Why Doesn't the Moon Spin?

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I've been asked on a number of occasions recently why the moon always seems to show us the same face -- the lunar nearside. I'm not sure why there's the sudden interest, but it's a very good and valid question, especially as tonight (March 27) is a full moon.

Look at the moon at any time and -- aside from the constantly changing phases that are caused by changing relative positions of the Earth, the moon and sun -- it does indeed show us the same face, constantly. Perhaps surprisingly, it's 'non-rotation' (from our perspective) comes from its interaction with the Earth.

Both the Earth and moon are big lumps of rock with the moon in orbit around the Earth or, more precisely, both objects in orbit around their common center of gravity -- known as the 'barycenter.' It just happens that this point lies very close to the center of our planet, so to all intents the moon orbits us.

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The moon's orbit is elliptical, taking it furthest away from Earth at a point called "apogee" (406,720 kilometers) and a closest point called "perigee" (356,375 kilometers) and as it orbits both objects gravitationally tug a little at each other. This tugging creates a bulge on the moon, and to a lesser extent on the Earth, which we call tides.

Common sense suggests that these two tidal bulges would be perfectly aligned with each other, but in reality, the rotation of the two objects tend to drag the bulge along a little. The end result is the extra pull from the slightly off-set bulge acts like a bicycle brake on the moon slowing its rotation to such a degree that it now takes the same time for it to orbit the Earth once as it does to rotate on its axis once.

Therefore, the moon does rotate, just once per orbit. Its a phenomenon known as 'synchronous rotation' and is commonly seen on other 'tidally-locked' moon systems in our solar system.

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So, it's the tidal forces exerted on both bodies that creates the bulges resulting in the same hemisphere of the moon facing Earth. Another effect from the tides is that the moon is slowly accelerating in its orbit, causing it to move very slightly further away from us at a rate of around 4 centimeters per year.

In reality though, we can see a little more than 50 percent of the moon's surface from Earth. That may seem to contradict everything I have just said about only one side of the moon facing us, but thanks to the orbital characteristics of our natural satellite we can take sneaky peaks just over the horizon.

The orbit of the moon is tilted very slightly to the plane of the Earth's orbit around the sun, which means that during periods of its orbit, the moon is 'lower' than usual and at other times it is 'higher.' During these periods we can see a little further over the north and south poles of the moon but only by a small amount. This is known as 'libration in latitude.'

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The elliptical nature of the orbit of the moon means its speed varies, as described nicely by Kepler's Law's of motion. Because of the varying speed we can sometimes see slightly further around the eastern and western horizon, a phenomenon known as 'liberation in longitude.'

These two effects allow observers from Earth to see a total of 59 percent of the moon's surface as it orbits us.

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