MadSci Network: Astronomy

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


Thanks for your question.  

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:

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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