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Old March 28th 11, 11:02 AM posted to sci.astro.amateur,sci.physics
Ben[_2_]
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Posts: 181
Default For Kids and Grownups -- Exploration3: the Incredible Shrinking Moon


http://en.wikipedia.org/wiki/Moon : "Libration also changes the angle
from which the Moon is seen, allowing about 59% of its surface to be
seen from the Earth".

I don't know whether that figure, dominantly at least due to apparent
"side to side" motion caused by eccentricity, includes the apparent
"nodding" motion due to tilts, or the extra half a degree all round due
to parallax and the size of the Earth.


There are three classes of libration, optical, physical and
topocentric. Jean Meuss in 'Astronomical Algorithms'
states:
However, apparent oscillations known as *optical librations*
which are due
to variations in the geometric position of the
Earth.......allow about 59%
of the surface to be observed from the Earth. (p.371)

He goes on to say that *physical libration* can never be larger than
0.04 degree in both latitude and longitude.
Well that's not very much and would be observable only under the most
rigorous conditions.

But concerning *topocentric librations* he states that the geocentric
values of the librations and the position angle
of axis should be reduced to the values at the place of the observer
on the surface of the Earth. "For the librations
the values may reach 1 degree and have important effects on limb
contour."

So by shifting one's position around the Earth, say from Hudson Bay to
Tierra del Fuego one could see perhaps
60% of the Moon's surface? I don't know. I've always held the 59%
factor as an adequate estimate.