jay r. yablon

For a change of pace on this Super Bowl Sunday, I have a question
totally unrelated to anything else I have talked about here.

We all know that the moon revolves about the earth and at the same time
rotates at virtually identical rates, so that one never sees the dark
side of the moon.

A few questions:

1) Has it been empirically determined to how many decimal places is
this so? If we are to come back 100,000 years from now, would today's
dark side still be out of view, or would there have been some
incremental change so that a fraction of what is now hidden from view
would rotate into view? One has to think that to some decimal place,
this is not totally exact.

2) From a standpoint of planetary evolution, based on physical
reasoning, what does this remarkable coincidence between the moon's
revolution about the earth and its rotation about its axis teach us, if
anything, about the origin and evolution of the earth-moon system?

3) Are their other planet / moon systems that we know of which have
this same remarkable coincidence between lunar revolution and rotation?

I also point out that when there is a solar eclipse, the moon appears to
be virtually the same size as the sun, which means that the ratio:

diameter(sun)/diameter(moon)
=distance(earth-sun)/distance(earth-moon)

I am curious how precise this is, and whether this also rather
remarkable coincidence tells us anything, based on physical reasoning,
about the origins and evolution of the solar system?

Thanks,

Jay.
____________________________
Jay R. Yablon
Email: jyablon@nycap.rr.com
co-moderator: sci.physics.foundations
Weblog: http://jayryablon.wordpress.com/
Web Site: http://home.nycap.rr.com/jry/FermionMass.htm

cl\.massé

"Jay R. Yablon" <jyablon@nycap.rr.com> a écrit dans le message de
news:6ummr7Ffj4icU1@mid.individual.net...


Nothing. When the rotation rates don't match, there is some friction in
both bodies because the change of direction of the gravitational field
changes their form (sea or earth tide.) The energy so damped is taken from
the rotation energy, since the bodies haven't an exact spherical form. That
way, the relative rotation of the moon has completely disappeared. Also,
the rotation rate of the earth decreases, as well as the rotation rate of
the moon around the earth.

That answers also 1)


It is probably the case of the moons of Jupiter, although I don't know
whether it's true.

--
~~~~ clmasse on free F-country
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oh no

Thus spake Jay R. Yablon <jyablon@nycap.rr.com>


In fact it is exact. The Moon's rotation is locked to its orbital
velocity by tidal forces. The Moon has developed a tidal bulge
on the side nearest the earth. This keeps this side facing the Earth.


We do not think that it was true at the time the moon was formed, but
rather that tidal forces slowed the rate of rotation of the Moon until
lock was achieved.


I don't know whether it is measured for other moons, or whether we
expect it to be true for gaseous moons, since it is not clear to me that
a gaseous moon can be said to have fixed sides.


Not very precise, but you can easily look up the numbers. Clearly not
true for other moons. One should avoid the notion that numerology can
tell us much about physics.

Regards

--
Charles Francis
moderator sci.physics.foundations.
charles (dot) e (dot) h (dot) francis (at) googlemail.com (remove spaces and
braces)

http://www.teleconnection.info/rqg/MainIndex

rich l.

I basically agree with this answer, but would like to add a slight
refinement. The reason the moon is locked to the earth is because it
is not perfectly spherical (mostly due to tidal forces). As a result
there is a (small) restoring torque when the moon rotates slightly
from the earth facing direction. I'd expect that if it were measured
precisely, one would find that the moon oscillates slightly back and
forth about this stable equilibrium. If the moon's orbit were
PERFECTLY circular, this motion too would damp out due to dissipation
in the moon (or even in the earth). Since the moon's orbit is not
perfectly circular, I would expect to see some small level of this
oscillation. At what amplitude would depend on the eccentricity of
the orbit (which would be exciting the oscillation) and the difference
in the oscillation frequency and the moon's orbit frequency. I'm not
aware that anyone has ever observed this motion.

Rich L.

michael j. strickland

I don't believe it is exact (i.e. I've heard that the moon oscillates
about
the point of tidal lock slightly. I don't know if the extent of this
oscillation has ever been measured.


I believe most orbits evolve toward tidal lock eventually. How long this
takes depends on the initial mass and rotation rate (angular momentum),
the homogeneity of its mass distribution, and the eccentricity of it
orbit. I believe that all other things being equal, tidal lock is an
indication of a very old orbit.

It is not precise. For the moon to completely block the sun during an
eclipse it must be near its closest point of approach to the earth
(within about 20-30% of its distance variation from earth, I believe).
When this is not the case, the "total" eclipse becomes an annular
eclipse (named for the annulus or ring of sun visible around the moon at
"totality").

Also, the diameter/distance coincidence is only a coincidence at the
current time. Most theories state that the moon was formed much closer
to earth and has gradually drifted outward since then.

....

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cl\.massé

"Michael J. Strickland" <qualityservices2@verizon.net> a écrit dans le
message de news:udGil.1487$eK2.1045@nwrddc01.gnilink.net...

As the orbit of the moon isn't spherical, its angular velocity around the
earth isn't constant. So its own angular velocity is an average and is
almost never equal to the former. Consequently, a narrow part of the hidden
side is seen periodically. The extent of the oscillation can approximately
be calculated from the eccentricity of the orbit.

--
~~~~ clmasse on free F-country
Liberty, Equality, Profitability.