Retrograde Moon?


Almost all the planets and satellites in the solar system rotate in the same direction as they revolve. However, there are some notable exceptions. For example, radar measurements made in 1961 showed that the planet Venus not only rotates very slowly (one rotation taking 243 days, so the "day" on Venus is actually longer than the "year"), but that the direction of rotation is retrograde, i.e., it rotates in the opposite direction to its orbital revolution. No one really knows why Venus has such an unusual rotation. Pluto is also believed to have retrograde rotation.


Of course, our own Moon (presently) rotates in the same direction as it revolves, since it is gravitationally locked into a 1:1 synchronization because the "tides" in the formerly molten rock have cooled and solidified, and the resulting ellipsoidal shape is permanently oriented with its major axis pointing toward the earth. This type of gravitational synchronization is fairly common in the solar system, almost always with a ratio of 1:1. The exception is Mercury, whose rotation is locked in a 3:2 coupling.


However, it's believed that in former times the Moon rotated more rapidly and was closer to the Earth. Over millions of years the Moon's rotation was slowed due to the tidal drag (with energy being dissipated in friction with the constant reshaping of the Moon), and to conserve angular momentum the radius of the Moon's orbit has increased. So at some time in the past the Moon had enough rotational momentum to spin it completely around relative to the Earth. For a while it may have been locked in various other rotation-to-revolution proportions like 3:2. But then one day it just couldn't quite make it all the way around, so it's rotation (relative to the rotating frame of its orbit around the Earth) actually stopped and reversed itself, and it swung back the other way. Then it couldn't quite get around that way either, so it "rang like a bell" for awhile until the oscillations finally dissipated (although they aren't completely gone, since the Moon still "librates" a little.)


This raises some interesting and perhaps unanswerable questions. The first time the Moon was unable to complete it's rotation relative to the Earth, did it then also fail to complete a rotation in the opposite direction? One might think so, since the potential "hill" it had to climb would be the same height in both directions, but with variations in the Earth-Moon distance and non-ideal shapes, etc., it isn't obvious that the Moon couldn't have successfully completed a rotation in the retrograde direction (again, relative to the frame of its orbit around the Earth).


This raises an even more fundamental question: In which direction was the Moon originally spinning? We usually presume it was the "forward" direction, but does the motion we observe today necessarily rule out the possibility that the Moon originally had retrograde spin? If it's original spin had been retrograde, tidal forces would have slowed its spin (relative to the earth) just the same as it would if it had forward spin, and when it finally couldn't quite complete a retrograde rotation relative to the Earth, it would have gotten locked into a 1:1 orbit that might be indistinguishable from what we observe today.


The only thing that might rule out the retrograde possibility is the idea that the Moon was originally closer to the Earth, and it's been moving farther away to conserve angular momentum. If the Moon originally had retrograde spin then its slowing would tend to shrink the Moon's orbit and bring it closer to the Earth. Of course, we know the Moon is slowly receding today, but thatís due to the slowing of the Earth's rotation. The Moon's spin is already 1:1 so it no longer has any secular changes in spin rate (aside from that due to the slowing of the Earth's spin).


Assuming a retrograde Moon, it could still have been the case that the Moon has been always receding, because the slowing of the Earth's rotation might be the dominant factor. On the other hand, even if the Moon's spin is the biggest factor, I'm not sure what evidence we have (aside from physical inferences based on the assumption of forward Moon spin) to tell us that the Moon's mean distance has changed monotonically. It's conceivable that the Moon was originally in a more distant orbit with retrograde spin, then approached as the Moon's spin slowed, and now is receding based on the slowing of the Earth's spin.


The answer to these questions may depend on the still unresolved issue of the Moon's origin, i.e., was it co-formed along with the Earth or was it formed elsewhere and then captured. In any case, the example of Venus shows that retrograde spins are not impossible. It's an interesting physics question to decide whether it is possible, even in principle, for us to infer the Moon's original direction of spin from it's present pattern of motion.


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