The Moon’s distance from Earth

Class practical

An estimate of the ratio of the Moon's distance from the Earth compared to its diameter.

Apparatus and Materials

• Coin, small (e.g. penny)

Health & Safety and Technical Notes

Great care must be taken not to look directly at the Sun. Wear sunglasses or use a filter to reduce the Sun's glare.

Read our standard health & safety guidance

Procedure

1. Hold a small coin just beyond arm's length and adjust its distance from the eye until the disc of the coin just obscures the disc of the moon. A partner can then measure the distance from the eye to the coin.

Teaching Notes

• A British penny has a diameter of 2 cm so a clamp stand will be needed to support it. The coin will be approximately 110 coin-diameters away. So the Moon's distance is about 110 Moon-diameters away.
• A similar technique can be applied to the Sun, preferably when it is somewhat veiled by mist or cloud.
• By chance, the Sun looks about the same size as the Moon so the Sun is also about 110 Sun-diameters away. The Sun and the Moon both subtend an angle of about ½ ° at the Earth.
• Nowadays scientists measure how far the Moon is from the Earth with radar, by timing a pulse of radio waves to the Moon and back or by placing a reflector on the moon and timing a flash of laser light there and back. Greek astronomers were able to make measurements of the radius of the Earth and the distance of the Moon and Sun with reasonable accuracy 22 centuries ago with no radar, no radio time signals, and no telescopes and with only a small part of the world explored.

This experiment was safety-tested in July 2007}.