### Re: Does the position of the lit portion of the moon change?

Date: Wed Nov 4 17:11:22 1998
Posted By: Everett Rubel, Degree in Physics
Area of science: Astronomy
ID: 909761424.As
Message:
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Sabrina,

The phase of the Moon is determined by the relative positions of
the Sun, the Moon, and the Earth.  If the Moon is between the Earth and the
Sun, then we have a new Moon with the shadowed side of the Moon facing the
Earth.  If the Earth is between the Moon and the Sun, then we have a full
Moon.  For a quarter Moon, the triangle formed by the Sun, Earth, and Moon
has a right angle in it, with the Moon either preceding the Earth in its
orbital path or following the Earth.  This is best understood by looking at
a diagram or a simulation.  A good one to look at can be found at:
http://tycho.usno.navy.mil/vphase.html

There is one additional factor that determines how a phase of the Moon will
appear to a person standing on the Earth. This is the person's position on
the Earth.  The phase shape and orientation is fixed for a given time, but
the orientation of the observer is not.  A person standing at the North
Pole would see an inverted Moon and phase compared to a person standing at
the South Pole.   A person at the Equator would see a Moon phase rotated 90
degrees compared to a person at a pole.  Observers at a pole of the Earth
would always see the cresent Moon with the points aligned nearly vertically
with the horizon.  Observers at the Equator would always see the cresent
Moon with the points aligned nearly horizontally with the horizon.  The
appearance is complicated by the fact that the Moon's orbit is tilted a few
degree with respect to the Equator.

Since you are in Florida, which is fairly close to the Equator, you will
tend to see the bottom or top half of the quarter Moon lit.  If you were in
Alaska, you would tend to see the quarter Moon oriented with the left or
right side lit.  It would be the same Moon, but you would be standing
"sideways" in one place compared to the other.

Best regards,

Everett Rubel

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