Why the same side of the moon always faces us

[carlthinks]

I think I finally figured out why the same side of the moon always faces us. But I'd like to hear it explained to me by an astronomer. So, why does the same side of the moon always face toward earth? Thank you for 'facing" this question.

 

[MeteorWayne]

The reason is because the tides raised on the (solid) lunar surface have over time (~ 4.3 Gy) stopped stopped the rotation at a rate matching it's orbit around the earth. The moon is doing the same thing to the earth, slowly slowing our spin rate, and the angular momentum is also causing the moon to slowly back away from the earth about an inch per year.

 

[carlthinks]

Hmmm . . . well that sounds authoritative, are you sure? That's not quite what I was thinking.

My own (layman-on-the-street) theory is that the moon is slightly out-of-round and its mass is unsymmetrical. (If it had a molten center, like Earth's, it would probably form itself into more of a true sphere.) So the Earth's gravity pulls the denser parts of the moon toward Earth, just like a fisherman's bobber always floats with the heavy side pointed down toward the Earth's center.

What do you think of that?

 

[MeteorWayne]

That might explain which side of the moon faces is, but would not have had sufficient energy to unspin it... Let me check the gravity maps.

 

[crazyeddie]

Hmmm . . . well that sounds authoritative, are you sure? That's not quite what I was thinking.

My own (layman-on-the-street) theory is that the moon is slightly out-of-round and its mass is unsymmetrical. (If it had a molten center, like Earth's, it would probably form itself into more of a true sphere.) So the Earth's gravity pulls the denser parts of the moon toward Earth, just like a fisherman's bobber always floats with the heavy side pointed down toward the Earth's center.

What do you think of that?

The moon is a sizable object, in comparison to the Earth, so it's gravity has a significant effect on it's primary. I don't believe the moon is "heavier" on one side, but Earth's gravitational pull does raise a "tide" in it's crust and mantle, which does have the effect of making it slightly oblong. The moon, in turn, creates a tidal bulge in the Earth's crust and mantle. Both of these bulges would point towards each other if the moon and the Earth did not rotate. Because both bodies DO rotate, the bulges in each body's surfaces could not quite keep up with their natural tendency to point towards each other. The Earth's bulge is always a little in advance of it's surface's closest point to the moon, and this causes the moon's gravitational attraction to, in effect, "pull back" on the Earth's bulge, which slows it's rotation. The Earth has the same effect on the moon, but because the moon is smaller and less massive, it had less angular momentum to work with, and consequently it's rotation became synchronized with it's orbital period. Eventually, billions of years from now, the Earth's rotation will become synchronized with the orbital period of the moon, and both will have the same hemisphere face each other at all times.....and the Earth and the moon will both take about 47 days to rotate once, if I recall correctly.....the same amount of time it will take the moon to orbit the Earth.

 

[MeteorWayne]

IIRC, both the earth and the moon will be incinerated within the surface of the Red Giant Sol before that happens....

 

[ZenGalacticore]

From what I understand of it, M.Wayne's first response was correct. It still fries my brain though when I try to visualize the Moon's spin being slow enough in its orbit around the Earth to where the same side faces us, tidally locked and all.

I wish I had one of those Earth-Moon system double globes that shows the orbital relation between the two. You know, they are connected and the natural dance of the two is displayed. And I'm pretty good at visualizing stuff, being a lefty and all. It still short-circuits my neurons!!

 

[MeteorWayne]

I understand that. I'm a 3D guy myself. Some things are much easier to get a handle on when you have a model in your hand, like the dance of the seasons.

I found it very enlightening this year when I was teaching an astronomy class to 8th graders when I had proprly sized models for the earth and the moon and showed the kids how far away from the earth the moon actually was.

If you'd like to do that, my earth was 3" in diameter (About the size of a large orange) the moon was about 3/4 inch (a large marble) and the moon orbits 7 feet away. I did that to show them how far away the brave men who went to the moon were.

Those double system globes always show the moon too close...with the typical sizes of the earth and moon on them, the moon would be tens of feet away!

Wayne

 

[ZenGalacticore]

Of course that is true, the double globe things don't try to show scale, just relative motion. This groovy chick I knew once wanted me to paint a representation of the solar system in florescent/black light paint TO SCALE in her basement rec room. :lol:

I told her that even with the Sun being the size of a basketball, she was going to need a MUCH larger rec room if she wanted a to-scale solar system! Pluto would have been 14 miles away! (Or something like that.) :lol:

 

[MeteorWayne]

That is certainly true...scale models of the solar system involving realistic sizes AND distances are hard to fit into a county, unless you're out west

 

[usn_skwerl]

http://www.spacevidcast.com/2009/09/15/ ... -09-09-15/

 

[aphh]

It still fries my brain though when I try to visualize the Moon's spin being slow enough in its orbit around the Earth to where the same side faces us, tidally locked and all.

I once read a good analogy for moon's spin;

consider you're in a room facing an object at the center of the room. Now start moving sideways around the object at the center still facing the object.

Does the background change while you move. If it does, you rotate when you go around the room and the object at the center. Compared to fixed positions in the heavenly sphere, the stars, the moon is in a constant spin.

That rotation, however, does not have impulse moment of it's own around moon's axis. All of that energy was spent when the Moon tried to stop the Earth from spinning. The rotation of the Moon is a by-product of the impulse moment the moon has when it circles it's center object.

