Moon Orbit Model

Mechanical model of Earth-Moon orbit around Sun.

What it shows:

A model to demonstrate the precession of the Moon's orbit relative to the ecliptic. It is useful for discussing the conditions necessary for the occurrence of an eclipse.

How it works:

A large aluminum disk represents the plane of the Moon's orbit about the Earth. The disk lies flush with the box surface it sits in; the plane of the box representing the Ecliptic. The Moon's own orbit is inclined at 5° to the ecliptic, and precesses with an 18 year period. You can set the disk rotating (it tends to be unstable at high speeds) and with a flashlight or projector representing the Sun, you can see how the precession aligns the three heavenly bodies to produce an eclipse.

The disk is a 3mm aluminum sheet 60cm in diameter. The Earth at the center is an aluminum hemisphere 6cm in diameter with a recessed steel pivot point so the disk pivots above its center of gravity. The precession pointer is a 1m length aluminum rod of 1cm diameter. The Moon, situated at the outer rim of the disk, is a brass hemisphere 2.5cm in diameter. It is counter-balanced with lead on the underside of the disk. The box, constructed of wood, has an inner floor 2.5cm deep allowing the disk to tilt a maximum of 55° to the horizontal. There is a 1cm clearance between the disk and the circular hole in the box.

figure 1. Side view of the Moon Orbit Model


figure 2. Side view of Moon orbit model


Although not designed to cast shadows, the use of a flashlight to represent the Sun will aid in showing the conditions for an eclipse, and that a solar eclipse can only occur at a new moon and a lunar eclipse can only at full moon. This model was designed and built by Harvard student Walker Stevenson as a project for Science A-17 in 1982.