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> In lugnet.space, Jason J. Railton writes:
>
> Actually, I seem to remember that the moon's pull on the tides is mutual
> (the moon is affected by the gravity of water on the Earth), and because
> tidal waters drag across the surface (thus slowed by friction), this is
> gradually decelerating the moon's orbit. So, it's orbit is very slowly
> shrinking...
Actually, you have this backwards. The friction of Earth's oceans against
its solid parts is slowing the Earth's rotation down. This translates into
a loss of angular momentum for the Earth. But angular momentum must be
conserved. The angular momentum is transferred to the moon, so the moon is
actually gradually moving *farther* from the Earth. IIRC the increasing
separation of the Earth and Moon has been measured quite accurately by
bouncing lasers off of the mirrors left behind by the Apollo missions.
When the Earth has slowed enough so that its period of rotation equals the
period of the moon's revolution, there will be no more tidal friction. The
Earth will cease to slow, and the moon will cease to move farther away.
Interestingly, at this point one side of the Earth will always point towards
the moon -- just as, right now, one side of the Moon is always pointed
towards the Earth. This state of affairs is known as "tidal locking." When
this finally happens, one Earth day will be somewhat longer than 28 current
Earth days.
This tidal locking will take a pretty long time. In fact, some recent
studies suggest that increasing solar radiation will cause Earth's oceans to
evaporate in the next 500 million to 1 billion years, sooner than tidal lock
is expected to be achieved. Tidal lock can also occur with an ostensibly
solid body (e.g., Jupiter's moons), but it's a slower process.
--
John J. Ladasky Jr., Ph.D.
Department of Biology
Johns Hopkins University
Baltimore MD 21218
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Message has 1 Reply:
Message is in Reply To:
 | | Re: Couldn't resist
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| (...) Whoa, that's something I never thought of before! It kind of raises an interesting idea, too. If this is always a trend for the satellite (i.e., "moon") of any planet with large amounts of liquid on the surface, then maybe over time there is a (...) (23 years ago, 7-Jul-01, to lugnet.space, lugnet.off-topic.geek)
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