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The
Changing Face of the Moon:
We
tend to think that we only
see one half of the Lunar surface, but in fact we see more than that for
2 reasons.
1)
The Moon's rotational axis is tilted by
7°. This actually means that
the Moon experiences seasons,
just as the Earth does (it does all 4 each
month!).
-
Because
of this tilt we can see, from the Earth, a bit past the North
Lunar Pole at one time of the month, and
a bit past the South Lunar Pole
at a different time.
2)
The Moon's orbital velocity changes over
a month. It is a property of orbits
that things move faster when they are near
what they orbit, and slower when they are
far away. So the Moon is fastest
at Perigee and
slowest
at Apogee.
-
Since
the Moon rotates at a constant speed,
the changes in its velocity mean that it sometimes rotates slower than
it is orbiting, sometimes faster. This permits us to
see a bit past East and West edges of the Moon.
These two effects let
us see about 59% of the Moon's surface.
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