ntt@dciem.UUCP (Mark Brader) (06/14/84)
The explanation of tidal drag stopping the moon's rotation, posted by several people, is correct. One of them also said that "the size of the orbit decays". No. Just the opposite. Presuming that at one time the moon did rotate relative to the earth-moon line, when the tidal drag stopped this rotation, the energy lost could be converted into heat, but the angular momentum could not be. The only place for it to go was into the earth-moon system as a whole. The result is that the size of the moon's orbit INCREASES (though it moves more slowly). Furthermore, since the EARTH still rotates with respect to the earth-moon line, there are tides on the earth; and the friction associated with these tides is gradually stopping THAT rotation, and further increasing the size of the moon's orbit. I believe I remember reading that its radius will have increased from the present 238,000 miles to 340,000 by the time the earth is also rotating once per month, that is, not rotating at all with respect to the earth-moon line. (And there is fossil evidence that years in the past had more days in them, which confirms that days are getting longer. So also does historical evidence of the PLACES that had eclipses on certain dates in the past.) After THAT, the tidal drag of the SUN on the earth-moon system will dominate, I remember reading that the moon will come closer again, but I don't understand why. Mark Brader