side of the moon

[Jade123321]

y do we always see the same side of the moon? I'm not doing this for school, but i just don't get it.

[kags]

This is tricky to explain - the moon revolves around the earth in around 27 1/2 days, and it revolves on its own axis in exactly the same amount of time, so we always see the same side (with a bit of variation round the edges). You can try this at home - but you need a friend. Stand facing your friend, and pretend you are the moon and he is the earth. Imagine the 4 poles around your friend, and you are at south - now move up to east, and turn 90 degrees to your left - you will still be looking at your friend. Now move to north and turn another 90 degrees to your left. you will now be looking at the back of his head. Now move again to west and turn another 90 degrees left. Move again back to south and turn again and you will see you have turned a full circle whilst orbiting him, but always faced inwards. Now do it again and ask your friend to revolve on the spot to mimic the earth's rotation, and you will see that whenever he sees you, he sees your face. When he is not looking at you it is because you are on the other side of the earth, but the people there are also looking at the same side.

[kags]

ooh, cut off in my prime! Hope that makes sense, I spent months trying to understand the geometry and could only get it it by doing this!

[bernardo]

The reason why the Moon is "stuck" in that position is because the Moon is not quite spherical; it's got a very slight bulge which sticks out towards the Earth and another bulge outwards on the other side. So the gravitational pull on the nearest bit makes it unable to rotate out of position. Imagine what would happen if you tied a pencil to a piece of string and whirled it round your head: it would end up pointing away from you in a straightish line. It's the same sort of thing. The Moon was probably first formed when a Big Thing crashed into the Earth and caused a Big Blob to come flying out. The Big Blob cooled down and became the Moon; it is still moving away from the Earth very slowly. That helps to explain why it didn't coalesce as a rotating thingy.