 

[ZenGalacticore]

appph- Oh yeah! That totally clears it up and illustrates it for me! :lol:

 

[Planet_Lubber]

Most moons in the Solar System - at least those close to their primaries - are tidally locked to the primaries, so the revolve and rotate at the same rate, always keeping the same side toward the primary, always having the same leading side and trailing side. That doesn't depend on the bulge in the primary, although Earth's bulge under the moon will eventually cause Earth's rotation to slow down until it is also tidally locked to the moon, and they face each other as they dance, just like Pluto and Charon.

The Earth's gravity pulls a little harder on parts of the Moon that are closer. The difference in these forces acts to pull the moon apart. That's called the tidal force. If the Moon has enough tensile strength to resist being pulled apart, then a torque (twisting force) is exerted on the Moon. This torque tends to turn the Moon toward the orientation in which the long axis of the mass distribution is toward the Earth. If the Moon was spherically homogeneous, that wouldn't have any effect, and there would be no tidal locking.

So the Moon slowly rocks back and forth about this equilibrium orientation. Over millions of years, the internal friction caused by the torque will slow the rocking and the Moon settles into a tidal lock, also called a 1:1 spin-orbit resonance. It can get more complicated: Mercury, for example, has a 3:2 spin-orbit resonance, which means it rotates 3 times for every 2 times around the Sun.

Tidal locking is also important for attitude control of spacecraft. I've heard that they like to fly the Shuttles nose-down for this reason. If no control is applied, the shuttle will slowly rock back and forth about this orientation.

The tidal lock of the Moon's rotation is prevented from being too perfect by disturbances from the Sun's gravity. The Moon actually wobbles back and forth in longitude by about 4 degrees each way. So there is not a sharp boundary between near side and far side. At the Lunar equator, 8 degrees is about 243 km.

PL

 

[ZenGalacticore]

I thought Mercury's day was the same length as its year, 88 earth days. Or maybe its day was TWICE as long as its year, something like that.

 

[MeteorWayne]

Nope, the information given above is correct.

 

[CalliArcale]

I thought Mercury's day was the same length as its year, 88 earth days. Or maybe its day was TWICE as long as its year, something like that.

Astronomers used to think it was. They also thought Venus was tidally locked to the Sun. In both cases, it was because the few observations made all showed them with the same face pointing at the Sun. This wasn't entirely coincidental, either.

Venus isn't tidally locked to the Sun, but it does have a strange resonance with respect to the Earth. Though tidal action cannot account for this (it's too distant), it seems to always have the same side facing Earth during its closets passes to Earth (the only time anyone can study it via radar from Earth). Its true rate of rotation wasn't revealed until orbiting radar-sounding probes visited the planet. It's not understood how gravitational forces could have produced this situation, and it is entirely possible that it is actually just chance. Just a very surprising chance. Venus rotates retrograde, which is weird enough, and its day is *longer* than its year. Oddly, this means that if you stood on the surface of Venus (and didn't get simultaneously crushed and boiled by the weather), it would only seem to go 117 Earth days from sunrise to sunrise, though Venus actually takes 243 Earth days to rotate. There is a span of 584 days between close Venus approaches to Earth, and oddly, this is almost exactly five sidereal Venus days.

Mercury was likewise initially thought to be tide-locked with the Sun. But this was based entirely on the two visits that Mariner 10 made to the innermost planet; on both occasions, it happened to be pointing the same face towards the Sun. This wasn't entirely coincidental, though. The nature of Mariner 10's trajectory was such that it saw Mercury not at the same point in Mercury's year, but at the same point in the odd resonance that Mercury's rotation has. For every three Mercurian days, it goes around the Sun exactly twice. This resonance is the result of tidal interactions with the Sun. It can't become tidally locked the way the Moon is because it orbit is considerably more eccentric, which, due to its nearness to the Sun, means that the tidal forces vary dramatically over the course of a Mercurian year.

 

[crazyeddie]

Mercury was likewise initially thought to be tide-locked with the Sun. But this was based entirely on the two visits that Mariner 10 made to the innermost planet; on both occasions, it happened to be pointing the same face towards the Sun. This wasn't entirely coincidental, though. The nature of Mariner 10's trajectory was such that it saw Mercury not at the same point in Mercury's year, but at the same point in the odd resonance that Mercury's rotation has. For every three Mercurian days, it goes around the Sun exactly twice. This resonance is the result of tidal interactions with the Sun. It can't become tidally locked the way the Moon is because it orbit is considerably more eccentric, which, due to its nearness to the Sun, means that the tidal forces vary dramatically over the course of a Mercurian year.

It's not often that I am in a position to correct you on anything, Callie, but the discovery of Mercury's true rotation had nothing to do with the Mariner 10 spacecraft. It's 59 day rotation was discovered in 1965 by astronomers at the Aricebo observatory. I remember reading about it as a kid, and wondering how my 3rd-grade science book could be wrong!

This website is a good primer on the topic:

http://www.uwgb.edu/DutchS/planets/mercury.htm

 

[CalliArcale]

I didn't say Mariner 10 revealed it; I said Mariner 10 *failed* to reveal it, because the same face of Mercury was illuminated on both passes. This is the main reason why so much of Mercury's surface was still unmapped as of the first MESSENGER encounter.

I did leave out the fact that radar observations revealed the truth of Mercury's orbit, but what I didn't realize was that they did so *before* Mariner 10 visited. So scientists were already aware of Mercury's rotational period when Mariner 10 visited. I did not know that! Cool! They must've been extra frustrated, then, that Mariner 10's trajectory would not allow them to see Mercury's other side.

EDIT: Correction: the same side of mercury was illuminated on all *three* Mariner 10 passes.