In many Islamic countries the religious calendar is actually based on more sophisticated algorithms which predict the first visibility
of the lunar crescent by rigorous calculation of the moons position with respect to the sun and the observers horizon. The
thus predicted dates can differ up to one or two days with those of the tabular Islamic calendar described here.
- The Umm al-Qura Calendar of Saudi Arabia the official calendar of Saudi
Arabia and several neighbouring states. This web site includes a calendar converter valid from 1356 AH (1937/1938 CE)
to 1500 AH (2076/2077 CE).
- The Islamic Calendar of Turkey the official Islamic calendar of Turkey
and Turkish communities living abroad. This web site includes a calendar converter valid from 1318 AH (1900/1901 CE) to 1444 AH
- Predicting the First Visibility of the Lunar Crescent this web site provides
detailed global visibility maps for the first visibility of the lunar crescent for the years AH 1430 to 1440 (2009 to 2019 CE).
- Hilal Sighting Committee of North America announces the start
of the current Islamic month based on the observed and calculated visibility of the young lunar crescent.
- Diyanet İşleri Başkanlığı web site of the Turkish Presidency of
Religious Affairs which provides dates for the Islamic lunar calendar based on the calculated visibility of the young lunar crescent.
- Islamic Crescents Observation Project (ICOP) comprehensive
web site which collects observations of young lunar crescent sightings and gives monthly predictions for the visibility of the lunar crescent.
- Moon Watch web site by HM Nautical Almanac Office which provides predictions for the first visibility of the young lunar crescent. The
predictions of previous new moon crescents are compared with sightings reported by various observers around the world. The predictions
are based on the 1997 Yallop criterion.
- Crescent Watch web site by the Zaytuna Institute with predictions for the first visibility of the lunar crescent based on the 1997 Yallop criterion.
- The Islamic Calendar web site
of Helmer Aslaksen on the regulation of the Islamic lunar calendar in Singapore and in many other Islamic countries.
- Calendrica based on
the calendar algorithms described in E.M. Reingold & N. Dershowitz, Calendrical Calculations: The Millennium Edition
(Cambridge, 2001) and E.M. Reingold & N. Dershowitz, Calendrical Tabulations 1900-2200 (Cambridge, 2002).