St. George Decoded - An Astronomy Lesson, page 7

An Astronomy Lesson

As the answer to the puzzle of St. George depends on the effects of precessional astronomy, a small diversion is required for those readers who are not familiar with its concepts. No maths is required to understand this. Those readers familiar with the precession of the equinoxes can skip straight to the section entitled Decoding the Legend.

The 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 3 "Movement of the poles caused by the precessional wobble" below. Just watch a toy gyroscope and you'll see this happen. This wobble is called precession. It takes the Earth's axis over 25000 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 in 7BC compared to its present position on Polaris (in the constellation of Ursa Minor), see Figure 2 below. The significance of the year 7BC will become apparent later.


Celestial north pole position in 7BC and in 2005AD

Figure 2. Celestial north pole position in 7BC and in 2005AD.

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. Almost half way around the cycle, in about 13000 years time, and about 13000 years ago, the pole star will be and was near the extremely bright star Vega. Figure 3 "Movement of the poles caused by the precessional wobble" below shows the complete cycle amongst the northern constellations.

Figure 3. Movement of the poles caused by the precessional wobble.

(Hotlink to a larger version of this figure.)