“A child’s top spins. It not only spins, it wobbles. After a short time, the handle of the top begins moving in a circle. This happens because of the uneven distribution of the weight (or mass) of the child’s top. A slight “nudge” allows the wobble, called precession. Our planet Earth acts like a child’s top.

The movement of celestial bodies was very important to the ancient Egyptians. Much of interior Egypt had been a desert for a very long time. The production of food was dependent on the flooding of the Nile, which laid down a layer of fertile sediment suitable for planting. This flooding was very regular and could be predicted using a calendar based on the sun, or solar calendar. Interestingly, the Egyptians used three calendars. One was their lunar calendar, used mainly for religious ceremonies and festivals based on the phases of the moon. They regulated, or calibrated, the lunar calendar by using a second and sidereal (star-based) calendar based on the rising of the star Sirius. A
third and mainly solar calendar was used as the civil calendar. Correction schemes in this third type of calendar were implemented from time to time as needed because of the inconsistency between lunar and solar time. These schemes corresponded roughly to our present-day rules for leap years.

The Earth rotates on its axis with a speed of about 1000 miles per hour (1600 kilometers per hour) at the equator. This causes the diameter of the Earth measured around the equator to be about 26.5 miles (42.4 kilometers) greater than its diameter measured around the poles. This is what we mean by the uneven distribution of Earth’s mass. The
gravitational forces of other bodies (mainly the sun and the moon) on the Earth provide the “nudge” that allows the precession of Earth’s axis (the imaginary line about which Earth rotates).

At present, Earth’s axis is tilted at an angle of about 23.5 degrees from the plane of its orbit around the sun. The axis currently points very close to Polaris, the current North Star. It is also known as the Pole Star, or Alpha Ursa Minoris. You may recall from a previous issue of Middle Wisconsin that this would make it the brightest star in the
Little Bear (the Little Dipper). As the axis precesses, the direction in which the axis points in the heavens describes a circle in the sky. It will move away from Polaris. In the 41st century, the axis will point near Gamma Cephei in the constellation Cepheus. Around the 91st century, the North Star will likely be Deneb in Cygnus the Swan, and in the 120th century, it will be the star Vega in the constellation Lyra.

Looking backward in time from the present, Polaris had not yet become the North Star. In 2500 B.C., around the time of the building of the pyramids in Egypt, the North Star was the star Thuban, or Alpha Draconis
in the constellation Draco. We must remember that the precession of Earth’s axis happens on a faster time scale than movements among the stars. In ancient Egypt, constellations and important stars (such as Sirius) would have risen at somewhat different times and places than expected today.

The precession of Earth’s axis has a cycle lasting 26,000 years. In about 260 centuries, Polaris will be the North Star once again.”