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From: rmartin@clear.com (Bob Martin)
Newsgroups: sci.space
Subject: Re: orbiting bodies
Message-ID: <1990Nov8.053319.11875@clear.com>
Date: 8 Nov 90 05:33:19 GMT
References: <129@ctbilbo.UUCP> <1990Nov6.010422.26534@uokmax.ecn.uoknor.edu>
Organization: Clear Communications, Inc.
Lines: 49

In article <1990Nov6.010422.26534@uokmax.ecn.uoknor.edu> jabishop@uokmax.ecn.uoknor.edu (Jonathan A Bishop) writes:
>pete@ctbilbo.UUCP (Pete Ritter) writes:
>
>Now, my question:
>Are the forces that synchronized the Moon's rotation and revolution the same
>forces that have nearly done so with Venus relative to the Sun?  
>Does this phenomena only happen in a narrow band?

The Moon's rotation is locked to its orbital period by the tidal forces
exerted upon it by the Earth.  These tidal forces deform the moon
so that it bulges directly towards (and dirctly away) from the Earth.
If the moon were rotating at a different rate from the orbital period
these bulges would rotate too.  Since the bulge has mass, and would
rotate out of the line connecting the center of the Earth with the
Center of the moon there would be a very slight tug exerted  on the
Lunar bulge by the Earth which would tend to change the Moon's 
rotational velocity to be closer to the rate at which it revolves.

This same effect is slowing down the rotation of the Earth and
driving the Moon into higher and higher orbits.  Very slowly of
course.

Venus must also experience tidal forces with the Sun and perhaps
the it is the Sun's tides which have given Venus such a slow
rotational period.  But the story is more complex.  Venus' rotational 
period is 243 Earth-days which is _longer_ than its 255 day period of 
revolution.  Thus Venus' rotation is retrograde.  There is 
circumstatial evidence indicating that Venus is tidaly locked with 
the _Earth_.  I turns out that when both planets are on the same side 
of the sun, Venus presents approximately the same face towards the 
Earth.  So it is possible that the Earth's meager (at that distance) 
gravity is in some way responsible for the small Retrograde rotation 
of Venus.

Another interesting result of tidal effects is the vulcanism of
IO.  IO is the innermost moon of Jupiter, which would certainly be
tidally locked to the planet if it were not for the influence of the
nearby moons of Ganymede and Europa.  These moons tug on IO and
force it to continue rotating even against Jupiter's tremendous
tides.  This means that the tidal bulge of IO is continually dragged
around causing IO to be alternately stretched and pulled.  This causes
frictional heating which has melted the interior of the moon and causes
it to spew forth its innards in copious volcanic flows.

-- 
+-Robert C. Martin-----+---------------------------------------------+
| rmartin@clear.com    | My opinions are mine.  They aren't anybody  |
| uunet!clrcom!rmartin | elses.  And thats the way I want to keep it.|
+----------------------+---------------------------------------------+