cycle to climate events such as the Ice Ages that caused huge ice sheets to cover parts of Earth for thousands of years.