An Orrery is a machine which simulates the motion of the planets (and some of their moons) around the Sun. Each planet orbits the Sun in its own special period of time. eg. The Earth orbits (travels once round) the Sun in 365 days or 1 year.

A strip of wood, two axles, a pulley and an elastic band can be used to make a simple ‘Sun, Earth and Moon’ orrery something like the one shown in this drawing.

Rolled up paper and tape are used to make the Sun and Earth.

The Moon is a ball of blutack on the end of a bent wire.

An elastic band transmits motion to the axle supporting the Earth and this causes it to spin on its axis.

The Sun remains still.
The Earth spins and rotates around the Sun.
The Moon does not spin and rotates around the Earth.

The orrery model shown above has been made by a primary school pupil (Year 4) as part of an investigation of how the motion of the Earth and Moon give rise to an eclipse.

The Sun (left) is removed and a torch is used to shine a beam of light towards the Earth and Moon as if coming from the Sun.

If the moon is in the correct position it casts a shadow on the Earth and this explains how a solar eclipse arises.

It’s great fun to draw a tiny person on the surface of the Earth and see the shadow cover the person as the Moon moves into position between the Sun (the light source) and the Earth.

Other materials or objects can be used to make the Sun and Earth

In the model above the Sun is a yellow sponge ball and the Earth some white packaging foam.

In the position shown above the light from the Sun is being blocked from reaching the Moon so the Moon would appear in darkness having been eclipsed by the Earth. This is a Lunar eclipse.

The parts needed to make the orrery are shown below

Once you have experimented with the orrery and can demonstrate how a Solar and Lunar eclipse occur you could try and work out if there is a problem with this simple model!

How good a simulation of real planetary motion does the simple model orrey give us?

Does the simple orrery model explain planetary motion accurately?

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