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Comment by SegfaultSeagull | Hacker News Reader

Can someone ELI5 how it is one side of the moon is never seen on Earth? The moon orbits and also rotates, does it not?

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.

But why does this not happen to planets themselves in relation to the sun?

I think the explanation is wrong: wikipedia offers a completely different explanation https://en.wikipedia.org/wiki/Tidal_locking.

Planets can become tidally locked to the sun- mercury is. Probably the timescale required for the other planets is just much longer

EDIT: Apparently mercury isn't actually tidally locked to the sun, TIL

I don't see how my ELI5 disagrees with the wikipedia article.

wikipedia's explanation works for uniform bodies, yours doesn't. And since tidal bulges seem to be symmetric, there's no away to explain away the deformation as making one side heavier

I don't think their explanation works for uniform bodies.

Moons get tidally locked because they're very close to their planet, so the planet's gravity is by far the strongest influence.

The planets have much more complicated gravitational interactions because in addition to the Sun's gravity, they influence each other. So you end up with things like orbital resonances instead.

A planet that's close to its star and far from other strong gravitational influences will tidally lock to the star.

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.)

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

thanks, that's probably the best animation I've seen explaining a tidal locked moon.

The moon is tidally locked to earth, we only ever see one side.

Is it 2-way? I guess moon see all around earth, or no?

If you are on the near side of the moon[1], you will always see Earth see around Earth as it rotates and as the moon orbits it. You will also see it in different phases, like how we see lunar phases from Earth. If you are on the far side of the moon, you will not see Earth at all as you will always be facing away from it.

[1] The Earth does move in the moon's sky a bit. If you are on the near side but getting close to the far side, the Earth will be below the horizon sometimes.

Consider what it'd mean if there were parts of the Earth that could not be seen from the moon, it would also mean those locations could never themselves see the moon.

Ignoring the orbital period implications, I think it'd be bigger news if either US or Europe, or Asia couldn't ever actually see the moon.

This is the best ELI5 explanation I have heard. Thanks !

It may one day be, far in the future, although that's predicted to be so far off that Sol might have become a red giant by then, making the issue moot.

But only Luna is tidally locked at the moment. Terra is not, and its rotation still has a long way to slow down before it becomes so.

To add: this happens because the parts of the Moon that are closer to the Earth are pulled in more by Earth's gravity, compared to the ones further away.

The missing piece of information: the moon is not perfectly sphere shaped, the bulge has more mass, and is attracted by the earth more.

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 orbits and also rotates, does it not?

and one cancels the other, yes

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