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Crescent Sighting Criteria |
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Based on data collected from late 1980s by The World Federation, a criteria has been developed for calculating the likelihood of crescent visibility.
This criteria has been continuously improved by the data collected for crescent observation.
A very effective tool to calculate ahead of time, whether the moon can be sighted.
Elongation
After the moon comes out of conjunction it takes some time for it to get away from the sun such that the crescent can be seen from the earth.
In 1931 the French Astronomer, André Danjon, suggested that scientifically it was not possible for the crescent to be seen when the angle of separation
(elongation) is less than 7°, regardless of what the age of the crescent is and regardless of any favourable observing circumstances that may exist.
Our data suggests that this angle of separation (elongation) between the sun and the moon must be at least 9.7° for the crescent to be sighted with the naked eye.
Altitude
At sunset the altitude of the moon above the horizon must be high enough above the glare of the sun for the crescent to be sighted.
A young crescent is usually sighted when the sun has dropped at least 5° below the horizon, which is about 20 minutes after sunset.
Therefore the moonset time has to be more than 20 minutes after the sunset time.
Crescent Age
Our records show that with a naked eye, while no crescent can be seen before it is 16 hours old, it may still be up to 23 hours old, or more in certain seasons, before it is visible. |
