NOVA Online > To the Moon > Lunar Puzzlers

Question 1 animation Well done! A moon day is 29.5 Earth days long.

Although from Earth's view the moon doesn't rotate, with respect to the sun it does. Take a look at the animation above. Look only at the moon. (The red dot marks a fixed point on the moon's surface.) See how it does rotate on an axis?

Photo of moon It seems odd that the moon doesn't rotate on its axis at a rate independent of its orbit around the Earth, doesn't it? If the Earth behaved as the moon does—that is, if one side always faced the object it orbited (the sun)—then one side of the Earth would always be in sunlight, the other, always in darkness.

The moon behaves this way because the side facing the Earth, the man-on-the-moon half, is denser, or has more mass. The Earth's gravity attracts this denser, near side more strongly than it does the far side, so the man-on-the-moon side remains pointed toward Earth.

The next lunar puzzler...

Photo: NASA

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