Does the moon rotates? Yes. The rotation matches exactly the orbit of the Earth. Which means in 28 days the moon makes one rotation. Shouldn't this be also happening with the Earth rotation around the orbit of the sun?



1 Answer 1


It is called tidal locking.

A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner. This causes one hemisphere constantly to face the partner body. Usually, at any given time only the satellite is tidally locked around the larger body, but if the difference in mass between the two bodies and their physical separation is small, each may be tidally locked to the other, as is the case between Pluto and Charon. This effect is employed to stabilize some artificial satellites.

The way this works is described in detail, too long for this page.

A hand waved summary the smaller orbiting satellite loses self angular momentum and gains rotational around earth angular momentum going to higher orbits until it locks.

Locking of the large body is also observed:

The tidal locking effect is also experienced by the larger body A, but at a slower rate because B's gravitational effect is weaker due to B's smaller size. For example, Earth's rotation is gradually slowing down because of the Moon, by an amount that becomes noticeable over geological time in some fossils.1 For bodies of similar size the effect may be of comparable size for both, and both may become tidally locked to each other. The dwarf planet Pluto and its satellite Charon are good examples of this—Charon is only visible from one hemisphere of Pluto and vice versa.

The mass of the sun is so much larger than the earth's and the distance so large, that tides generated by the earth on the sun are very small and ineffective up to now to produce locking, fortunately for us.

  • $\begingroup$ but if the gravitational force from the sun is so weak that can't lock us then it shouldn't be able to hold us in it's own orbit. According to Newton's rotation law we should be shoot out of the sun orbit? $\endgroup$ May 12, 2014 at 14:16
  • $\begingroup$ I suppose that tides are dissipative: thermal energy must have been generated in the Moon due to tides caused by Earth. Once locking was achieved, the dissipation stopped. Does this dissipation play a significant role in tidal locking? $\endgroup$
    – garyp
    May 12, 2014 at 14:32
  • $\begingroup$ @garyp Dissipation is a part of the transfer of momentum to the earth. The dissipation happens even after locking, just the two bodies are in a sort of lowest state, the angular momentum exchanged such that the lock remains. The moon could have had earth tides when it was liquid or had a liquid core on which the crust could move , as the earth's does with the tides ( called earth tides to distinguish from sea tides). $\endgroup$
    – anna v
    May 12, 2014 at 14:55
  • $\begingroup$ In this spaceacademy.net.au/library/notes/solastrl.htm there exists a table with relative effects of planets on the sun. It is the changes in the gravitational force that will create the tides ( they are really second order deformations of pliable mass) and these are very small on the surface of the sun, if I remember correctly even Jupiter is of the order of milimeters. $\endgroup$
    – anna v
    May 12, 2014 at 15:10
  • $\begingroup$ this is weird: Venusian solar days, making approximately the same face visible from Earth at each close approach. It's funny, looks like it's locked to the Earth, lol $\endgroup$ May 12, 2014 at 15:40

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