This thought hit me the other day. I tried to duck, but it got in my brain, anyhow! Ok, maybe someone can explain:
It is said that our moon, and many others in the solar system, have been observed to keep the same side rotated towards their parent planet. In other words, their rate of rotation equals that of one revolution. It is said this is no coincidence, but due to laws of conservation of momentum.
Yet, why can’t the rate of rotation drop below a 1:1 relationship with period of revolution? For example, a solar day on Venus is longer than a solar year on Venus! So, why don’t we find moons where a “day” would be longer than a revolution?
I don’t see why you couldn’t have that. I think it’s unlikely but possible. It would seem that eventually a small body orbiting a large body will sync to 1 rotation/ revolution if it started with greater then or less then 1 r/r.
Just it’s unlikely that a body will form with such a low rotational speed (IMHO).
You know what the moon does to our oceans? The gravity gradient of the moon does this, all that sloshing back and forth, its called the tides. They are more complex than they first appear. They are caused by the fact that the earth feels more of a tug from the moon on the side (of the earth) that is closest than the side that is farthest. That gradient tries to stretch the earth like a rubber ball. The oceans (being liquid) stretches more than the earth, thus the rise and fall of the tides.
You are probably wondering how I am going to tie it in to the rotational speed of the moon. Its comin’.
If the moon does that to the earth, imagine what the earth does to the moon. It litterally stretches the moon to be slightly egg with one of the ends pointing at the earth. When the moon spun at a rate other than its orbital period, that constant stretching into different shapes actually heat up the moon.
From the conservation of energy we deduce that the energy to heat up the moon had to come from somewhere. The only place it could come from was from the energy of the moon’s spin, thereby taking energy from the spinning motion and reducing its speed of rotation. This puts the brakes on to the moon’s speed of rotation until it is spinning at exactly the right speed to keep one side always facing the moon.
Because it equally valid to consider the deceleration of the moon’s rotational speed as an acceleration in the other direction, we can deduce as well that if a moon was spinning slower than one revolution per orbit, the same effect was bring it rational speed up to the matching rate as well.
This reminds me of a question I have had for some time. The moon was once only a tenth as far away from the earth as it is now. In the process of moving from, say, 23,000 miles to 230,000 miles out, it has obviously gained quite a bit of gravitational energy. Imagine how much fuel it takes to move a spaceship from 23,000 miles (hey, that is nearly geosynchronous) to the moon’s orbit. That energy can come only from the earth. But what is the mechanism by which that energy is transferred?
To the OP: The Earth’s rotational speed is slowing down, too, from the same mechanism that produced the Moon’s synchronous behavior. Eventually, there will be just one side of the Earth that faces the Moon.
To Hari Seldon: The tidal bulge that the Moon produces in the Earth and in its oceans is not lined up directly underneath the sub-lunar point. (I’m ignoring for the moment Solar influences.) Friction between the Earth and the tidal bulge “drags” it in the direction of the Earth’s rotation. With the tidal bulge out ahead of the sub-lunar point there is a component of gravitational force tending to pull the Moon forward in its orbit. Increasing the energy of an orbiting object increases its orbital radius.
This argument makes sense to me, but raises a couple questions. First, the same logic would apply to planets orbiting the sun and moons orbiting other planets. It would explain why one side of Mercury essentially always faces the sun, especially given that Mercury is so close to the sun and so small, so the effects you describe would be correspondingly greater.
But what about the rest of the moons of the solar system? I was under the impression that the earth’s moon was unique in terms of having only one side towards its primary. Don’t the moons of Jupiter and Saturn rotate out of synchronization with their primaries?
I believe that Velikovsky or some of his friends put forth such a theory, but as far as I know it is not accepted by ‘conventional’ (i.e non-crackpot) astronomers.
As far as I remember (and I might well be wrong) the moon is actually getting closer and closer, as its influence on earthly tides slows it down ever so slightly.
