Precession of the Equinoxes, page 1

A Wobble in the Axis

The planet Earth is a spinning object, just like spinning tops and gyroscopes. It is much larger, more massive, and it takes all of 24 hours to make one revolution. Nevertheless, like all spinning objects, its axis wobbles in a circular manner, as shown in Figure 1 below.

  

Wobble in the Earth's axis

Figure 1. Movement of the poles caused by the precessional wobble.

(Hotlink to Figure 2 for a larger version of this figure.)

The wandering pole

Just watch a toy gyroscope and you'll see this happen. This wobble is called precession. It takes the Earth's axis nearly 26000 years to complete one cycle of this wobble. Precession is subtle, but measurable, and its major effect is to point the Earth's axis at different points of the polar skies. The following sky shot, from RedShift, shows the position of the Celestial North Pole 2000 years ago compared to its present position on Polaris (in the constellation of Ursa Minor), see Figure 3 below.

  

Path of the celestial north pole over its 26,000 year cycle

Figure 3. Path of the celestial north pole over its 26,000 year cycle.

Tracing the path of the pole

The north and south poles each describe a circle across their polar skies over that 26000 year cycle. In the Northern Hemisphere, our planet's axis points toward a star in Ursa Minor (the small bear) called Polaris. This is called the 'pole star', but Polaris has had that privilege only recently. 2000 years ago, the axis pointed into empty space between the constellations of Draco and Ursa Minor. Some 4000 years ago it crossed Draco by a star called Draconis Thuban. Almost half way around the cycle, about 13000 years ago, and in about 13000 years time, the pole star was and will be near the extremely bright star Vega. Shown against the constellations of the north polar sky, the celestial north pole traces the path shown in Figure 3 above. Today it points to Polaris.