From: Amara Graps (Amara.Graps@mpi-hd.mpg.de)
Date: Mon Jan 08 2001 - 08:18:42 MST
From: Anders Sandberg <asa@nada.kth.se>, 07 Jan 2001
>"Ross A. Finlayson" <raf@tiki-lounge.com> writes:
>> Also, on Mercury, as it doesn't spin as it revolves around the Sun but points
>> towards it, half of Mercury is always pointed towards the Sun and the other half
>> dark, like Earth's Moon. So, human industrial and settlement activities could be
>> started on the borders of the dark and light halves to take advantage of the
>> properties of both halves.
>Actually, this is not true. This was how people believed Mercury
>rotated before 1962, when radar observations showed it is a 3:2
>resonance between its orbit and rotation. It rotates three times in
>two Mercury years.
Not sure if you really corrected Ross, Anders.
To Ross:
Please...
*There is no such thing as the "dark side of the Moon"*
It's a common misunderstanding among those in the public who haven't
had an astronomy course. On this list though, I would hope that folks
know it, especially if they are going to be speculating about advanced
astronomy topics.
You can look up "tidally-locked" in your astronomy textbook, and it
will be explained in some detail, but I append part of a FAQ, to
get you started.
Amara
>From the sci.astro FAQ
http://sciastro.astronomy.net/sci.astro.5.FAQ
===========================================================================
Subject: E.13.1 Why doesn't the Moon rotate?
Author: Laz Marhenke <laz@leland.Stanford.EDU>
In fact the Moon *does* rotate: It rotates exactly once for every
orbit it makes about the Earth. The fact that the Moon is rotating
may seem counterintuitive: If it's always facing towards us, how can
it be rotating at all? To see how this works, put two coins on a
table, a large one to represent the Earth, and a small one to
represent the Moon. Choose a particular place on the edge of the
"Moon" as a reference point. Now, move the Moon around the Earth in a
circle, but be careful to always keep the spot you picked pointed at
the Earth (this is analogous to the Moon always keeping the same face
pointed at the Earth). You should notice that as you do this, you
have to slowly rotate the Moon as it circles the Earth. By the time
the Moon coin goes once around the Earth coin, you should have had to
rotate the Moon exactly once.
This exact equality between the Moon's rotation period and orbital
period is sometimes seen as a fantastic coincidence, but, in fact,
there is a physical process which slowly changes the rotation period
until it matches the orbital period. See the next entry.
------------------------------
Subject: E.13.2 Why does the Moon always show the same face to the
Earth?
Author: Laz Marhenke <laz@leland.Stanford.EDU>
When it first formed, the Moon probably did not always show the same
face to the Earth. However, the Earth's gravity distorts the Moon,
producing tides in it just as the Moon produces tides in the Earth.
As the Moon rotated, the slight elongation of its tidal bulge was
dragged a bit in the direction of its rotation, providing the Earth
with a "handle" to slow down the Moon's rotation. More specifically,
the tidal bulge near the Earth is attracted to the Earth more strongly
than the bulge away from the Earth. Unless the bulge points toward
the Earth, a torque is produced on the Moon.
If we imagine looking down on the Earth-Moon system from the north
pole, here's what we'd see with the Moon rotating at the same rate as
it goes around the Earth:
Earth Moon
__
/ \ ____ ^
| | / \ |
\__/ \____/ Orbiting
this way
Tidal bulge *greatly*
exaggerated.
What if the Moon were rotating faster? Then the picture would look like:
Earth Moon
__
/ \ ___ ^
| | / ) |
\__/ (___/ Orbiting
this way
Rotating
counterclockwise;
Tidal bulge *greatly*
exaggerated.
If it isn't clear why the tidal bulge should move the way the picture
shows, think about it this way: Take the Moon in the top picture, with
its tidal bulges lined up with the Earth. Now, grab it and rotate it
counterclockwise 90 degrees. Its tidal bulge is now lined up the
"wrong" way. The Moon will eventually return to a shape with tidal
bulges lined up with the Earth, but it won't happen instantly; it will
take some time. If, instead of rotating the Moon 90 degrees, you did
something less drastic, like rotating it one degree, the tidal bulge
would still be slightly misaligned, and it would still take some time
to return to its proper place. If the Moon is rotating faster than
once per orbit, it's like a constant series of such little
adjustments. The tidal bulge is perpetually trying to regain its
correct position, but the Moon keeps rotating and pushing it a bit out
of the way.
Returning to the second picture above, the Earth's gravitational
forces on the Moon look like this:
___
F1 <-----/ )
F2 <-------(___/
F2 is larger than F1, because that part of the Moon (the "bottom" half
in the drawing, or the half that's "rearward" in the orbit) is a bit
closer to the Earth. As a result, the two forces together tend to
twist the Moon clockwise, slowing its spin. Over time, the result is
that the Moon ends up with one face always facing, or "locked," to the
Earth. If you drew this picture for the first case, (where the Moon
rotates at the same rate that it orbits, and the tidal bulges are in
line with the Earth), the forces would be acting along the same line,
and wouldn't produce any twist.
Another way to explain this is to say that the Moon's energy of
rotation is dissipated by internal friction as the Moon spins and its
tidal bulge doesn't, but I think the detailed force analysis above
makes things a little clearer.
This same effect occurs elsewhere in the solar system as well. The
vast majority of satellites whose rotation rates have been measured
are tidally locked (the jargon for having the same rotation and
orbital periods). The few exceptions are satellites whose orbits are
very distant from their primaries, so that the tidal forces on them
are very small. (There could be, in principle, other exceptions among
some of the close-in satellites whose rotation rates haven't been
measured, but this is unlikely as tidal forces grow stronger the
closer to the planet the satellite is.)
Pluto's satellite Charon is so massive (compared to Pluto) that it has
locked Pluto, as well as Pluto locking Charon. This will happen to
the Earth eventually too, assuming we survive the late stages of the
Sun's evolution intact. :')
--
*********************************************************************
Amara Graps | Max-Planck-Institut fuer Kernphysik
Interplanetary Dust Group | Saupfercheckweg 1
+49-6221-516-543 | 69117 Heidelberg, GERMANY
Amara.Graps@mpi-hd.mpg.de * http://www.mpi-hd.mpg.de/dustgroup/~graps
*********************************************************************
"Never fight an inanimate object." - P. J. O'Rourke
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