Young moons and The Islamic Calendar

Yearly calendars used by most of the world's inhabitants are based on the Earth's orbital period. Yet the hundreds of millions who follow Islam mark the events of their lives using a strictly lunar cycle. The history and significance of the Islamic calendar, introduced in 638, have been researched by Waheed A. Muhanna (Ohio State University) and Mohib N. Durrani, astronomical co-ordinator for the Islamic Societies of North America.

By tradition, each of the 12 lunar months of the Islamic calendar, or Hijri, begins the day after a declared sighting of the young crescent Moon. No intercalation is done, and the months are unrelated to particular seasons. Thus the Islamic year works out to be approximately 11 days shorter than the Gregorian year. The year 1 A.H. (Anno Hegira) commemorates the migration of the prophet Muhammad from Makkah to al-Madinah, to establish the first Muslim city-state in AD 622. The current Islamic year 1417, therefore, denotes the number of lunation years that have elapsed since July 16, 622, the date chosen as the starting point.

Muhanna and Durrani note that a predicted sighting of the lunar crescent is not considered as definitive or conclusive as some other predetermined moment governed by astronomical circumstances, such as a conjunction. Since conflicts sometimes arise as to when a young Moon is first sighted, Muhanna concedes, "This makes it difficult to produce Islamic calendars that are reliable."

In the late 1970s Islamic scholar Mohammed Ilyas introduced the standardising concept of an International Lunar Dateline (ILD). Subsequent seminars and papers refined the concept. In April 1988 the Islamic Society of North America's Fiqh Council of religious scholars adopted the doctrine of Ikhtileaf al-Mataal'i (Different Horizons), which allows each locality to make its own crescent sightings. This differs from the doctrine of Ittehaad al-Mataal'i (Unity of the Crescent), which used sightings on other continents to determine the start of the Islamic month for North America. Durrani says both doctrines are equally valid under Islamic law.

Lunar-crescent sightings by Muslim observers on North America are now forwarded to four North American groups for analysis. A determination is made and promulgated through regional clerics, imams, and other teachers and advisers who regulate the calendar for Muslim communities. Record sightings, most of them made by amateur astronomers, are taken into account in these calculations. In fact, Islamic communities around the globe often request astronomers and observatories to supply refined times of lunar conjunction and predictions of crescent visibility.

Like the conventional International Dateline, the ILD divides the world into days. By definition, anyone within a specified zone of uncertainty along the dateline has a 50 percent chance of sighting the Moon's crescent. The provability of success increases to the west and diminishes to the east. Since the crescent is theoretically visible to the ILD's west, the lunar month starts on that night. Observers farther east cannot see the crescent, and their month begins the following night.