Has this thought ever crossed your mind? Look to the night sky on any night of the year, and staring back at you is the same side of the moon. Does it have only one side? Is it fixed in space? Does it spin about any axis? are some of the questions that come to mind.
Well, worry no more. This little piece of mystery is about to be explained, sit back and enjoy!
The truth is that the moon is not fixed, it is rotating, just like the earth. But its rotation period is the same as its translation period; this means that the time it takes the moon to spin about its own axis, is the same time it takes to rotate the earth. Hence, 1 moon day = 27/28 earth days.
If the Moon didn’t spin at all, then eventually, it would show its far side to the Earth while moving around our planet in orbit. However, since the rotational period is exactly the same as the orbital period, the same portion of the Moon’s sphere is always facing the Earth.
You can check the validity of this by doing a little experiment with a friend. Imagine you are the Earth and you stay on the same spot, while your friend is the moon rotating around you. Ask that your friend go in circles around you, but to keep watching you all the while.
If your friend does this, you can’t see the back of your friend, only his front side. When a circle is complete, your friend would have rotated 360º on himself, and 360º around you. Then his translation period would be the same as his rotation period, exactly the same as the Moon. This movement is called Lunar Libration
It wasn’t always this way though. Millions of years ago, the Moon spun at a much faster pace than it does now. However, the gravitational influence of the Earth gradually acted upon the Moon to slow its rotation down, in the same way that the much smaller gravitational influence of the Moon acts upon the Earth to create ocean tides. This influence slowed the rotational period of the Moon to match that of its orbit – about 27.3 days – and it is now “locked in” to this period.
In the solar system, a object rotating around a massive one experiences the same effect. For example, Mercury has a synchronized movement and always shows only a side to the Sun. Venus has a near synchronized movement with the sun, while Callisto (one of Jupiter’s moon) has a synchronized rotation with Jupiter.
Hence, the phenomenon of the translation period of the moon being equal to its rotation period is also evident in these other planetary bodies, and is not as unusual as it sounds. And you need not wonder anymore why the moon presents the same side of itself to observers on earth night after night.
