And the planets generally? Most of the moons are, and I remember reading that most binary stars were also, so it can’t be the relatively small distances between planets and satellites that causes it. Why is the Earth and the planets not so?
You’re mistaken, no moon is tidally locked to the Sun- the planet they orbit exerts far more tidal influence. It hasn’t been that long since there was a tidally locked planet… but Pluto is no longer considered a planet, and it is locked to Charon rather than to the Sun.
The earth would tidally lock to the moon first, as the sun exerts only 46% the tidal force that the moon does. However, this process is slow enough that it won’t be complete before the sun expands to envelop the Earth-Moon system, so even that won’t happen.
Sorry, I missed writing the bit about tidally locked around their planet.
So, could you please explain why
- Most moons are tidally locked around their planet
- Most planets are not so tidally locked around their planet or their sun
- Most binary stars are tidally locked.
I think it is just a matter of time. The Earth will eventually become tidally locked to the Sun (assuming something like the Sun blowing up does not happen first), but as yet we have not lost all the angular momentum that keeps us spinning relative to it.
I think you may be starting off with an incorrect assumption that all of these things have had enough time to be in equilibrium- they haven’t.
The torque that tides exert is generally very small, so tidal locking can be slow even on the age of the solar system timescale. Whether or not a given body is tidally locked depends on how large the tides influencing it are and how fast it was rotating initially. (Other things matter too, like shape and density and whether there are oceans, etc.)
The tide that a nearby planet exerts on a moon can be very large, so often there’s been enough time for them to lock.
The tide that the Sun exerts on planets is relatively small, there’s not been enough time for them to lock (and Earth is the exception anyhow, as the Moon exerts more tidal force on it than does the Sun.)
Mercury is in a 3:2 orbital resonance that’s stableish for now, rotating exactly 3 times every two revolutions in an eccentric orbit around the Sun. Venus rotates retrograde but very slowly, and is slowing measurably.
- Again, it depends on the initial spins and masses and distances, as well as on age of the system in question. In some cases there’s been enough time for them to lock.
So given enough time, all systems will lock?
The Earth would become tidally locked with the Moon first, then, much later, both worlds will become locked to the Sun; at that point the Earth and Moon would start to orbit the Sun separately, as two planets. I say would, rather than will, because even the tidally locking of the Earth to the Moon would happen some billions of years after the Sun has expanded into a red giant and detroyed them both.
The date when these two tidal locking events would occur can’t be predicted exactly, since it depends on the amount of tidal friction on Earth, which in turn depends on the tides in the oceans, their shape and depth, and probably on the movement of the crust as well.
As I understand it, it’s because the Moon is so strongly gravitationally bound to Earth due to being so large and so close that the Sun is working not just against the rotation of the Earth, but the forces of the entire Earth/Moon system. Which is much larger; the Earth likely would be tide-locked like Venus if it was moonless like Venus is.
Among other things, the amount of time for a body to become tidally locked is proportional to the 6th power of the distance between the locked object and the locking object (http://en.wikipedia.org/wiki/Tidal_locking#Timescale). Since the Earth is 400 times further from the Sun than the Moon is from the Earth, that gives a factor of 400^6 (or about 4 million billion) times longer for the Earth to become tidally locked, than for the Moon to become tidally locked (there are other factors, but the one that is to the 6th power tends to win)
And to address the point that binary stars are often tidally locked: First of all, the time for locking also depends on the size of the objects, and second, binary stars can sometimes be much closer than any planet is to the Sun (some of them are even literally in contact with each other).
Interesting… reading around, it looks like there’s a star (Tau Boötis) known to be tidally locked to one of its planets. (Of course, that planet is six times larger than Jupiter and orbits at a distance similar to that between Mercury and Sol.)