Sunday, January 8, 2012

Moon Brain: Why Do We Always See the Same Side of The Moon?

The Question:

Why do we always see the same side of the moon?

The Background:

Remember the man in the moon?  Ever wondered what the back of his head looks like?  Ever wonder why no one except a couple of Apollo Astronauts have ever seen the "dark side" of the moon?

It can be a little hard to wrap your head around the fact that the same side of the moon always faces the Earth.  Try it like this.  Look up at the moon.  Imagine that the side of the moon that you are looking at were to come crashing straight down into you and through the Earth. When it pushed its way out the other side, the folks in China would see the back side of the moon as it flew back up into space right?  Right.  Only that's not what happens.  Folks in China see the same side of the moon we do.    Observe this picture of the full moon rising over the National Stadium in China:

It's the same side of the moon!   WTF?!!

So What's the Answer?

The answer to this question comes in two parts.  

First, the moon is turning at exactly the same rate it orbits the Earth.  Weird huh?  But it makes sense if you think about it.  In order your you and me, on opposite sides of the planet, to see the same side of the moon, it must be turning at exactly the same rate that it is orbiting the Earth.  You can do this with your fingers.  Hold a finger on one hand still.  Now spin a finger from the other hand around it slowly, turning it so that the same side is always facing the first finger.    That's what the moon is doing.  

The orbital period for the moon is roughly 27.321583 days.  Roughly.

The moon rotates on its axis roughly once every 27.321583 days as well.

How trippy is that?  Even weirder is why.

It turns out that the gravitational drag between two spheres will eventually slow their rotation so that their rotation matches their orbital periods.  This weird force is called tidal locking.  When the moon first formed, it did spin but the drag of gravity has slowed it till it always shows us the same side.  Since the moon is not a perfect sphere, the heaviest part of the moon settled toward us.  If you turned your bike over, put a weight on the back wheel, and then spun the wheel, when it slowed the weight would settle toward the Earth right?  Same thing happened to the moon.

Tidal locking with the moon has slowed the Earth too but the Earth is much more massive so we still spin once every 24  hours...Roughly.

We see the same side of the moon at all times because tidal locking has pulled the heaviest side toward us and it now spins at exactly the right speed to keep the heaviest side toward us.  


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