How far is the far side of the moon?

As I’m sure many of you are aware the same side of the Moon always faces out towards us, which is why it looks the same every night. This is because the Moon is ‘tidally locked’ and the time it takes to turn on its axis (29.5 days) is the same as it takes to go round the Earth.

It wasn’t always like this though and it didn’t just happen by chance. Since the Moon was created the Earth has been pulling on it and over time this pulling slowed down the Moon’s turning until it got to the state it is in now.

Libration of the moon

This animation shows how the lunar surface appears to wobble over a few nights, giving us a little glimpse of the dark side of the Moon.

You might be thinking that we’ll only ever see half of the Moon with our own eyes, seeing as how holidays to the Moon aren’t looking likely any time soon, but that’s not quite true because of something called libration. The Moon orbits around the Earth in a slightly eccentric orbit, meaning it goes in an ellipse rather than a circle. When the Moon is closer the Earth pulls on it more and it spins faster. When the Moon is further away it turns slower. Over the 29.5 days it takes the Moon to go around the Earth it will rotate once but this slowing down and speeding up means that what we see in the sky wobbles a bit. If you look at the night by night animation on the right you can see what I mean.

If you look at the Moon every night from new Moon to new Moon you’ll actually get to see 59% of the Moon’s surface. The other 41% isn’t completely dark to us though. We’ve sent enough missions around the Moon that we’ve got some pretty good images of it. Personally I prefer our side. Apparently a man lives in it, though I’ve never managed to see the bloke myself…

The far side of the moon.

The far side of the moon, imaged by NASA’s Lunar Recon Orbiter

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