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Name: Naomi
Status: student
Grade: other
Country: United Kingdom
Date: Spring 2012


Question:
Positions of Sun and Moon.

I often walk in the early morning, and on a clear day sometimes both the sun and moon are clearly visible. On the days when the moon is at half phase, effectively a semi circle , the visible lit side is illuminated by the rising sun. So, using a set square aligned along the moon's visible diameter, and a ruler at right angles, in contact with the square, I expected that the ruler would point directly at the sun. On the days I have tried this the sun always seem to be lower down in the sky than the ruler suggests it should be. Maybe, I guess, as much as 20 degrees off target. Why does the lit half of the moon not seem to always point straight at the sun?


Replies:
You cannot be lining your set-square correctly with the terminator (the day/night line). Do not forget that you are dealing with an object that has a diameter of only 0.5 degrees (very small), and it is VERY easy to mis-align things.

Howard Barnes Astronomer.


Dear Naomi,

Good question. The Moon does not have to point toward the Sun to be illuminated by it. A light in the center of a room need not point to specific objects in the room to light them.

Sincerely David H. Levy



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