Why do we only see one side of the moon when the other planets rotate, is the moon just a space station?

Answer 1

Because of tide locking, the moon rotates about its axis once per month. Therefore, the same side of the moon is always facing the Earth.

Answer 2

A couple of additional things:

item #1:
The statement that we can see only 41% of the moon's surface at one time because of its spherical shape is wrong. One can easily calculate that we see about 49.8% of the moon's surface at one time. The 41% figure from truly_indian refers instead to the fraction of the moon's area that we never see from earth. In short, we see about 49.8% of the surface at a given time (ignoring the fact that some portions are not illuminated), but we can see 59% over a longer time because of libration.

item #2:
The moon is tidally locked to earth, and the earth's rotation is slowing down and eventually will be tidally locked to the moon. It's easy to say that this happens because of gravity and tidal bulges, but what does that really mean? (I heard this for a long time before I finally figured it out.)

Consider why the earth is slowing down. The moon exerts a tidal force on earth. The usual picture in textbooks is two symmetric bulges -- one on the side facing the moon, and one on the side opposite the moon. Symmetric bulges, however, would not slow down the earth. The key is that the earth is made of materials that don't respond instantly, because they have their own internal forces. The bulge that forms on the side facing the moon takes a while to build and takes a while to dissipate; the upshot is that the greatest bulge is not on the part of the earth exactly facing the moon, but on the part that was facing the moon a little while ago. (I don't know the exact delay, but maybe half an hour or an hour.) The same is true of the bulge on the other side of the earth.

If the bulges were exactly in the earth-moon line, there would be no force that changes the rotational period of the earth; but because they are slightly offset, the effect of the moon's gravity on these bulges causes the earth to slow down very gradually until it eventually becomes tidally locked to the moon. (Because angular momentum is conserved, the moon moves farther away from the earth as the earth's rotation slows down.)

Just as the moon is now slowing down the earth's rotation, the earth long ago slowed down the moon until its rotation period matched its orbital period. Once that happens, the bulges on the moon should become permanently located on the earth-moon line, so there is no more asymmetry to change the moon's rotation period.

-- added later:
I need to make one correction to the long explanation above. As the earth rotates, it has two bulges a little away from the earth-moon axis, and the moon exerts a force on each of these. The force on the near bulge produces a torque that tends to slow down the rotation of the earth, and the force on the far bulge produces a torque that tends to speed up the rotation. Because gravity diminishes with distance, the force on the near bulge is slightly greater; so the net effect is to slow down the rotation.

Answer 3

Actually, early in Earth's history, the moon DID rotate. (probably).

The moon's center of gravity is *not* at it's center - it's about 150-200 miles out from the center, closer to Earth. As the moon rotated, this off-axis center made the moon act like a dumbbell in space- two weights out from the center. Everytime the heavy end came by Earth, Earth's gravity slowed it's rotation, just a little, until the next time around. After a few hundred million years, the moon finally stopped turning altogether - and only faced one side towards us.

Answer 4

I'm not sure what you are saying by the moon being "just a space station."

We hope to turn it into a space station someday, because it will be a lot easier to launch craft from the moon than from the earth. Of course, there's not much up there right now that you wouldn't have to bring from the earth anyway, so this will be a tough project to get off the ground, so to speak.

Answer 5

The moon does rotate; it rotates once per revolution around the earth. If it didn't rotate we would see both sides of the moon as it revolved around us. The moon causes tides on the earth but the earth also causes tides on the moon. These tides have caused the moon's rotation to become 'locked' in relation to its orbital period so the same side of the moon always faces us.

Answer 6

The moon is a space station - you are correct. But not 'just' a space station as you put it. The moon is where all sorts of people like Bush and Blair come from.

Answer 7

The moon rotates on its axis at 10.3 mph and moves around Earth at 2,286 mph. Earth rotates on its axis at 1,000 mph. These three velocities combine to cause only one side of the moon to face Earth.

Answer 8

Relative to the Earth, the Moon makes one rotation every 29.5 days. That happens to also be the time it takes for the Moon to complete one revolution around the Earth. This might seem like a coincidence, but it's not.

In the past, the Moon used to rotate much faster than it does now. But over millions of years, the effect of the Earth's gravity has slowed down the Moon's rotation until it became gravitationally locked to the Earth. This is why we always see the same side of the Moon.

It would seem logical to say that at any one time we can see 50% of the Moon's face. If the Moon were flat, that would be correct, however we know the Moon is a sphere. And the spherical shape of the Moon hides the area close to the perimeter and we can, at any one time, see only 41% of the Moon's face.

Even though the same side of the Moon's faces us, we do see a bit more than half of the Moon's face. Over time, because of librations, we can see up to 59% of the Moon's surface.

Librations are irregular motions of the Moon in its elliptical orbit around the Earth. They are measured using longitudinal and latitudinal coordinates. Both are computed from a central point that is at a fixed geographical location on the lunar surface. This point is in the Sinus Medii, a small plain just below and to the right of he large rayed crater Copernicus. Two meridians emanate from this point: the Central Meridian runs north and south, and the Lunar Equator runs east and west.

Different librations affect different sides of the Moon and each contribute added surface area. The added viewable surface area changes from day to day and month to month. Some librations overlap, but the maximum amount of lunar surface that we can see over time is 59%.

The longitudinal libration is produced by the elliptical orbit of the Moon. Because of the nature of the elliptical orbit, the speed of the Moon changes depending on which part of the orbit it is in. When moving from its fastest point (closest to Earth) to its slowest point (farthest from Earth,) the Moon's speed is slowing down. But, because the Moon's rotational speed stays the same, for a period of time the Moon's face is not pointed directly at us, and this "lag" effect allows observers to see an extra bit of the lunar surface. In effect, we are "peeking" around the edge of the Moon! When the Moon is one quarter of the way around the Earth, it is 97 degrees through its rotation.

This libration is called longitudinal because the extra surface areas exposed are along the lines of longitude (perpendicular to the equator.) The total extra surface area we can see from longitudinal librations is about eight degrees.

There is also a libration that is latitudinal. The plane of the Moon's orbit is titled 5 degrees to the ecliptic. For half the lunar cycle, the Moon is above the ecliptic and for the other half it is below the ecliptic. Each of these half cycles expose an extra bit of the Moon. The extra surface area shown during these librations is at the top of the northern hemisphere or the bottom of the southern hemisphere. The total extra surface area we can see from latitudinal librations is almost 7 degrees.

There is one more librations called Diurnal Librations. This librations occurs every day. Observers can "see" over the top of the Moon as it is rising. Likewise they can see under the bottom when it is setting. Diurnal librations are caused because the radius of the Earth adds an extra 4000 miles of height for looking "over" or "under" the Moon when it is on the horizon. This daily libration gives us an extra one percent of lunar surface area for viewing.

Answer 9

The long winded answer by truly_indian is correct, as is the much shorter answer by jgoulden. And some moons of other planets, such as Saturn's moon Iapetus, are tidally locked to their planets too; it isn't only our Moon that does this.

Answer 10

*sigh* Yes, that's right, it's actually a Soviet Space Station.

OR the moon is rotating on it's axis at the same rate that it orbits the earth hence the we always see the same side.