Not a very challenging or exciting thread but can someone please explain to me how we only see one side of the moon?
I know it is gravitationally locked but what does this mean?
Also, how can it rotate and still show us just one side, if it is rotating then we should see all sides surely?

I had it explained to me last evening at a viewing session at my local observatory, but having a few other people also listening i was too embarrassed to say, huh? Which is what i was thinking!

Much appreciated,

Steff.

The moon takes around 28 days to orbit the Earth, during which time it rotates once. If you imagine the system from above you will see that if it only rotates once during its orbit, it will always have the same face pointing at the Earth.

You might imagine it using the hands of a clock. If you stick a coin on the tip of the minute hand, as the hand moves around the clock the same part of the coin is always pointing towards the centre of the clock-face. After the minute hand has been around the clock once, the coin will have rotated once.

Here's some more simple thoughts:
One side is heavier than the other. The heavier side will tend to point toward the most gravity which would be the earth.

Similar to how an out of balance suspended bicycle wheel will settle to the heavy side down. Now if you move that wheel around the Earth, it wall always point that way, thus making one full circle (from a non-Earth point of view).

Tie a bucket of water to a rope and spin it around. It's rotating, yet you are only seeing one side of it because you are in the center.

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To put it simply, the Moon's rotation period just happens to be equal to its orbital period around the Earth. This is why we only see one side of the Moon.

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In the case of the moon, the "heavy side" of the moon is not pointed directly towards the Earth. It's a few tens of degrees off, if I recall correctly
It's not just a coincidence though, a lot of moons are locked to their larger planets. The induced tides can slow down a satellite.

I like pictures:

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Actually, I never heard that. Darn media and public schools have always said that to me.
So; the tidal forces were more of a predomonent factor? Although, won't the moon eventually settle with the "heavy side down" (millions of years or more).

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I am so frustrated right now because i cannot seem to grasp this idea. I'm a smart person but i'm still lost! I appreciate you guys for replying.

I can sort of see what you guys are saying, still have difficulty with the same side facing us. Unless it turns with us as we turn an dthat just dosen't make sense to me.

It rotates with the exact same period as its orbit.

Try this: Put a lamp on the floor in the middle of the room, and stand facing it. Now shuffle along in a circle around the lamp, but keep facing it as you are going around it. That is what the moon is doing. You will have rotated once at the same time you went around the lamp once.

Nick

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Nick Theodorakis

Try speedfreek's idea. Seriously. You don't have to have a clock if you don't have one, just use a coin in one hand and move it around your other hand. Let the heads side of the coin face your other hand--notice that as you move it around the back side (tails) faces opposite parts of the room--it's rotating. Or, try Nick's idea.

Short answer: Try it!

Here is a simple diagram.

Attached Images

Moon's orbit.JPG (20.7 KB, 23 views)

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Not necessarily. In free fall it will be stable with either the heavy or the light side toward the Earth. Whichever is lower will accelerate faster, thus tending to align the long axis of the mass distribution with the gravity vector. That is different from the case of an unbalanced wheel on a stationary bearing. In that case the bearing places a nongravitational constraint on the motion, a constraint that is not present with a satellite in free fall.

Clear as mud? I realize this can be hard to visualize from words alone.

It appears that this reputed feature referred to as the "heavy side" does not accurately depict the actual mass distribution.

Can you identify a source for that remark?

Looked for one a couple hours ago.

How about yours?

ETA: From Nine Planets Which is what I've always heard, but that also it's not directly towards the Earth. I'll see if I can dig up something else.

Am i confusing rotation with orbiting? If i (the moon) will have rotated once by the same time i went around the lamp (which is earth), how is there not a time when my back is facing the lamp?!

Myself and my bf are killing each other trying to simulate the moons rotation in our living room

Nick Theodorakis said:Steffanie said:Read again the bit in bold in Nick's quote. Keep facing the lamp as you shuffle around it. What you're doing is facing the same wall as you shuffle around the lamp.

There is a difference between the two actions.

