What is the relevance of Elongation in crescent sighting?
Elongation is the angular distance between the moon and the sun. At The Astronomical New Moon this angle is zero and all the sunlight reflected by the moon surface is directed back towards the sun. With passage of time, the moon in its circular motion around the earth, moves away from the alignment with the earth and the sun. The angular distance between the moon and the sun increases, and some 18 to 24 hours after the Astronomical New Moon, the moon will have moved sufficiently for the sunlight reflected off its outer edge to be directed towards the earth. The new crescent moon can now become visible on earth. Elongation is a critical factor in determining the crescent’s visibility, and this depends on more than just time. In the early 1930’s, the French astronomer Andre’ Danjon deduced (from detailed consideration of the geology of the Moon) that no illuminated portion of the crescent can be seen from the earth when the Moon is 7° or less from the Sun. No sighting can penetrate this
