Originally posted by: Descartes
Look at it just above the horizon will make it appear even bigger. I used to spend a lot of time out in the middle of nowhere with my telescopes, and a perigee moon just peeking above the horizon looks probably 3-4x bigger than a normal apogee moon at zenith. It's really incredible.
The reason the moon looks bigger at the horizon
than overhead is that the moon is bigger at the
horizon than it is overhead. There are sound
physical principles that explain why this happens.
As you know, the moon is in orbit around the earth
and if you recall your physics you will remember
that objects in orbit maintain constant potential
energy. You will also recall that potential energy
in a gravitational field is a function of an object's mass and
altitude relative to a reference plane. When the
moon is on the horizon, lower to the ground, it
has a certain, defined potential energy. As the
moon rises, it must conserve energy and therefore
must shed mass. If the moon did not shed mass, it
could not rise or it would be in clear violation
of well established physical laws. The moon continues
to shed mass until it reaches its maximum altitude
at its azimuth (named for the famed English mathematician/
automotive designer Sir Isaac Azimuth who invented the
differential and who should not be confused with the
other famed Azimuth, the sci-fi writer and spectular
humorist{one who tells off-color jokes}). After
passing through its azimuth, the moon begins to
accumulate mass at just the right rate to maintain
its orbit. Now you may well be asking yourself
"what happens to the mass the moon has shed and
how does it get it back?" The answer to the first
part of this question was first answered in this centruy
by Einstein's famed equation which shows that moon mass
is converted to energy and shed it the form of moonbeams.
The second part of the question "how does the moon regain
mass?" That remains a mystery as yet unassailed by science.
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