The Islamic calendar is principally a lunar calendar. The determination of the first day of any Islamic month is not a simple matter, but rather a complex one. The question is how soon after the new Moon can one spot the lunar crescent in the evening twilight? Several civilizations before the Muslims faced the same question, and several criteria were introduced for a possible sighting. The most common one is that the thin lunar crescent should be at least one day old at the time of sunset. Each succeeding day the Moon sets later, increasing the chance that it will be seen. Sightings of the Moon within 20 hours of its new phase are extremely rare. However, some records have been set such as the naked-eye visibility of a 15.4 hours crescent in 1871, a 14.9 hours visibility in 1972, and a 13.5 hours visibility in 1988.
The sighting of the thin lunar crescent depends on several factors such as the atmospheric clarity, the sky brightness, and the sensitivity of the eye during the evening twilight. The uncertainty in the weather adds more problems in the exact determination of the Islamic months. One general method used by earlier Muslim astronomers for possible sighting is the time lag between sunset and moonset. The thin crescent can be visible if the Moon sets at least 48 minutes after sunset. This limit, however, can be either shorter or longer depending on the location and the season. Another equivalent method is that the angle of separation of the Moon and the Sun should be at least 12 degrees at sunset.
For the purpose of having a quick handy Hijra calendar for any year, we have adopted the following simple two criteria to set our calendar. We have chosen the Moon's age and the moonset lag time as our main two criteria to set the first day of the twelve Islamic months. If the Moon's age is below 12 hours and the lag time is less than 25 minutes, the first day of the Islamic month is set two days after New Moon. Otherwise, it will be the next day. Again, we stress the point that this is just to have a quick calendar. The rule of deciding the first day of any Islamic is based upon a direct observation of the thin lunar crescent Moon through any mean, i.e., a naked eye observation or a telescope.