precession - obliquity of ecliptic - gravitational perturbation
What it shows:
Due to the oblateness of the Earth, the gravitational force between the Earth and the Sun sets up a couple which causes the Earth's axis of rotation to precess. An adapted globe shows what is meant by precession.
How it works:
An old 8" (19cm) globe has been modified 1 to allow it to precess on its axis. A 23° cone is cut into the south pole, and a cone of metal supported by a metal equatorial ring has been inserted. This makes the globe bottom heavy (and gives it a total mass of 6kg) with a precession period of about 1.5 seconds.
A frame of reference is provided by a brass equatorial ring 5cm wide supported by a fork type mount; the metal equatorial belt also protrudes from the globe to provide a reference. A wire pointer rises and loops down towards the north pole of the globe, to be met by a metal rod pointer from the globe itself.
figure 1. Side view of the precession globe.
Discovery of the Precession of the Equinoxes is attributed to Hipparcus. The axis sweeps out a cone with a tilt angle of 23° 27' and makes a complete revolution every 25800 years. The Moon and planets also have a smaller oscillatory effect on the Earth's rotation which is called Nutation. Rating **
1 the original globe is by W. & A. K. Johnston Ltd., Edinburgh. The precession model is from Eastern Science Supply Co., Boston MA. The model is quite old.