Quote:
Originally Posted by Rockin Robbins
The phase of the moon is directly related to its rise and set times.
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No, it isn't. The phase of the Moon is a function of its apparent angular separation from the Sun. The percent of illumination can be calculated as the haversine¹ of the separation angle in degrees.
The apparent angular separation can be calculated using the following formula:
θ = acos(sin(Dec1)⋅sin(Dec2)+cos(Dec1)⋅cos(Dec2)⋅cos(G HA2-GHA1)) (Not sure why there's a space inserted into "GHA2". I can't remove it.)
If the Moon was only 10 degrees from the Sun, it
should be a crescent. A very thin sliver at 0.7% illumination, but a crescent nonetheless.
¹Haversine = (1-cos)/2