Lunar and Planetary Institute

# Facing the Moon:Moon Rotation and Revolution

Students discover that the Moon’s period of rotation (on its axis) is identical to its revolution around the Earth, which is why only one side of the Moon faces the Earth..

The Activity

• Does it orbit something?  What?  How long does it take for one orbit?

Confirm that the Moon goes around the Earth, and that it takes about a month (27.3 days).  Then ask the students about the Moon’s rotation.

• Do we ever see the far side of the Moon?
• Does the Moon rotate?  Why don’t we see the far side?

Have one student volunteer to be the Earth, and a second to be the Moon.  The Earth should stand still, while the Moon slowly orbits the Earth, facing the Earth the entire time.

Ask the other students who are observing to discuss the Moon’s motion every few steps of the Moon.

• Is the Moon turning?  How much has the Moon rotated with respect to you?  Give an answer in degrees.
• How long does it take for the Moon to completely turn on its axis?

Once the Moon volunteer has completed two revolutions, have the students pair up and repeat the demonstrations for themselves, taking turns as the Earth and as the Moon.