Why is the moon still up there?

Question

Or: why are orbits stable?

For every orbital height, there's a linear speed you need to move at to keep yourself in that orbit. Any faster, and you fly off into space. Any slower, and you crash.

Question is: how can this be stable? Surely going even a tiny bit too fast or too slow will result in flying or falling out of your orbit? So any slight perturbation (crashing into a bit of space junk, whatever) should knock you out of orbit.

Furthermore, the moon, and all other satellites, have been losing energy for as long as they've been up there, by dragging the oceans over the surface of the earth in the form of tides. So they must be slowing down, and ought to crash.

Same applies to the earth around the sun, the sun around the galactic centre, etc.

Why is this process stable?

Answer 1

First of all, there's a slight misunderstanding in your first paragraph. Varying the speed of the orbit will *not* cause it to fly off into space or crash. It *will* make the orbit elliptical instead of circular.

Let's say that "X" is the speed that one object needs to orbit a larger body. If the object is moving at slightly less than speed X, it starts to lose altitude. As it gets closer to the larger body, it picks up speed until it is moving faster than X, so it starts to gain altitude. The result is an orbit shaped like an ellipse instead of a circle.

The Moon's average distance from the Earth is about 248,000 miles. The actual distance varies a few thousand miles above or below the average as the Moon orbits the Earth.

Answer 2

It is stable because a small change as you mention will cause a reaction to oppose this change, in this case, not reinforce the change. In this case if you push the orbiting moon out a little it will be going faster than the average orbit speed at that location, so it will go a little further out. But it is moving against gravity and will slow down to the same as the new orbit speed. That is a slight simplification. But it is stable.
The pull of the oceans is very small, but will affect the orbit over millions of years.

Answer 3

This is the idea of Isaac Newton on the Law of Universal gravitation. "Newton realized that if the moon did not fall, it would move off in a straight line and leave its orbit. His idea was that the moon must be falling around the earth. Thus the moon falls in the sense that it falls beneath the straight line it would follow if no force acted on it. He hypothesized that the moon was simply a projectile circling Earth under the attraction of gravity. It has a component of velocity parallel to Earth's Surface. This sideways or tangential velocity is sufficient to ensure nearly circular motion around earth rather than into it. If there is no resistance to decelerate, the moon will continue "falling" around Earth indefinitely. "

Answer 4

Moon is going away from Earth at a very slow rate, I think it is something as 1 cm every thousand years or so, due to the tide effect.
Te consecuence of small differences in kinetic energy are small differences in the orbit. No problem of chaotic consecuences.

Answer 5

the moon is actually moving away from the earth as well as slowing down the earths spin(we will have a 36 hour day in about 3 billion years) so the process is far from stable on a cosmic scale

Answer 6

Like you said, the faster you go, the higher the orbit. The orbit will always settle down into the one that matches the speed of the satellite.

Answer 7

It's not in the long run. The earth is moving closer to the sun and the moon is moving further away from the earth every day.

Answer 8

Magnetational Pull in Relative Motion in -0- inches of Vacuum.

Answer 9

This is a question worth thumb up.
i may not now much but i don't think the rotational motion of the moon is affected by it's energy rather what keeps it in motion is the original angular momentum that was there in the solar nebula from which it is formed.

Answer 10

Gravity