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.
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.
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.
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