Just to clarify, you complete a rotation on your axis once you've faced all four walls in the room. That will happen to take as long as it takes for you to complete one orbit of the lamp.

Stand against a wall and watch your boyfriend as he moves around the lamp. He'll always be facing the lamp, but you'll see all sides of him as he goes around. He's spun around once from your point of view, so you've seen him go through one full rotation. He's also gone through one full orbit around the lamp, and it took the same amount of time.

Edit: Read what Peter B. said. Make your boyfriend face toward the lamp the whole time he's moving around it.

I can imagine the possibility that irregular, lumpy concentrations of heavy material deep inside the Moon could displace the center of mass diagonally from the geometric center, rather than merely 2km along the geometric long axis of the surface. That could cause the best-fit prolate ellipsoid to be misaligned from the Earth.

Just imagine the possibility that exaggeration of the asphericity gives us something more like a potato than an egg.

I think I'm half way there knowing that the center of gravity is no longer the center of rotation for the wheel.

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Try this. You'll need a room and a friend, or object that can serve as one.

The friend, or friend-like (FLO) object is placed in the middle of the room. You will represent the Moon. To that end, you face the FLO. On the other side of the FLO you will see a wall. This wall is now "Wall 1".

Now take one step to your right and turn as needed to keep facing your friend. Each time you take a step, adjust slightly to keep your facing consistent. Soon, you will be 25% of the way around the room.

If you don't rotate to face the FLO you will crash into a wall.

At the 1/4 mark, You should be facing a new wall. "Wall 2". Wall 1 will be to your right. Keep doing this until you end back in the original position, or near it.

Along your trip around the room, you should have had the FLO centered in your view, while the background walls kept changing. By the end of your trip, you will have seen the FLO in the foreground of every wall in the room. But the FLO will have only seen your face. Never the back of your head.

Now, step into the same place as the FLO, or as close as you can get. Spin one time, slowly, to your left, and you will see the walls pass around you in the same way they did when you "orbited" your friend.

When dealing with the real moon, the background stars are the walls, and the Earth is the Friend. During a full moon, the sun will be behind the Earth as seen from the moon. The view will be the same as on the Earth at mid-day.

During a new moon, the view will be the same from the moon, as it is from the Earth at mid-night. These two times take about two weeks to pass, so there will be a slight change because of the orbit, but from the moon, the sun will rise, move across the sky, and set, just as it does on Earth, It will just take days instead of hours to do it.

Does that help?

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There are two different shapes involved here--one is the actual surface shape, the other is the mass distribution, which is reflected in the shape of the gravity field. On the Earth, the gravity field equipotential surface (the geoid) has a relief of a few hundred meters after the twenty kilometer equatorial bulge is accounted for, whereas the surface obviously has a much greater relief (Mt. Everest is almost nine thousand meters above sea level).

The moon shape is can be represented by spherical harmonics ("Degree 359 shape model of the Moon derived from the USGS Unified Lunar Control Network 2005 "), the coefficients of which are:The degree n (n,0) coefficient is always zero (actually it doesn't exist), except for n=0. The (0,0) coefficient is the average radius, which the table reports as 1737 kilometers (about a half kilometer different from what appears at Planet Scapes). The (1,n) coefficients represent a shift of the center of mass from the geometric center, and can be resolved into a single shift along an axis. The (2,0) coeffiicient represents the equatorial bulge, and the (2,2) coefficients the "pinching" of the equator. The (2,1) coefficients represent how much the degree two shapes deviate from axial symmetry. The (3,0) coefficient is the so-called "pear-shape" coefficient.

I'll see if I can dig up the lunar gravity field data.

ETA: The same website has some coefficients (Spherical harmonic coefficients of the lunar potential field LP150Q (Konopliv et al., 2001))Hmm, these seem to be in a bit different scale. There are no (1,x) coefficients (they're identically zero), because when the center of mass is the center of coordinates, there is no shift.

