[meteorite-list] Quickie

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Sun, 20 May 2012 01:47:48 -0500
Message-ID: <F3B138803D52445992F77166C43F0EDF_at_ATARIENGINE2>

Pete, James, Stuart, List

Long answer to a quick question.

1. The Moon DOES rotate on its axis. If it didn't,
we on the Earth would have a slow month-long
changing view of every spot on the Moon. There
would be no "near" side and "far" side. If you were
looking at what we call the near side tonight, in
two weeks you would be looking at the "far" side.

The sidereal (with reference to the stars, rotation
period of the Moon is 27.321582 days. The orbital
period of the Moon is 27.321582 days. In a word,
the orbit is synchronous. That's relative to the
stellar background.

The synodic (relative to the Sun) orbital period of
the Moon is different, 29.530589 days. In case that
puzzles you, the cause of the difference is explained
here:
http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question32.html

2. James, the strength of the tides is determined
by the Moon's MASS, hence its gravitational influence.
Given the same orbit, the tides would be the same
whatever the period of rotation, 27 days or 27 hours.

Now, you may be referring to the fact that the Moon's
center of gravity is displaced toward the Earth slightly,
and if it rotated rapidly (or didn't rotate at all), it would
slightly alter the gravitational pull and the tidal effect
from it, but effect would be incredibly small. The center
if gravity is only offset about two kilometers!

3. It has been hypothesized that without our large and
prominent satellite, humans would have been a much
longer time figuring orbital mechanics. Remember it
was idly trying to figure out how fast the Moon was
"falling" around the Earth that gave Newton his first
push into the theory of gravity while he was back home
to avoid the plague while a young student.

The Moon's orbit is incredibly complex, full of tilts and
wobbles of every kind.
http://en.wikipedia.org/wiki/Orbit_of_the_Moon

The full calculation of the equation of the Moon's orbit
(where it will be at a specific time) is one of the most
computationally intensive tasks ever done. Men have
devoted their entire working life to it and still not
finished the job. The last to do it was E. W. Brown:
http://en.wikipedia.org/wiki/Ernest_William_Brown

4. But even IF the Moon had a new, non-synchronous
rotation, tidal braking would slowly return it to its old
synchronous rotational period. OR, if it had no rotation
at all, tidal acceleration would spin it up again to the
synchronous period.

The full mathematical theory of tidal fiiction and the
evolution of the lunar orbit was worked out by the
XIXth century physicist George Howard Darwin
(Charles Darwin's son). Brief explanation here:
http://curious.astro.cornell.edu/question.php?number=124

5. The strength of tidal forces on the Earth's rotation
and the Moon's recession (moving away from the
Earth) is more dependent on the shape of the continents,
the width of the continental shelves, and the depth
of the oceans than any other factor. An Earth with many
low-lying continents, broad ocean shelves, and shallow
oceans would have been slowed to a "day" much longer
than 24 hours by now. And the Moon would have ended
up much further away than it is.

In the past, the "day" was shorter and the number days
in a year much greater than it is now. I appears that at
formation, 4.5 billion yars ago, the year was about 800
"days" of nearly 12 hours each:
ftp://ftp.ecgs.lu/public/publications/jlg/jlg90/JLG90_Denis.pdf

6. Tides are far from simple. In Tahiti, for example, the
actual experienced tides are almost entirely a product
of the Sun's gravity. You get a good approximation by
ignoring the Moon altogether. There's a high tide at
noon and midnight and lows at 6 am and pm. Why?
http://tahitiexpeditions.typepad.com/travelblog/2010/07/tides-in-tahiti.html

7. We now have a short list of people on this List with
nothing better to do on a Saturday night... I suppose
especially me who wrote the longest.


Sterling K. Webb
---------------------------------------------------------------------------
----- Original Message -----
From: "Stuart McDaniel" <actionshooting at carolina.rr.com>
To: "James Beauchamp" <falcon99 at sbcglobal.net>;
<pshugar at messengersfromthecosmos.com>
Cc: "The List" <meteorite-list at meteoritecentral.com>
Sent: Saturday, May 19, 2012 10:41 PM
Subject: Re: [meteorite-list] Quickie


>I thought the Moon did rotate??
>
>
>
>
>
> *****************************
> Stuart McDaniel
> Lawndale, NC
> Secr.,
> Cleve. Co. Astronomical Society
>
> IMCA #9052
> Sirius Meteorites
>
> Node35 - Sentinel All Sky
>
> http://spacerocks.weebly.com
>
> *********************************
> -----Original Message-----
> From: James Beauchamp
> Sent: Saturday, May 19, 2012 11:31 PM
> To: <pshugar at messengersfromthecosmos.com>
> Cc: The List
> Subject: Re: [meteorite-list] Quickie
>
> I would say less. The highest density of the moon stays on the earth
> side now, so the 1/r^2 magnitude of the gravity effect is maximized.
>
> If it were rotating, the average pull would always be less than it is
> now.
>
> Sent from my iPhone
>
> On May 19, 2012, at 10:16 PM, <pshugar at messengersfromthecosmos.com>
> wrote:
>
>> I have a quetion.
>> Would the moon's effect on the tides be more or less
>> if the moon still had rotational spin?
>> And why?
>> Pete
>>
>>
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Received on Sun 20 May 2012 02:47:48 AM PDT