ചന്ദ്രക്കല ദര്‍ശനത്തിനു വേണ്ട മിനിമം സമയം ??/ Prediction of the Dates of First Visibility of the New Crescent Moon

From ASTRONOMICAL INFORMATION SHEET No. 6 
Prepared by 
HM Nautical Almanac Office 
THE UNITED KINGDOM HYDROGRAPHIC OFFICE 
Admiralty Way, Taunton, Somerset, TA1 2DN 

A Note on the Prediction of the Dates of First Visibility of the New Crescent Moon 
"There are various parameters that are used to predict first sighting. For example, the age of the Moon at sunset, measured from the time of astronomical new moon, is often used. As a rough guide the new crescent moon cannot be seen without optical aid when the age is less than 14 hours, and it is usually visible by the time the age is 30 hours. This parameter often gives reliable predictions, and it is simple to calculate, however, it cannot always be relied upon. The lag time in minutes between moonset and sunset is another indicator that is often used, it is slightly more difficult to calculate, and again it cannot always be relied upon. A lag time of 50 minutes or more is usually a good indication that the new crescent moon can be seen during twilight. A geometrical rule that applies in most situations is that it is unlikely that the new crescent moon will be seen with the unaided eye unless the elongation exceeds 10° and the altitude of the Moon exceeds 5° when the depression of the Sun is 3°. Another important fact that should be taken into account is that the new crescent moon can always be seen 30 days after the previous one and in half the cases it can be seen 29 days after, because the length of the synodic month is 29·53 days." 

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 Courtesy : HM Nautical Almanac Office , THE UNITED KINGDOM HYDROGRAPHIC OFFICE http://astro.ukho.gov.uk/nao/aisinfo/ais006.pdf