Amara Graps wrote:
> 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
Hello,
Thank you for further information. The moon has a sidereal day.
It makes me feel that I am learning when there are obviously many, many people with high
skills and amounts of practical knowledge in these areas where I have interest. I am
looking forward to removing my foot from my mouth for "tidelocked Mars" and "dark side
of Moon."
Ross
-- Ross Andrew Finlayson Finlayson Consulting Ross at Tiki-Lounge: http://www.tiki-lounge.com/~raf/ "The best mathematician in the world is Maplev in Ontario." - Pertti L.
This archive was generated by hypermail 2b30 : Mon May 28 2001 - 09:56:17 MDT