General Question

intro24's avatar

Why do we always see one side of the moon?

Asked by intro24 (1399 points ) November 30th, 2010 from iPhone

I have been told it’s because the moon just happens to rotate and orbit the earth so that we always see the one side. However, I find it hard to believe that the moon has kept this coincidental motion for as long as it’s been there. I would think it would eventually shift so that we see different parts but that’s obviously not the case. So how does it work the we always have and always will supposedly see the same side of the moon from the earth?

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22 Answers

Qingu's avatar

It’s tidally locked to Earth.

This happens to a lot of moons orbiting planets. It can also happen to planets orbiting stars. Mercury is tidally locked to the sun.

RareDenver's avatar

It hasn’t kept that motion for as long as it’s been there, it used to rotate faster. Also it doesn’t “just happen to rotate and orbit the earth so that we always see the one side” and it isn’t “coincidental motion”.

As the moon effects the tides of the earth by dragging a huge bulge of our oceans around the planet as it gravitational force sweeps across the surface when it rotates us, so our planet also exerts its force on the moon and that force is stronger on the near side than the far side, tidally locking it to us. That’s how I understand it anyway, but I’m neither an astrophysicist nor mathematician so maybe someone else can come along and explain it with more finesse.

erichw1504's avatar

Nice! I just coined a new sentimental quip:

“Baby, I’m tidally locked to you!”

…on the other hand, she could take it the wrong way and think that you’re calling her fat.

Qingu's avatar

I don’t know if there’s any intuitive way to explain it (Wikipedia certainly doesn’t attempt).

I guess you could think of it like this: stars, planets, moons, etc are always pulling on each other (that’s the force of gravity). But gravity gets weaker over long distances.

So over long distances, the pull isn’t strong enough to stop the body from spinning on its own axis (like most planets do).

But over short distances—like between moons and their planets—the force of gravity is strong enough that it acts like a “lasso.” Not only does it hold the moon in orbit, it also holds the moon still as it orbits.

This is probably way off base when it comes to the actual physics, but it’s the best I could come up with.

Qingu's avatar

Also, correction: Mercury is not tidally locked to the sun. (it’s close, but not quite).

But some gas giant exoplanets are tidally locked to their stars, I believe.

RocketGuy's avatar

Tidal lock: the Moon is not perfectly balanced. Like an unbalanced bicycle wheel, the heavy end will eventually point to the center of the Earth.

CyanoticWasp's avatar

I can confirm what @RocketGuy is saying about the tidal lock thing and the heavy end pointing to the center of the Earth… I’m noticing the same thing in my own life now.

Don’t ask.

Rarebear's avatar

Interestingly, the moon also undergoes libration where the moon wobbles a bit. Moon photographers use libration to study lunar features that aren’t always visible.

bkcunningham1's avatar

The moon is rotating. It orbits around the earth. The earth is rotating.

The time it takes the moon to rotate once on it’s axis is the same amount of time it takes it to rotate around the earth once. Another way to say this is, one moon rotation on its axis equals one moon orbit around the earth or approximately 271/3 days. The moon’s rotation and revolution are synchronized so the moon is in a synchronized orbit.

By the time it comes around again, it is in the same position it was before.

Rarebear's avatar

Also, the moon is slowing the Earth’s rotation as well. In a gazillion years, the Earth will be tidally locked to the moon.

erichw1504's avatar

@Rarebear Wow, I hope to live long enough to see that! ~

AstroChuck's avatar

@Qingo- Right about everything except for your info on Mercury. Mercury is not tidally locked.

DancingMind's avatar

All the physics aside—it’s that the man on the moon just can’t bear to look away from our wonderful Earth! He misses it. Mad that he got blasted off of it when it formed from those two colliding rocky planets. [ :

(More about our moon, it’s also slowly getting farther from the Earth. I don’t know exactly what’s happening on a mathematical level, but it’s something to do with and gravitational effect of the ocean tide it pulls.)

Rarebear's avatar

@DancingMind Correct. It’s a question of angular momentum. The moon rotates around the Earth and as the Earth absorbs its angular momentum, the Earth slows down and the moon moves farther away. Once both the Earth and the moon are tidally locked, the moon will stop moving away.

LostInParadise's avatar

The most intuitive way of picturing it is to imagine there is a string that goes from the Earth to the moon. As the moon revolves around the Earth, the string forces the moon to always face in the same direction. The gravitational pull of the Earth eventually slowed the rotation of the moon so that now it only rotates just enough to face the Earth, one rotation per revolution.

gondwanalon's avatar

How about this idea…Our Moon’s rotational axis is always pointing directly towards the Earth. Picture a clown at a circus spinning a basketball on top of a stick held on his nose. When the clown looks up at the basketball he only sees the same half of the ball as it spins.

RocketGuy's avatar

Libration is what an unbalanced bicycle wheel does a few seconds before it comes to a complete stop (slightly rotate back and forth). It must have been really cool to see the Moon when its librations were very large.

CyanoticWasp's avatar

I don’t always see one side of the moon. Some nights I don’t see it at all.

Rarebear's avatar

@RocketGuy Libration is also what I look like after I’ve had one too many beers.

fredTOG's avatar

@bkcunningham1 your the only one who has it right ,I guess you know this.

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