The Islamic Calendar

Astronomers at the Istanbul Observatory
Astronomers at the Istanbul Observatory

Islamic Astronomy Software

The Islamic calendar is based on visibility of the crescent Moon. This is a very difficult scientific problem. We know perfectly well where the Moon and the Sun are at any given time, but how light must the Moon be and how dark must the sky be before we can see the crescent? And what if the weather is bad?

If you want to get a reasonable estimate for when the Moon will be visible, the are several nice pieces of software available.

HM Nautical Almanac Observatory in the UK makes very nice lunar visibility charts. Global First Sighting of New Crescent Moon information is given for the last couple of years. The charts for the current month are also available at Moon Watch.

For many years I have used MoonCalc, developed by Dr. Monzur Ahmed. Unfortunately it is a DOS program, and the lunar visibility charts are done in DOS graphics full screen, so under Windows XP I can no longer capture screenshots.

The web page of the Islamic Crescents' Observation Project (ICOP) has a number of interesting articles. They also host the software Accurate Times by Mohammad Odeh. It is a Windows program for the Islamic calendar, Qiblah direction, and prayer times. There are samples of the lunar visibility charts on Visibility of Ramadan Crescent.

Rules for the Islamic Calendar in Different Countries

In spite of the difficulties, some Muslim communities base their calendar on actual Moon sightings. This is done in India, Pakistan, Bangladesh and the US.

In Saudi Arabia and most of the Gulf countries they have simplified the calendar. They start the lunar month if the Moon sets after the Sun on the 29th day of the previous month, as seen from Mecca. If you don't believe this, please check out The Actual Saudi Dating System discussion on the web site of the Islamic Crescents' Observation Project (ICOP).

In Egypt they require moonset to be at least 5 minutes after sunset.

MoonCalc image
MoonCalc image

The Islamic Calendar in Singapore

Before 1974, the Muslim calendar in Singapore was based on sightings from Sultan Shoal, the southernmost part of Singapore. MUIS (Majlis Ugama Islam Singapura), the Islamic Religious Council of Singapore, then decided that the new month starts if at sunset on the eve of the 29th day the Moon is above the horizon. (The Muslim calendar in Singapore is based on latitude 1 20' 34'' N and longitude 103 51' 08'' E.) In the 80s they decided to follow a variation of the 1978 Istanbul criteria and require that the altitude of the Moon should be more than 5 degrees at sunset. In the 90s they switched to 2 degrees. This was part of an attempt to coordinate the major holidays with Malaysia, Brunei and Indonesia, which occasionally may lead to different results.

This is part of the MABIMS criteria, which require that

  1. The altitude of the Moon at sunset is more than 2 degrees.
  2. The arc of light (elongation of the Moon from the Sun or the apparent angular distance) is more than 3 degrees.
  3. The age of the Moon is more than 8 hours.

In practice, however, MUIS only looks at the first of the criteria.

MUIS is the sole and final authority on the Muslim calendar in Singapore. It is very convenient that they use predictions and let the public know their criteria. Unfortunately, these criteria are not based on science. The minimum possible values of the three variables in the MABIMS criteria are approximately:

  1. Altitude > 6
  2. Arc of light > 7
  3. Age > 16

The article Re-evaluation of Hilaal Visibility in Indonesia by Thomas Djamaluddin gives interesting background to the MABIMS criteria. He looks at 38 reported sightings from religious courts. He rejects 27 of them because they claimed to have observed the Moon after the time of moonset or similar problems, but he (somewhat generously in my opinion) gives the remaining 11 the benefit of doubt.

Islamic calendar for Singapore by Majlis Ugama Islam Singapura (MUIS), the Islamic Religious Council of Singapore. MUIS seems to move the location of their calendar around from year to year. If the link doesn't work, go to their homepage and look under "Downloads" or do a search for Takwim (or Taqwim). This will give you MUIS's prediction for first visibility of the lunar crescent. But it is based on a dubious method, so you will often not be able to see the Moon on the day when MUIS claims that you will.

