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After reading the Q&A Is the moon moving further away from Earth and closer to the Sun? Why? about the tides transferring energy to the Moon and pushing it from Earth, I have a question:

How is that energy actually being transferred to the Moon? The creation of tides requires energy, so I would expect that this should take energy from the Moon, slow it and cause it to eventually fall back to the Earth. Why isn't that happening?

Finally, if this is the general mechanism, would other moons that orbit around planets with a liquid surface and causing tides, be receding from their parent planets?

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1 Answer 1

You have correctly identified that the tidal forces are transferring energy from the Earth to the Moon. This energy causes the Moon's orbit to get larger thus slowing it down.

It's a bit counter intuitive, but if you think about it the Earth spins a rate of 1 spin per day The Moon is orbiting the Earth with a period of approximately 27.3 days. If it were to speed up it's orbit would actually decrease bringing it closer to the Earth.

To answer your final point all other moons cause tides on their parent planets and are moving away from them, but the effects are much smaller due to the larger difference in sizes. The Earth/Moon system is unique in the Solar System as the ratio of the sizes of the bodies are relatively close to each other.

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The other planet/moon systems do also have this property though. The moons slowly spin outwards. –  Rory Alsop Sep 30 '13 at 19:10
I still don't get how speeding up the Moon would decrease its orbit, from what I remember, the faster the body moves, the more distant is the orbit... –  РСТȢѸФХѾЦЧШЩЪЫЬѢѤЮѦѪѨѬѠѺѮѰѲѴ Sep 30 '13 at 19:30
@ŁukaszLech That's a misconception, the square of the orbital period is equal to the cube of its average distance (Kepler's 3rd law), but as the size of the orbit only scales linearly with distance, the velocity scales as r^(-1/2), i.e. decreasing with distance. –  Guillochon Sep 30 '13 at 19:43
Examples: At 150 miles up, orbital speed is 17000mph. At 22000 miles up it is only 7000mph. –  Rory Alsop Sep 30 '13 at 19:52
Interesting. But when the body is slowing, the centrifugal force reduces, therefore is no longer able to match gravity, and the body falls? This is what happens to low orbit satelites? –  РСТȢѸФХѾЦЧШЩЪЫЬѢѤЮѦѪѨѬѠѺѮѰѲѴ Sep 30 '13 at 20:55

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