alt Hacker NewsCan someone ELI5 how it is one side of the moon is never seen on Earth? The moon orbits and also rotates, does it not?
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It's because Earth pulls on the bulge in the Moon created by Earth's gravity.
It has to do with the tides. Except in this case it isn't ocean tides - it's lunar tides. Just as the Moon's gravity creates a bulge in Earth's oceans, Earth's gravity creates a bulge in the material that makes up the moon.
If Earth and the Moon didn't rotate, the bulges would "point" directly at the other body. But with rotation, the tidal bulge is a little bit offset in the direction of rotation. And the Moon used to rotate.
That offset creates a torque. Earth's gravity tries to pull the bulge into perfect alignment. Over time this slows the rotation of the moon until it stopped rotating at all.
(Technically the Moon does rotate, but it does so at the same rate that it orbits Earth. So it doesn't rotate from our perspective.)
One consequence of that is that the Earth, as seen from the nearside of the Moon's surface, just hangs there, never moving, apart from a small wobble.
The moon is tidally locked to earth, we only ever see one side.
> The moon orbits and also rotates, does it not?
and one cancels the other, yes
I believe it's caused by even the slightest imbalance in mass. Because the moon is so close to Earth, the imbalance causes gravity to be slightly stronger on one side than the other side. Eventually, that leads to no rotation at all.
I imagine most bodies rotating around a second object will eventually lose their angular velocity.
Yes and its rotation is synchronized to earth. See the animation at https://en.wikipedia.org/wiki/Tidal_locking
The gravitational pull of the moon lifts up the ocean to cause tides. Well the Earth's gravitational pull is so strong on the moon that the heavier side of the moon always faces the Earth. This is called tidal locking. So the only way to ever see the far side of the moon is to go there. Humans have gone there before, but almost always during an Earth "full moon", which means the far side is unlit. We do have full images of the far side of the moon from remote probes, but the 21% of the far side that was lit had human eyes on it for the first time ever.