ETA: This article from 1969 (Science, Kaula) is online, and it shows that the 4902 figure is GM, the product of the gravitational constant and the mass of the moon. The other figures are fairly close to the Michael et al. figures in its Table 2 (the (2,1) coefficients are smaller in the above--the size of the (2,1) coefficients result in what is known in the Earth as the Chandler wobble).

You are definitely doing it wrong!

Instead of killing each other, hold hands, both hands, facing each other, arms extended so that you're pulling each other. Now, have the bf turn around while you go around him in a circle. You will face him continually. But notice that sometimes during the circle you can look out a window, other times you have your back to the window--that means you are rotating.

That one seems to be in every "chick flick". Except the camera is usually in a retrograde orbit.

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Nicely described

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This visualization might help.

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Sometimes there is no substitute for face to face interactions with other people. It's easiest to understand the moon rotation thing if somebody demonstrates it to you. The header says you are in Cork, Ireland. So I suggest you contact the Cork Astronomy Club. This is one of the things that amateur astronomy groups are really good at. You can learn a lot about this, and all manner of topics in astronomy, through personal involvement in an astronomy club or society. So I suggest you take this problem to them and they can show you how it works.

And don't think you should be embarrased about "not getting it". In fact most people "don't get it", and your question is a common question. I took part in a workshop some years ago where we as astronomers taught elementary school teachers about the rotation and phases of the moon. It was not an easy task; we were teaching teachers, not students, and at the end of the day we still had a couple of teachers who still had problems with it. So in fact it really is harder for non astronomers to understand the rotation of the moon than it is for those of us who are more involved in astronomy.

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Also, to build a little onto what Tim said, almost every astronomer I've met LOVES to answer questions and describe how the sky works. These kinds of questions never get old. That's one of the main reasons this message board even exists. Astronomers love to talk about astronomical stuff

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I was just sitting here contemplating the immortal words of Socrates who said, "I drank what?"

"Think of the rivers of blood spilled by all those generals and emperors so that, in glory and triumph, they could become the momentary masters of a fraction of a dot." --Carl Sagan "Pale Blue Dot"

I think the problem is due to relativity! You have to adopt a different point of view in order to appreciate that the Moon is rotating as it orbits the Earth. From the point of view of someone on Earth, the Moon always presents the same side to them.

Imagine you are standing on the moon as it orbits the Earth. You are standing on the side of the Moon that always faces the Earth. As you watch the Earth over a long period of time you will see that the Earth is rotating. In the time it takes the Moon to do one complete orbit, the Earth will have rotated 28 times.

Now move your viewpoint out into space so you can see both the Earth and the Moon. Over that same 28 days you will see the Earth rotate 28 times whilst the Moon makes one orbit. If you watch the Moon as it makes that orbit, you will see that the same side of the Moon always faces the Earth - the Moon will be slowly rotating as it orbits, so as to keep the same side facing the Earth.

If we call the side of the Moon that is visible from Earth the "near side" and the other side the "far side", and move our viewpoint way out past the Moon and then look back, we will be seeing all of the "far side" of the Moon with the Earth behind it. 7 days later the Moon will have made a quarter of an orbit and will be to the left of the Earth and we can now see half the "near side" and half the "far side" of the Moon. Another 7 days and the Moon will have moved round behind the Earth as it has made half of its orbit, so now the "near side" of the Moon is completely facing us (but we cannot see it if the Earth is in the way, of course. If our viewpoint were a little higher we could look over the top of the Earth and see the "near" side of the Moon facing us in the distance). 7 days more and the Moon is now three quarters of the way through its orbit, it is now to the right of the Earth and we can see the other half of the "near" and "far" sides. Once the Moon completes its orbit we can see only the "far" side once more.

From this external viewpoint, all sides of the Moon have been facing towards us during the orbit, so the Moon has rotated once.

Oh dear god i have seen the light! How could i have been so silly! I admit guys it was the link Argos suggested that clinched it for me. I knew i would have to see something visual, thanks Argos Also to everyone else you have the patience of a saint.

In response to Tim Thompson i am a new member to the Cork Astronomy club since December

Well done! i had similar trouble getting it when i first tried to understand how it works!

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