Why is the Moon receding from the Earth due to tides? Is this typical for other moons?

  • 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?

    Note the recession only happens when the primary is spinning faster than the satellite. When the satellite is moving faster than the primary (like Phobos and Mars), then it spirals in, not out.

    @РСТȢѸФХѾЦЧШЩЪЫЬ: Um, "closer to the Sun"? It also gets further from the Sun, half the time...

  • It's pretty simple, actually.

    tidal bulging

    The Moon creates tides. Due to tides, the water bulges out towards the Moon (and also on the opposite side).

    But the Earth also rotates pretty fast (once a day), faster than the Moon orbits the Earth (once a month). There's friction between the rotating Earth, and the watery bulge created by tides. The rotation of the Earth "wants" to rotate the bulge faster.

    In effect, the rotation of the Earth drags the tidal bulge forward - the bulge is always a bit ahead of the Moon. When the Moon is at meridian, the tide is already decreasing.

    So there's a bit of extra watery mass on Earth, a little bit ahead of the Moon. This watery bulge interacts gravitationally with the Moon.

    This has two effects:

    • it slows down the rotation of the Earth, gradually sucking energy from it (the Moon pulls the bulge, and therefore the Earth, "back")

    • that energy is dumped into the Moon's orbital motion, effectively "pulling" it forward

    When you dump energy of motion into an orbiting body, it settles into a higher orbit - higher orbit means more energy. Therefore, the transfer of energy from Earth' spin to Moon's orbit gradually makes Moon's orbit larger and larger.

    This only happens because the Earth is spinning faster than the Moon orbits it. If the Earth was tidally locked to the Moon (spinning exactly as fast as the Moon orbits it), then no transfer would happen. If the Earth was spinning slower than the Moon's orbit, then the transfer would be opposite (from Moon's orbital motion to Earth's spin).

    Note: Counterintuitively, a satellite with more energy actually moves slower, but in a higher orbit. The extra energy goes into raising the orbit, not into making its speed faster. Why this happens exactly is a whole 'nother discussion.

    Is there a tidal wave in the Earth's molten mantle too?

    The whole Earth, including the "solid" crust and its soft interior, experiences a tide due to the Moon; on a planetary scale there are no true solids. It's called Earth tide. The amplitude is on the order of dozens of centimeters. http://web.ics.purdue.edu/~ecalais/teaching/eas450/Gravity3.pdf

    @user104372 Energy does not just exist in the form of kinetic energy. In this case, the total energy (kinetic plus potential) of a wider orbit *is* larger. This is *really* basic physics that you are arguing about.

    Re *It's pretty simple, actually.* It's not that simple. The true picture is much more complex than is this simple picture. The tidal bulge as depicted in the image does not exist. If it did exist, high tide would occur shortly after lunar culmination (and then 12 hours and 25 minutes after that). This is very rarely observed. In fact, that tidal bulge cannot exist. To get the correct picture, one would have to integrate the effects of the oceans on the Moon over a long period of time (preferably 18 years or more). Our models aren't there yet.

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Content dated before 7/24/2021 11:53 AM