Astrolabe by Norman Greene
Astrolabe by Norman Greene

The Date of Eid ul-Fitr (Hari Raya Puasa) in 2000

Eid ul-Fitr (Hari Raya Puasa) was celebrated in Singapore on January 8 2000. To see why this did not correspond to an actual sighting, you can consult Ramadhan & Eid ul-Fitr 1420AH (1999/2000) by Dr. Monzur Ahmed.

Eid ul-Fitr (Hari Raya Puasa) was celebrated in Singapore on 27 December 2000. To see why this did not correspond to an actual sighting, you can consult Ramadhan & Eid ul-Fitr 1421AH (2000CE) by Dr. Monzur Ahmed.

The Arithmetical Calendar

Some sources describe an arithmetical (tabular) Islamic calendar. It is sometimes used for approximate conversions for civil purposes, but is not used for religious purposes by Sunnis or Twelver Shi'ites (Ithna Asharia). However, it is common among Sevener Shi'ites (Isma'ili), including the Bohras (Musta'lis) and Nizaris (Isma'ili Khojas, Aga Khanis). It seems to have been designed to be closer to new Moon than to the first visibility of the lunar crescent, so it often runs a day or two ahead of the regular Islamic calendars. There are currently about one million Bohras and about 15 millions Nizaris, compared to over a billion Sunnis and close to a hundred million Twelver Shi'ites. Both of these groups are today primarily Indian Muslim groups, but they trace their history from the Fatimid Caliphate that ruled Egypt from about 970 to 1171. The calendar was put into practice by Imam al-Hakim (985-1021) and is therefore sometimes referred to as the Fatimid or misr (Egyptian) calendar. The calendar is sometimes attributed to the famous astronomer Al-Battani (850-929) and an alternative version to Ulugh Beg (1393-1449). It is also sometimes referred to as hisabi. It is possibly also used by the Qadianis (Ahmadiyyas), but they also seem to use a solar calendar, and they are not considered Muslims by other Muslims.

The average lunar year is about 354 11/30 days, so you get a reasonable lunar calendar by using a cycle of 11 leap years (kasibah) with 355 days in a 30 year cycle. The odd numbered months have 30 days and the even numbered months have 29 days, except in a leap year when the 12th and final month has 30 days. There are several versions for how to space out the 11 leap years. The most common rule is the one followed by the Nizaris Isma'ili, which uses years 2, 5, 7, 10, 13, 16, 18, 21, 24, 26, 29, but some replace 16 by 15 and the Bohras replace 7 by 8, 18 by 19 and 26 by 27. Imran Hussain has both a Bohra and a Sunni version of his Islamic Diary. For more information about the different leap year rules, see Islamic-Western Calendar Converter (Based on the Arithmetical or Tabular Calendar) by Robert H. van Gent.


Qibla - the direction to Mecca

Determining the direction to Mecca was together with determining the first visibility of the lunar crescent and computing prayer times one of the central problems in Islamic astronomy. It requires a clear understanding of spherical trigonometry and knowing how to find longitude. However, you would think that at the moment it would be a trivial computation. I was therefore quite surprised when I started to receive e-mails from Muslims in the US and Canada asking me about it. I then realized that it is a major controversy among Muslims in North America.

All the famous Muslim astronomers and mathematicians in the past, like al-Khwarizmi (780-850), al-Battani (858-929) and al-Tusi (1201-1274) agreed that qibla should be measured by great circle. However, some people who are used to looking at Mercator maps and don't understand that lines of sight are represented by great circles argue that you should draw straight lines on a Mercator map, i.e., follow rhumb lines. They strongly attack people who follow the traditional method of great circles. For examples of this, see Direction of Al-Qiblah, Determining the direction of the Qibla and For a scientifically correct discussion, please see, the excellent paper The Correct Qibla by Kamal Abdali or Which Way Is Jerusalem? Which Way Is Mecca? The Direction-Facing Problem in Religion and Geography by Daniel Z. Levina.

Prayer Times

I have a separate page of additional calendar links.

Calendar Measurement

Back to Helmer Aslaksen's page on Calendars in Singapore.

Helmer Aslaksen
Department of Mathematics
National University of Singapore

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