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A simple count of the days in a year shows that a common year contains 354 days and a leap year contains 355 days. In a cycle of 30 years, there are thus 19 common years and 11 leap years, and therefore (35419) + (35511) = 6,726 + 3,905 = 10,631 days. The Islamic leap year rule functions like a kind of Metonic cycle rule, in that it defines an equivalence between a certain number of years and a certain number of lunations. Specifically, there are (1230) = 360 lunations in a 30-year cycle. The synodic period of the moon is 29.53059 days, and (36029.53059) = 10,631.0124 days. The agreement between the two values is thus fairly close. |
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A major consequence of a strictly lunar calendar is the fact that no attempt is made at producing equivalence between the length of the calendar year and the actual length of the solar year. The months of the Islamic calendar thus do not occupy fixed dates with respect to annual seasons and slowly migrate through all seasons of the year, because the Islamic year is approximately 11 days shorter than the actual solar year. |
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Thus, the Islamic calendar tracks the moon fairly well, but fares poorly with the sun. A calendar such as the Persian Solar calendar, by contrast, tracks the sun quite well, but essentially ignores the moon. |
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It should be remembered that local climatic and geographic conditions may play a strong role in the construction of a calendar. The four seasons familiar to dwellers in temperate zones are not universal features in all lands. Areas closer to the equator generally experience a wet/dry alternation, in which the association of a particular month with a particular season and its characteristics is simply not as important. Second, the desert areas in which the Islamic calendar originated experience much less cloud cover, making the sighting of first lunar crescents more a matter to be routinely expected than a source of marvel. |
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The Islamic calendar defines a day as beginning at sunset. This rule is a common one in many parts of the world and is especially common among cultures in the Middle East. Geography again is a factor. Areas closer to the equator experience less variation in the length of night and day over the course of a year than do areas at higher latitudes. Special rules for the definition of day are required whenever such calendars are used in areas of higher latitude. Note that there is a general correspondence, but not exact equivalence, between a date in the Islamic calendar and a date in the Gregorian calendar. |
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The epoch of the calendar is the date corresponding to A.D. 622 July 16. The date marks the first visibility of the lunar crescent of the first month of the year in which the Hegira of the prophet Mohammed from Mecca to Medina occurred. There are several errors and misconceptions surrounding the word, its meaning, and the events it describes. These are treated at greater length in the following section. |
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