Earth-Moon Relationship

[ezapf]

I know this was touched on not long ago, but ...
How did it come to be that one rotation of the moon on its axis takes exactly the same amount of time as one revolution of the moon around earth, allowing us to only see one side of the moon?

[bobclean]

The moon used to spin on its own axis, but over time the gravitational 'drag' exerted by the earth has gradually slowed the spin of the moon, until the present day, when it has stopped spinning, hence we see only one side from our viewpoint on earth.

[mibn2cweus]

 Just as the moon affects the earth�s tides, gravitational pull from one direction on a rotating body rearranges material within creating heat from friction. This acts like a break slowing the rotation. Over billions of years this effect of the earth�s gravitational pull on the Moon has slowed its rotation until its rotation was in sync with its revolution around the Earth. This is called tidal lock and is why we only see one side of the Moon as it travels around the Earth.

[gen2]

The same thing has happened to Mercury in its orbit around the sun.

[ezapf]

Oh, now I get it. Fascinating! Thanks.

[JudgeJ]

Just to clarify bobclean's answer, the moon continues to spin on its own axis, but it takes the same time to make one revolution on its axis as it takes to orbit the earth. Hence we can only ever see one side of it.

This feature is known as "tidal locking" and was touched upon in an earlier answer. The tidal forces which exist between them tidally lock bodies. This term is not to be confused with the tides we observe in the sea. In fact those tides stole their name from the astronomical term because they are cause by the tidal forces which exist between the earth and the (mainly) the moon.

The tidal effect caused by the moon’s gravitational “pull” causes the earth to “bulge” towards the moon. This happens to all celestial bodies but can be seen quite clearly on Earth by the twice daily movement of the sea known as the “tides”.

Eventually the Earth will become “tidally locked” with the moon and the same point on earth will always face the moon. The moon became tidally locked with the earth much sooner because it has only one hundredth of the mass that the earth has.

[Smiggle]

So JudgeJ, does this mean that when the earth becomes tidally locked with the moon, then only one side of the planet will be able to see the moon?  What happens to the other side?  Does that mean at night time it will be complete darkness in the sky?