Ah! I must contrive that the world should do three things. It must roll over once a day, roll round the sun once a year, and yet wriggle about in another way at the same time.—Stop a bit—I have it. My top turns round itself, and pretty quickly, too; it shifts about on the floor almost in a circle as it turns round; and I see it, especially when it is quietly spinning, have a slow roll of the head, like a sailor when walking ashore, as if its head were a little giddy.
You have hit it exactly, my good fellow. The three motions of your top are much like the three motions of the world. That top-heavy slow swinging of your top while it is spinning in full force is like a sort of head-rolling of the earth. The poles seem to have a roll of their own, independent of the regular roll.
Yes; and that would make the Polar stars seem as though they were swinging. But how long are they before they are in their regular places again?
Astronomers calculate 25,850 years.
But will not this changing make the star-charts of the ancients all wrong?
Indeed it does. Aries the Ram, for instance, as a sign, is the first thirty degrees on the celestial globe; but as a constellation it has shifted to between the thirtieth and sixtieth degree on the ecliptic. This is called the precession of the Equinoxes.
How is that? I know when the equinoxes are—March 21st and September 23rd—when it is equal night all over the world.
The European vernal equinox took place at the first point of Aries. But by this precession, or more properly recession, or going backward of the stars, the Equinox takes place when the sun is in Taurus the Bull, two thousand years after it was in the Ram.
I see. The stars not only get earlier four minutes a-day to accommodate the earth in its annual motion, but make a change to accommodate the swinging of its pole.
Can you tell me, James, how much the stars slip back in a year?