Precession Globe
Publication information:
Abstract
Globe pivoted so north pole can precess.
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.
Comments:
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.
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.
Full text
Globe pivoted so north pole can precess.
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.
Comments:
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.
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.