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Q: moon rotation ( Answered,   3 Comments )
Subject: moon rotation
Category: Science > Astronomy
Asked by: alb-ga
List Price: $2.00
Posted: 04 Nov 2002 13:47 PST
Expires: 04 Dec 2002 13:47 PST
Question ID: 98625
Why does our moon always face the earth while others rotate with
respect to their planets?  How exactly does ours face us; that is, is
there ANY change over the centuries, however miniscule?
Subject: Re: moon rotation
Answered By: alienintelligence-ga on 05 Nov 2002 00:19 PST
Hi alb

This is a really good question. The answer lies in
the same reason we have high and low tides on Earth.
The mass of the Earth tugs on the Moon in the same
way the moon pulls on the Earth. Since the Earth is
so much greater in size it is able to distort the
sphere of the moon. The Earth draws mass from the Moon
towards it. This elongation of the Moon reduces its 
rotational velocity and converts it to heat. Eventually
the spin is nearly nullified.

The elongation is drawn along an axis pointing to the
Earth. It results in permanent tidal bulges which makes it
dynamically most stable if one end of the bulge is always 
pointed towards the Earth.  

This process is called Tidal Locking. A very gradual
process. The dynamics of it are explained in detail

"[21.06] Tidal Despinning Timescales in the Solar System"
C.F. Chyba (SETI Institute and Stanford University), P. J.
Thomas (U. Wisconsin, Eau Claire)
[ ]

"Planets and satellites in the Solar System despin to a
spin-evolved end-state due to tidal dissipation. The usual
derivation for the despinning timescale sets the change in spin
angular momentum equal to the gravitational torque acting on
the object's tidal bulge (MacDonald 1964, Goldreich and Soter
1966, Peale 1974, 1977). The despinning timescale is found to
be proportional to the difference between the initial and final
spin angular velocities, and is finite. However, this
approximate derivation ignores the orbital mean motion n of the
despinning object, and is less and less satisfactory as the
object's spin angular velocity w approaches n. We have instead
calculated tidal despinning times by applying the formalism of
Peale and Cassen (1978) to calculate tidal energy dissipation
due to tides raised on a non-spin-locked object. Tidal heating
in the latter case is larger than tidal heating in the spin
locked case by a factor (1/7)[(w-n)/n](1/e2), where e is the
orbital eccentricity. This factor is initially greater than 104
for many objects in the Solar System. Calculating despinning
times from energy loss, we find that the despinning timescale
includes a previously neglected term that goes to infinity
logarithmically as w approaches n. In this sense all despinning
timescales are in fact infinite. We therefore define an
effective despinning timescale as the time required for despin
tidal heating to fall below tidal heating due to orbital
eccentricity. For many satellites in the Solar System,
including such major moons as Io and Europa, the neglected term
in the despinning timescale is in fact the dominant term. For
some especially short-period satellites, such as Phobos or
Amalthea, the resulting despinning timescales are one to two
orders of magnitude longer than those previously accepted."


Since that's a big chunk of info here is a simpler explanation:

"Tidal locking, or Orbit-spin resonance" by Anthony Lawson
`Physics of the Solar system'. Feb 9, 1999 
[ ]
"Although the Moon changes phase throughout a month if we look
carefully we can see that it actually keeps the same face
directed towards us the whole time. This doesn't mean that it
is not rotating, but that it has a rotation period the same
length as its orbital period (this is the sidereal period, 27.3
days, not the synodic period). The Moon is in orbit-spin
resonance, or tidally locked, with the Earth. How did this
"As seen above the Earth creates a tidal bulge on the Moon which
lies on the Earth-Moon line. However, if we imagine a time in
past when the Moon was rotating faster, the rotation would tend
to carry the bulge away from the Earth-Moon line. As it did so
the Earth's gravity tried to hold it back slowing the rotation
of the Moon ever so slightly. Over a long period of time this
would slow the rotation of the Moon until eventually the spin
period of the Moon matched that of its orbital period. When
this point is reached the tidal bulge is always pointing
towards the Earth and the rotation rate remains constant. This
is the situation we see today. There are a number other
examples of this tidal locking in the Solar system. Just like
the Moon, the Galilean satellite Io has a spin period which is
equal to its orbital period around Jupiter."

This chart shows the orbital data of satellites in our 
Solar System.
[ ]

Please ask for a clarification if there is a point you do
not understand.

I have provided a few search links to do additional research.

"tidal locked" OR  "tidally locked" moon
[ ://

"tidal locked" OR  "tidally locked" satellite
[ ://

"Tidal Despinning"
[ ://


Request for Answer Clarification by alb-ga on 07 Nov 2002 06:28 PST
Thanks.  I'm getting the idea but what about the second part --  how
exact is the lock?  From the explanation, it would seem that it would
never be perfect, just getting better all the time.  Also, what was
your search strategy?

Clarification of Answer by alienintelligence-ga on 11 Nov 2002 20:54 PST
Hi again alb...

For clarification you asked about the
exactness of the moon's tidal locking.

The term for this is libration.
[ ]
"Although the Moon always presents the same face towards the Earth,
due to its rotation and revolution being locked to the same period,
the combined effect of these different librations allows us over time
to see some 59% of its surface. "

This is a great animation of it...
[ ]

The Inconstant Moon can provide some more data for 
you on our lunar friend
[ ]

The search strategy for this question was
at the end of my original answer, I will 
repost for clarity. Search terms first 
then URL.

"tidal locked" OR  "tidally locked" moon 
[ ://
"tidal locked" OR  "tidally locked" satellite 
[ ://
"Tidal Despinning" 
[ ://

For your clarification I did these searches:

moon faces earth percent
[ ://

libration moon
[ ://

Subject: Re: moon rotation
From: javit-ga on 05 Nov 2002 06:29 PST

The relationship between Pluto and its moon Charon may be of interest
to you. Charon is in a synchronous orbit around Pluto. This means that
they keep facing EACHOTHER the same way. Unlike Earth, which rotates
if observed from the Moon, Charon and Pluto always look the same from

Now, if we go back to the Earth-Moon case, the gravitational
attraction of Moon on Earth causes tides and as Earth rotates there
has to befriction between Earth's crust and the oceans. This friction
is called the tidal friction and causes Work to be done against it.
Therefore the angular momentum is NOT conserved and is lost. In other
words, the rotation of Earth around its own axis is SLOWING DOWN due
to friction mainly between the oceans and the crust. Eventually, this
will cause the Earth and Moon to have a synchronous orbit just like
Charon and Pluto.
Actually, this will never happen because, by the time comes the Sun
will have already become a red giant which threatens even the very
existance of Earth and Moon.
Subject: Re: moon rotation
From: neilzero-ga on 05 Nov 2002 08:44 PST
Both answer and comment are good. There is a bit of wobble that allows
us to see about 51% of the moon's surface from Earth over a period of
months. I should think the bulge in the moon shifts steadily about one
degree per billion years or so.  Neil
Subject: Re: moon rotation
From: thenextguy-ga on 06 Nov 2002 06:00 PST
I don't think Io can be "locked" to Jupiter because of Europa.  My
understanding is that Europa's interference keeps Io from having a
perfectly circular orbit.  The lack of a circular orbit will prevent
synchronization (because orbital speed will change via Kepler's 2nd
law, but axial rotational speed can't change), and the dragging of the
tidal bulges back & forth across Io provide the heating necessary to
power its volcanoes.

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