The Mysterious Moon

Why do we only see one side of the moon?

What is it about the moon? Why does it only present one face? These questions are part of the most popular myth in astronomy. Whenever you take a look at the moon, you will always see the same side. Aside from its familiar phases, it looks as if the moon doesn't rotate! This is a difficult concept for many people to understand.

How the moon rotates is an easy science experiment to do, using human models in a crowd setting. First, designate one volunteer as the moon and another one as Earth. Anyone else in the group will observe. Place the moon volunteer about eight feet away and facing the Earth volunteer. Now, move the moon as if it were revolving (orbiting) the Earth, taking care to make sure the moon faces the Earth at all times. Have the Earth spin very slowly in one spot to demonstrate its 24-hour axis rotation. Each time the Earth completes one rotation, the moon will have moved further in its orbit. The surrounding crowd should carefully watch the positions of the moon as it revolves around the Earth.

Observers will see that to keep the moon facing the Earth, the moon has to rotate as it goes around the Earth. As a result, the moon rotates once every orbit. If you don't rotate the moon, it will show all its sides to the Earth as it goes around. Feeling dizzy yet?

In another frame of reference, the Earth volunteer will always see the same face (near side) of the moon. From the oberservers point of view, the moon is clearly rotating once as it makes an orbit around the Earth.

The toughest part of this is to explain "why?" The reason the moon's near side is always visible from the Earth is because it spins on its axis in precisely the same amount of time it takes to revolve around the Earth. These two intervals have been equal for all of recorded history and probably for millions of years or longer!

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