Why Do We Always See One Side Of The Moon?

When you look up at a full moon at night, it always looks the same.  The view never changes.  The same craters in the same position, always.  It might lead you to believe that the moon doesn’t rotate like Earth or other planets do, but in fact the moon does rotate.  Would you believe that the exact amount of time it takes the moon to rotate on its axis is the exact amount of time it takes the moon to orbit around the earth, thereby making it so the view never changes?  Well its true!

Think about this, if the moon didn’t rotate, you would see all sides of the moon eventually over time.  Don’t believe me?  Try an experiment.  Get 2 balls, one larger than the other.  The large ball represents earth and the small ball represents the moon.  Tie a string to the small ball in such a way so the ball cannot spin while you hold the string.  Move the small ball (the moon) around the large ball and notice that a different side of the moon would always be facing a different side of the earth.

Forces at Work?

The moon orbits the earth about once per month, or 29 days.  The rotation of the moon on its axis has become “tidally locked” with the earth, as earth being larger than the moon, it has a great deal of gravitational influence on the moon.  Millions of years ago, the moon rotated at a much faster pace, the earth and its gravity slowed it down to is present rate.  Even though the moon is smaller, the moon does exert some gravitational influence on the earth, we mainly see this effect in the form of tidal waves at the beach.

So the old phrase (and song) “the dark side of the moon” is a misnomer, because it suggests that there is one side that is always dark when in fact the moon experiences day and night in the same way that the earth does.  However a single moon day is a lot longer, being about one month long (29 days).

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Category: Science, Space