Internationale Zeitschrift für Fortschritte in der Technologie
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The circumstances controlling visibility of the Hijri lunar months

Mohamed Ghareeb Rashed

The 22% of the world population that holds the Islamic faith still use the lunar phase’s cycle as the base for their calendars. They begin their months with the first sighting of the crescent Moon.

The month in the Islamic statute which known as the Hijri lunar month requires two conditions to be fulfilled: First, The conjunction i.e. the celestial longitudes of the Moon and Sun are equal which occurred before the Sunset on the day of conjunction. Second, the new Moon crescent is observed above the horizon soon after sunset on the day of conjunction. That is to say, the Hijri month is a synodic lunar month starting from the first midnight following the observation of the new Moon crescent after the sunset. The complexity of the problem concerning with the first sighting of the new lunar crescent. The cause is attributed to various astronomical, astrophysical and geographical factors.

These conditions are: - Weather conditions (clouds, etc.), the atmospheric optics during observations, the sky background around the crescent, the crescent lag after sunset, the age of the crescent, the apparent altitude of the Moon above the horizon at Sunset, Azimuth difference between the Moon and the Sun (The relative azimuth) at Sunset, The Moon–Sun celestial elongation, the crescent width and geographic location (the latitude and the longitude of the location).

Analytical studies of the available samples of sighting the new Moon data taken during the period (1859 to 1996) confirm the theoretical conclusions about the circumstances controlling the first new crescent sighting. The studies show that the probability of the crescent visibility increases by the increase of the crescent lag, lunar age criterion might be considered virtually useless, The probability of the crescent visibility increases with the increasing of the altitude of the Moon above the horizon, Higher probability of the crescent visibility occurs when the relative azimuth is relatively small ranging from -2 to 2 degrees, If the Moon-Sun elongation is less than 7 degrees, the Moon will be invisible (agree with Danjon limit). The most of the observation has been carried out at elongation 9