Islamic Calendar

Tabular Dates

The Islamic Calendar is a purely lunar calendar in which months correspond to the lunar phase cycle. There are rules for calculating this calendar which is very accurate over long periods of time. Dates in this calendar are called tabular dates.

In the calculation of the tabular dates a prediction of first crescent sighting is made, based on the width of the crescent Moon, the time of sunset, the time of moonset, the age of the crescent Moon, the altitude of the Moon at sunset and its bearing from the Sun.

There are times when one or more of these parameters is quite useless for predicting whether the crescent Moon will be visible to the naked eye or not. However based on experience we simplify the calculations by saying that:

The crescent Moon has to be a minimum of 20 hours old at sunset

The sighting does depend upon the distance of the Moon from the Earth. When it is close to the Earth, ie at perigee, sighting will be early, say at 18 hours old, but when it is at apogee sighting will not occur until the Moon is at least 28 hours old.

The moonset time is not less than 20 minutes after sunset

The visiblity of the crescent Moon depends on the obsrever's eyesight, clarity of the atmosphere, local weather, pollution level, degree of illumination around the observer and numerous other small factors

Using binoculars does not help very much. It may enlarge the Moon, but it reduces its brightness and it is the contrast between the twilight sky and the crescent that matters.

The Moon's elongation from the Sun is greater than 10 degrees

This is based on an ancient Babylonian rule, and is consistent with the criteria laid down by Bruin in Vistas in Astronomy (1977), Vol 21, pp331-358, Pergamon Press.

It is interesting that the Moonwatch observations indicate that about 8 degrees could be possible and 7 degrees may be possible but maybe only once in a lifetime.