What is the minimum angular separation required to see the new crescent? How can I calculate it for a particular place and date for a new born moon?
Minimum separation (elongation) for visibility, statistically known, is about 10.5 to 11 for naked eye, and about 7.5 to 8 for powerful telescopes. Remember, these are for perfect atmospheric conditions, which do not exist most of the times and places. To calculate this separation is not easy. It requires a lot of mathematical terms to calculate the positions of sun and moon based on local horizon in question, and then their separation angle is calculated using spherical trigonometry. The more accurate lunar theory and calculation methods you use, the better the results would be. 10.5 Question: How much time is required for the 1st hilal to remain in the sky after sunset for visibility according to your findings? Why it is invisible if it remains above the horizon for more than 40 minutes after sun set? (Jan 20, 1999) Answer: Time varies for locations and months. On one particular evening a crescent may be thinnest in Japan but the same crescent will be 15 hours to 18 hours thicker in
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