The earth’s rotation is also slowing due to friction caused by the tidal effect of the moon. At some point, that rotation will match the moon’s revolution and the two will be tidally locked. For a whole ton of bonus points, when will that happen and which side of earth will wind up perpetually facing the moon?
The side with Eurasia on it?
If I remember, though, it won’t happen until after the Sun burns out, so we don’t have to worry about a moonless North America any time soon. Come to think of it, the continents will have moved by then anyway.
North America will always have Walmart…
The Moon is receding because the Earth is rotating, not because the Moon was once rotating faster. If the Moon had initially had no rotation (so appeared to rotate once per revolution, as seen from Earth), I’m fairly sure it would gain rotation until it was tidally locked to the Earth.
I agree with this, it’s possible the moon had a slower rotation rate originally and tidal forces sped up the rotation so it would be locked with the Earth.
The only way tidal forces would slow down the orbit of the moon is if the moon was in an orbit that was faster than Earth’s rotation - in other words, if one revolution of the moon was less than one Earth day. This is the case with one of Mars’ moons, Phobos. At some point in the far distant future Phobos’ orbit will decay enough to crash into Mars. Mars’ other moon, Deimos, is in a higher orbit and is pulling away from the planet for the same reason as Earth’s moon.
Most things in the solar system that ARE NOT tidal locked don’t rotate “slowly”…so just from a statistical point of view the odds are the moon did not start off as a very slow rotator. Just off the top of my head though…
True, and I doubt the moon did either - I’m just agreeing that if it did rotate slower then tidal forces would have sped it up. As I mentioned in a previous post, I think the moon was probably tidally locked with the Earth right from the start since it formed in a fairly low orbit out of debris from a giant impact.
Again, Earth and Moon are a system. Tidal forces are internal forces of the system, so they cannot change momentum. If the moon started rotating slower than now, it would vertainly speed up rotation to get tidally locked, but this would mean increasing the system’s momentum. Something would have to loose momentum, either the Earth rotating more slowly (that is true) or the Moon approaching Earth (that is false).
Since the Moon started as part of the Earth, it is certainly getting farther from the planet, so there is a great increasing of momentum there. If all this momentum and the momentum gained by the speeding up of the Moon’s rotation had to be compensated by a slowing of Earth’s rotation, the planet should be rotating very fast at the beginning.
There are three contributions to the angular momentum of the Earth-Moon system; The Earth’s rotation, the Moon’s rotation, and the two bodies orbiting their common center of mass. There’s going to be negligible direct coupling between the Earth’s and the Moon’s rotation. If the Moon had begun with no rotation, it would speed up at the expense of the orbital period, not the Earth’s rotation.
(Of course, the Earth’s slowing of its own rotation would also couple into the orbital period, overcoming the effect of the Moon.)
It seems the intended original question was never answered, nitpicking aside. How long ago did the Moon become tidally locked. The answer is calculated on the following page as a maximum of 16 million years. It assumes the maximum initial spin rate which is probably not unreasonable but also assumes the current distance between Earth and the Moon. That would have been much less and the time depends on the 6th power of the semi-major axis so overall it was probably much less, just a few million perhaps.
Another minor point: the moon’s rotation isn’t linked to ocean tides on earth. The tides here would be the same whether the moon appeared to us to turn on it’s axis or not.
The times of the tides are primarily driven by the Earth’s rotation and the Moon’s orbital period so they would have been much more frequent than the ~12 hour separation at present.
I seem to recall that the Earth has been slowing down due to this tidal locking phenomenon. When the Earth was young, a day was only ~13 hours. That would give us tides every 6.5 hours, presuming no other differences, but the Moon may have revolved faster then, too. Not sure. Anybody know what the tides were like when Earth was young? “Young” being, let’s say, less than a billion years old?
To clarify, the calculated time is 16 million years forward from the formation of the moon, not 16 million years ago. If the moon was formed 4.51 billion years ago, then tidal locking occurred about 4.50 billion years ago, practically as soon as it was formed.
The oldest day lengths actually recorded (via sedimentary deposits that have recognizable variations on a daily and yearly rhythm) is a 21.9 hour day around 620 million years ago. Earlier day lengths are based on computer simulations, with an estimate being a day length of around 6 hours just after the formation of the moon. The simulations also suggest an original orbital distance for the moon of around 25,000 kilometers. That’s around 15 times closer than the moon is today. (See this.) Tidal effects vary by the inverse cube of distance, so tidal effects would have been more than 3000 times higher than they are today.
Interesting claim. I note the Roche limit for the Moon / Earth system is very roughly 6,000 miles from the Earth center or 2000 miles from the Earth surface. Before doing the calcs I’d have guessed it was farther out.
Still, at 25000km ~= 15000 miles, that early Moon was not that far outside the Roche limit. As such the accretion process would have taken a long time.
So when in the last 4.5 billion years did the back side of the moon face the earth? I phrase it that way because I assume the tidal locking would have been asymptotic.
I have read that originally, the moon was about a tenth of the distance it is now and the earth day was only about 5 hours. Since tidal force varies at approximately the cube of the distance, this suggests mile high tides every 2.5 hours. Incredible.
The Roche limit isn’t one single limit, but depends on the density and shape of the orbiting body. But if all bodies are spheres, and the orbiting body is the same density as the parent, it’ll always be proportional to the parent body’s radius, about 1.5 times it.
Tides of what? Water? That’s seems to be controversial.
This article gives the two sides, that water was present from the beginning and that water arrived later from icy asteroids. It says that a recent study found that the water in earth’s oceans more closely resembles asteroidal water.
It doesn’t explain how water already present would have survived the crash and the boiling off that would have entailed. I hope somebody here can do a better job.
The two bodies probably took time to settle down into spheres, but the bulging of solids would have been far more pronounced on the moon than the earth. In fact, that seems to have left permanent bulges on the moon.
Whatever went on for those first few tens of millions of years was no doubt spectacular. But I can’t figure out how water tides would be involved.
Yeah. More probably tides of Earth’s molten-but-slowly-cooling surface.