Hi Charlie,

We only see one side of the Moon because it rotates exactly one time per revolution around Earth. This physical phenomenon is called synchronous rotation.

Wait a second, you may be thinking... if the Moon rotates, then shouldn't we see its backside eventually? Nope! If the Moon did NOT rotate, then we would get to see the Moon’s entire surface at some point as it orbited Earth. But, it rotates at just the right rate to keep the same side facing Earth.

You can try this for yourself. Find a friend and have him or her pretend to be Earth. You be the Moon, and "orbit" your friend. If you keep facing one wall of the room as you orbit, you won't be rotating, and your friend (Earth) will be able to see all sides of you. But, if you orbit your friend and keep facing him or her, you will rotate yourself one time per revolution.

It may sound like an amazing coincidence, but synchronous rotation occurs naturally as objects in space gravitationally interact. Most moons in the solar system are in synchronous rotation around their planets.

To learn more about the Moon, check out my blogpost about Moon misconceptions.

- Jesse Richuso

Morehead Planetarium and Science Center Educator

www.moreheadplanetarium.org