Since the earth is spinning faster than the orbital period of the moon, it will absolutely do two things.
Slow the earth’s rate of rotation.
Add energy to moon’s orbit.
Adding energy to moon’s orbit will put it further away.
As to how much the distance has changed over what period of time, we need to calculate just exactly how much energy is being transferred per unit of time.
I understand the mechanism. But, I am not quite up to the calculations required to quantify it. If you can make those calculations, you would have the historic orbits of the moon.
I would expect there would be some quantities that needed that would need to be more or less guessed at along the way. (for example, the amount of mechanical friction of a planet body in stretching its shape slightly?) That will tell you how much heat your are making. If you don’t have a good way to measure or calculate that, it would be pretty tough to put an exact figure on any of it.
Being off by even a factor of two would probably create a much larger than factor of two amount of error in the calculated rate of orbit change.
Thank you scotth! I had to think for a while, but of course you’re right!
I went agoogling, and there seems to be a consensus that the distance to the moon (currently) increases by 3.8cm/year. (as measured with laser ranging technology!)
This has been seen by creationists as a ‘proof’ of creationism :rolleyes:
There also seems to be people believing that the proper solution to the three-body problem will ensure that the moon will end up in the sun, even disregarding tidal forces. (here, nut I don’t quite understand all of their premises.)
That link was just the first one I found. The great Bad Astronomer has an excellent page on Mercury, that deals with tides and rotational periods, too.
As toadspittle said, Mercury is in a 3:2 rotational resonance. This is because Mercury’s orbit is quite elliptical, and when it’s in the part of its orbit closest to the Sun (perihelion), it moves considerably faster than when it’s farther away. Since its rotation rate has to stay pretty much constant, it can’t rotate faster when it’s closer to the Sun to keep one face toward the Sun, so it kind of compromises, and keeps its long axis (it’s almost a sphere, but not quite) pointed at the Sun when it’s at perihelion, when the Sun’s gravity is strongest–but the next time it comes around to perihelion, it will have the opposite side pointed toward the Sun, though this means that its long axis is aligned with the Sun again.
Actually the only moon in the Solar System known not to be in synchronous rotation is Hyperion, a moon of Saturn, which tumbles chaotically due to perturbations from other saturnian moons. There may be others among the smaller moons, but their rotation rates have not all been measured.
Tidal locking of moons is a very efficient process, taking less than a million years, so, with just the one known exception, they’re all in synchronous rotation.
Pluto and Charon are both tidally locked, making Pluto the only planet (if you consider it a planet ) in the Solar System in synchronous rotation with its moon.
Too bad they don’t show their work. It is my guess that they aren’t accounting for the fact that at closer orbits, the orbital period is much closer to a geosynch rate. In a geosychronous orbit the tides would not move and no energy would be transferred. That implies that if the moon is going away from us now, it must have ALWAYS been at least a geosynchronous orbit distance away.
Oh wait, they do show their work at the bottom and they don’t take the difference between orbital period of the moon and the rotation period of the earth into account.
The formulas they show should account for the tidal coupling efficiency as a function of distance, but it simply does not account for the amount of energy that would be available.
In close the coupling is better, but there is less energy difference to couple.
Of course, they don’t expect anyone buying their story to want to or have the ability to check their figures.
Guess they figure if they can provide faith by authority, it is equally valid to provide science by authority.
Another interesting thing to note concerning these equations.
If the moon had ever been in a lower than geosynch orbit, the tidal coupling would have added energy to earth’s rotation speed and removed it from the moon’s orbit. Thereby pulling the moon into the earth.
The author, Don deYoung, teaches at Grace College, a mid-western religious college and seminary. They don’t have a physics department, so I’m not sure what the reference to his being a professor of physics there is about. They do have all of two(!) “science” courses listed in their Fall schedule, one of which is taught by Mr deYoung.
) in the Solar System in synchronous rotation with its moon.