The Islamic Calendar
The Islamic calendar, also known as the Muslim or Hijri calendar, is utilized by Muslims to determine the day on which to celebrate Islamic holy days. It's the official calendar in countries around the Gulf region, including Saudi Arabia, although other Muslim countries use the Gregorian calendar for civil purposes and also the Islamic for religious ones. As with the Hebrew calendar, these holy days are fixed on the exact same day of their calendar each and every year, even though they appear to shift in Gregorian terms simply because the two calendars aren't synchronized.
This calendar is really a lunar one - the only widely utilized lunar calendar in the world - and is based purely on the cycles of the Moon. Every month starts on the day of the new moon, but although most lunar calendars use precise astronomical calculations for this, the Islamic one has retained an observational definition of the new moon, starting the new month when the crescent moon is initial seen. This makes it impossible to be certain in advance of precisely when a particular month will begin, which means the initial day of Ramadan is never known ahead of time, even though it could be estimated within a day or two simply because the astronomical new moon could be calculated. If the weather is cloudy in Saudi Arabia when the new moon is expected, individuals are sent up in the sky in airplanes to observe the moment the moon appears.
The Islamic year is made up of 12 synodic months. As a synodic month is 29.5306 days - or just over 29 days, 12 hours, 44 minutes; in length, the months generally alternate between 29 and 30 days, even though this isn't specifically prescribed, as a month's length depends on when the next new moon is sighted.
This makes their year 354 days in length; just over 11 days shorter than a solar year - so the calendar isn't fixed to the seasons. This means that the spring equinox, for instance, will fall on a various date each and every year, and also the seasons will drift out of sync with the calendar dates. For instance, the equinox on March 21, 2005, Gregorian, fell on Safar 10; March 21, 2006, fell on Safar 21; and March 21, 2007, is on Rabi I 2.
In a lunar calendar, the pattern of the lunar months will repeat each and every 33 years, and holy days will fall in the exact same season and on the exact same Gregorian day as they did 33 years ago. Thus although occurring on the exact same day in the Islamic calendar each and every year, a holy day that falls in the middle of winter one year will probably be celebrated approximately 11 days earlier seasonally each and every year after that, occuring in the middle of summer 16 years later, and not falling at midwinter for a further 16 or 17 years.
As the Islamic year is 11 days shorter than a solar year, it advances more rapidly than a Gregorian year, so 33 Islamic years correspond within a couple of days to 32 Gregorian years. For instance, in the Gregorian year 1968, the month of Ramadan began on November 22, just prior to the begin of winter in the Northern Hemisphere. Sixteen years later, in 1984, Ramadan began on June 1, the begin of summer. In 2000, Ramadan began on November 27. Thirty-two Gregorian years passed in this cycle, although 33 Islamic years did. (All conversions from the Islamic calendar to the Gregorian are accurate to within a day simply because the begin of a new month is dependent on the new moon being observed, not calculated.)
The passing of time is also marked in a various way in the Islamic calendar. The calendar utilized today was introduced in 638 c.e. by Umar ibn Al-Khattab, a companion of the Prophet Muhammad, in an attempt to rationalize the numerous systems utilized until then. It was retroactively decided that the calendar would begin from the year 1, or 622 c.e. in Gregorian time, which is when the Hijra occurred. The Hijra, which chronicles the migration of Muhammad from Mecca to Medina, is the central historical event of early Islam.
Prior to this a lunisolar calendar was utilized, with an intercalary month inserted as needed to maintain in step with the seasons. Nevertheless Muhammad forbade the intercalary month, releasing the calendar from the seasons. The initial day of the Islamic calendar, Muharram 1 in the year 1 a.h., corresponds to July 16, 622 c.e. in Gregorian years. Years are designated either "h" or "a.h." - the latter being an abbreviation of the Latin Anno Hegirae, meaning "in the year of the Hijra." In the year 2007 in the Gregorian calendar, 1,385 years will have passed since the advent of the Islamic calendar, but 1,427 years will have passed in the Islamic.
Simply because of its acknowledgment of the starting date of the new Muslim chronology, the Islamic calendar has a deep religious and historical significance, and is more than just a method to date religious events and also the passing of time.