The Islamic Calendar
The Gregorian date (Anno Domini/Common Era) is the present Christian/World calendar based on birthdate of Christ; the Hijri date (Anno Hejira) is the lunar Islamic calendar based on the year in which the Mohammad moved from Makkah to Madinah, the 'Hijrah'. The date 1 A.H. corresponds to either 15th (astronomical epoch) or 16th July (civil epoch), in the year 622 A.D./C.E. so the astronomical date is one day after the civil date. The moon rises progressively later than the sun for locations further west, so western Muslim countries are more likely to celebrate a holy day one Gregorian day earlier than eastern Muslim countries.
Day of the week:
The Islamic calendar is purely lunar and consists of twelve alternating months of 30 and 29 days, with the final 29 day month extended to 30 days during leap years. Leap years follow a 30 year cycle and occur in years 2, 5, 7, 10, 13, 16, 18, 21, 24, 26, and 29. Days are considered to begin at sunset.
Problems with the Islamic Calendar.
Islamic months begin at
on the day of
of the lunar crescent. Even though visual sighting is necessary to determine the start of a month, it is useful to accurately predict when a crescent is likely to be visible in order to produce lunar calendars in advance. Although it is possible to calculate the position of the moon in the sky with high precision, it is difficult to predict if a crescent will be visible from a particular location. Visibility depends on a large number of factors including weather conditions, the altitude of the moon at sunset, the closeness of the moon to the sun at sunset, the interval between sunset and moonset, atmospheric pollution, the quality of the eyesight of the observer, whether or not optical aids are used.
There is also a problem as to whether 1 Muharram, 1 AH is assumed to be 15 July, 622 CE (known as the astronomical or Thursday epoch) or 16 July, 622 CE (the civil or Friday epoch. Our converter uses the civil epoch.
Khalid Shaukats Suggestion for a World Islamic Calendar.
This can be pre-calculated with a convention that if the moon is born before 12:00 UT (i.e. 15:00 Makkah time), then the month begins at sunset of that day everywhere in the world (starting from International Date Line (IDL), where the moon would be born before that day starts at IDL. This moon will be about 18 hours old at a point just East of International Date Line at local sunset. If the moon is born after 12:00 UT, then the month begins on the next day's sunset everywhere. You will not need any calculations for this. Only the time and date of new moon birth is needed, and that is easily obtained from Almanacs, Newspapers, or the Internet. This convention has a basis of visibility at a point just East of the IDL, and is born before the day begins at just West of the IDL. [A lunar IDL has been proposed but, since this is not only curved but also not a fixed line, this only serves to complicate the situation].