Subject:
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Re: Evolution of Earth and moon (was: Couldn't resist)
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Newsgroups:
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lugnet.space, lugnet.off-topic.geek
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Date:
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Sun, 8 Jul 2001 16:03:21 GMT
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Viewed:
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8561 times
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Hi,
Long time no see, John! Have you just been lurking about?
(I'm finally back from Europe myself.) Coming to Brickfest?
How's JHU?
In lugnet.space, John J. Ladasky, Jr. writes:
>
> > 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.
Weren't there other methods used recently as well? I'm not
sure that any would be as accurate as a laser, given that
the international meter standard is based on the speed of
light (as of the 1980s, I think). Somehow using radar sticks
in my mind, but that might just be a holdover from earlier
measurement in the 1940s and 1950s.
> 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.
I wasn't aware both faces had to be locked for the term
"tidal lock" to be valid. For example, I've heard the
statement made that Mercury is tidally locked to the Sun--
true in that the same face of Mercury is sunward, but not
true for the Sun, if you can really call that a "face".
And locking is not fixed tight--both Mercury and the moon,
like Jupiter's satellites, and presumably Pluto and Charon
(which *are* a tidally-locked double planetoid system, like
Hector in the Belt) too, "librate"--they basically wobble.
But whether this is the settling of a golf ball in the cup
or it's being powered from outside, I don't know offhand.
> 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.
Of course, this does assume that no weird momentum-altering
things happen (collisions, the unexpected expulsion of a
gaseous shell from the Sun, etc). But all of those kinds of
things might make our discussion a little bit, um, "academic."
Not that there's anything wrong with that.
Re: the oceans evaporating: I wonder if we can look at solar
output in past aeons? It may be that Earth was only warm
enough for multicellular life at a certain point--and that it
may be different enough *now* that if one brought, say, an
eryopsid labyrinthodont (big, giant, mega-amphibian) to the
present day, it would cook or suffocate somehow. I know that
there's a lot of work being done on the sheet-of-ice planet
idea--where only the equator regions were ice-free, sort of
a super Ice Age.
rambling,
LFB
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