By Fatima Chaudhry
Introduction and History
The Islamic calendar is a lunar calendar that is still used today by Muslims all over the world to determine on which days certain Muslim holidays fall. One calendar year consists of 12 lunar months and approximately 354 days making it about 11 days shorter than the solar calendar. The first year of the calendar is the marking of Hijra and all years are marked with H or AH (The current year is 1429 AH). The calendar that was used by Arabs before the Islamic calendar was a lunisolar calendar. This meant that it used lunar months but synchronized with the seasons by adding an intercalary month, when it was thought necessary. Therefore it is possible that the months on the Islamic calendar from the years 1 AH to 10 AH (year when intercalary months became prohibited) can contain a few intercalary months.
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Calender Scale on Islamic Astrolobe
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Astronomical Basis
A month on the Islamic calendar begins at the time of conjunction. This is when the moon is located on a straight line between the Earth and the Sun. Usually the first day of the month is determined by the sighting of the lunar crescent during sunset. However, this practice can make it hard to determine whether or not the crescent moon will be visible from a certain position on the globe. Also there may be other extraneous factors that limit the viewing of the moon such as weather conditions and altitude. The beginning of the month varies from country to country depending on when the crescent moon is seen.
There are 354.37 days in one year. The number of days in a month is the average duration of the rotation of the Moon around the Earth, which is 29.53 days. Therefore the months range from having either 30 days or 29 days.
Crescent Moon During Conjunction Viewed from Jordan
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The Tabular Islamic Calendar
Because of the 29.53 days in one month, there are about 44 min left over when the month is made either 29 or 30 days long. This left over time adds up to a day in 2.73 years. The tabular calendar includes an extra day every three years, making that year 355 days compared to the regular year of 354 days. It is similar to the leap year in the Gregorian calendar which takes place every 4 years. The tabular calendar used a more mathematical approach, as opposed to the traditional astronomical and observational approaches.
Lunar Crescent
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Sources
http://en.wikipedia.org/wiki/Islamic_calendar