Re: Libration of the moon

[P3D <CJMCE@xxxxxxxxxxxxxxxx>]

John R. said:

>As I understand it, a fairly simple explanation of the phonomenon is as
>follows:
>  The moon rotates about its axis at an essentially constant angular
>  velocity. However its orbit around the Earth is not a perfect circle,
>  so in accordance with Kepler's laws of motion, the moon does not travel
>  around the Earth at a constant angular velocity (v. and a.v. are greater
>  when the moon is closer to the Earth). Because the orbital and rotational
>  angular velocities are not always of the same magnitude, the moon appears
>  to rotate back and forth slightly from the viewpoint of an observer on the
>  Earth.
>
>Complicating factors:
> - The moon's orbit is also tilted with respect to the Earth's axis of 
>     rotation.
> - The moon's center of gravity is not at its geometric center, which
>     causes the moon's rate of rotation to be more strongly tied in to
>     its motion around the Earth.
>And many more.

Yup, that sounds correct.  The entire range of nutations of the moon's node,
its pertubations in orbit & the resultant libration takes one metonic cycle.
One metonic cycle is 18.6 years, and is the minimum period of time to observe
the rise and fall of the tides to compute mean sea level at a tide gauge.

After the Army Map Service did the stereophotogrammetric mapping of the moon 
from observatory glass plates in the early 1960's, the Air Force Aeronautical
Chart & Information Center (ACIC) did a series of Lunar Aeronautical Charts
(LAC series) at 1:250,000 scale in the middle 1960's.  These were stereo
compilations from the Lunar Orbiter I-IV return beam vidicon (RBV) imagery.
AMS did the photo triangulation and ACIC did the compilation.  Subsequent to
that, in the late 1960's the AMS and U.S. Army Topographic Command (TOPOCOM)
used the Apollo orbital photography (metric frame & panoramic cameras) to do
the photo triangulation & stereocompilation for the actual Moon Landing maps.

I was a brand-new 2nd Lieutenant in the Extraterrestrial Branch at TOPOCOM in
1968.  I worked on the photogrammetric triangulation of the Apollo imagery.

Clifford J. Mugnier  (cjmce@xxxxxxx)
Topographic Engineering Laboratory
Department of Civil & Environmental Engineering
University of New Orleans
New Orleans, Louisiana  70148
Voice: (504) 280-7095
FAX:         280-7095      


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