Why is it possible to see the same spot on the moon every night?

Answer 1

Because it is "tidally-locked" to the Earth. The Earth's gravity slows down the moon's rotation, so that it rotates at the same speed as it revolves around the Earth. Imagine a person who walks around a center pole, but shifts their body so they are always facing it as they walk.

Just as the Moon's gravity exerts a force on the oceans of the Earth, the Earth's gravity exerts a force on the Moon. For the Earth's oceans, the tides make the water shift around, rising and falling.

The Moon is solid, without any liquid that could flex and deform under the gravity of the Earth. So for the Moon, the effect was to slow down its rotation, and eventually made it equal to the orbital period. The same effect, at a much smaller scale, is affecting the Earth, and the Earth's rotation is slowing down. In a few million years, the day will be 25 hours long.

Short :

(1) initially, when the earth-moon system was young, the force of gravity from the earth produced bulges on the moon (the gravity force in this case is called a "tidal force") that MOVED across the moon's surface like a wave (in a VERY exaggerated view, the moon looked like a football with one end pointing to the earth and the other end pointing exactly opposite--in actuality, the tidal bulges are not large and not as pointy as on a football)

(2) the bulge-wave though didn't move at the same rate as the line connecting the earth and the moon moved on the moon's surface (in other words, the football can't initially always have one of its two pointy ends pointed at the earth)

(3) over TIME, though, the bulge-wave slows down its progress across the moon's surface as the moon's football shape "learns" to lock itself into step with the earth so the football's end (one of the two ends that is--doesn't matter which) EVENTUALLY will always point at the earth and the 2 bulges STOP moving across the moon's surface (there is a torque on the football caused by the earth's gravity that makes it WANT to point to the earth)

(Note that the mass imbalance between the near side and the far side of the moon is also thought to contribute to the tidal locking but NOT entirely. The fact is that the near side now IS heavier than the far side because of a greater frequency of volcanic eruptions on the near side. Those eruptions throw (heavy) basalt onto the moon's surface and create the lunar "seas" on the near side (the huge areas on the moon that look flat from earth). The far side did not have as much volcano activity (it has fewer plains or "seas") and so not as much mass at its surface. Since the near side is heavier, it is attracted more strongly to the earth than the far side.)

This is because the moon is in synchronous rotation with the Earth. It takes exactly the same time to orbit us as it does to spin on its axis once. See the Web Link to the left.

Interesting side note: Because the moon's orbital velocity is not constant due to the eliptical shape of the orbit, while its rotational velocity is constant, an interesting thing called "libration" occurs. Observed carefully over the course of a month or two, the moon will appear to wobble to the left and right. This is east-west libration. There is also a north-south libration related to the inclination of the orbit to our equatorial plane. Combining the effects of these librations over a cycle of about 15 years, a few per cent more that 50% of the moon's surface is actually observable from earth.

Because the moon is tidally locked to the earth, it does in fact rotate on its axis but at the same rate as it orbits the earth (If you think about it it would have to be doing this in order to keep the same face pointing at the earth).

When the moon was first formed it rotated faster than it does now but because it is smaller than the earth the tidal forces cause by the earth gradually slowed it down. Of cause this works the other way too, the moon is slowing the earth and as it does so is receding from the earth. The eventual end state will be for one face of the earth to always face the one face of the moon as it orbits the earth. This end state will not happen for billions of years into the future.

In one theory of the creation of the moon, there was a planet impact (likely Earth and another planet) millions of years ago and the debris kicked up from the immensity of the impact gathered around in the orbit of Earth until it balled to form the moon. It is thought that the reason the moon continually faces Earth is that while the molten rock was cooling, the internal core of the moon was pulled toward the gravitational force of the Earth and cooled in that way, thus holding the face of the moon in our direction via the immense pull of Earth's gravity.

Much like a buoy in the water floats with the heaviest part down, so does the moon against the Earth's gravity with the heaviest spot facing toward us.

Answer 2

The Moon rotates on its axis at the same rate it orbits the Earth, creating a synchronous rotation that keeps the same side facing Earth at all times. This means the same spot on the Moon is visible from Earth each night.

Answer 3

Because the Moon is "tidally locked". Tidal forces induce a drag on satellites, which tends to eventually either slow or speed their rotation so that it matches their period of revolution. Most moons in the Solar System are either known to be tidally locked to their primaries or suspected to be so.

Answer 4

Because the Moon rotates on its axis with the same period as it revolves around the Earth. (Mercury does the same with respect to the sun.) This was caused by tidal forces on the Moon from the Earth.

Answer 5

The rotation of the Moon must have gotten slower and slower, due to tidal forces, until it showed us the same side all the time. Many other moons in the Solar System always show their planets the same side; Earth's rotation is also getting slower and slower, until one day in the future we will show the Moon the same side all the time.

Answer 6

All of the stars move; some faster than others. But they are all SO FAR AWAY, that even over the course of a lifetime (or several lifetimes!) nobody could notice the differences. In a thousand years, the constellations in the sky will look a little different; in 10,000 years, they'll look a LOT different.

Answer 7

You only see one face of the moon because it rotates at a speed where the orbit along with rotation keep one side facing Earth. However, there is day and night on both sides of the moon depending on where it is relative to the Sun and Earth.

Answer 8

Because of the proximity of the moon to the earth and the effect of the earth's gravitation on the moon, the moon has experienced tidal locking and is now in what is called synchronous orbit about the earth. We always see the same side of the moon because it makes one rotation about its axis for every orbit it makes about the earth. Exactly. The curious person would follow the related links below to get more of the particulars.

Answer 9

Pretty much. People who live in rural areas where there are few city lights see far MORE stars than we townfolk do, but the ones that we see are the same ones that a country person does.

Want to see something REALLY impressive? Go wilderness camping. Lay out on a mountaintop at midnight and see the stars as they truly are.

Answer 10

The way the Earth rotates on its axis. It will make you see the same stars until the Axis has changed a significant distance that you will see different stars. That is why you can see Orion more in the fall then in the summer.