KDE Core/Astronomical Calendars/Islamic

Introduction

The Islamic calendar (or Hijri calendar) is a lunar calendar. It contains 12 months that are based on the synodic motion of the moon, which makes an Islamic year to have 12 x 29.53=354.36 days, the Islamic calendar is consistently shorter (11 Days) than a solar year, and therefore it shifts with respect to the Gregorian calendar. There has been a reference in the Quran about the number of months in an Islamic year: "Surely the number of months with Allah is twelve months in Allah's ordinance since the day when He created the heavens and the earth."

Months

The 12 months of the Islamic calendar are:

 1. Muharram
 2. Safar
 3. Rabi' al-awwal (Rabi I)
 4. Rabi' al-thani (Rabi II)
 5. Jumada al-awwal (Jumada I)
 6. Jumada al-thani (Jumada II)
 7. Rajab
 8. Sha'bad
 9. Ramadan
10. Shawal
11. Dhu al-Qi'dah
12. Dhu al-Hijjah

The month starts when the lunar crescent is first seen after the new moon. This sighting is, however, subjective and depends on factors such as weather, the optical properties of the atmosphere, and the location of the observer. Furthermore, some Muslims depend on a local sighting of the moon, whereas others depend on a sighting by authorities somewhere in the Muslim world. Both are valid Islamic practices, but they may lead to different starting days for the months.

Year

The Era for the Islamic calendar begins on the evening of the Prophet Mohammad's flight from Mecca to Medina. It is believed that the migration occurred at the time of the first visible crescent of the New Moon, on the first day of the month of Muharram, or 16 July 622 AD (Julian Calendar). The word "flight" in Arabic is the Hijra, so the Era of the Moslem calendar is called that of the Hijra or, in English, the Hegira which defines the epoch of the Islamic calendar in terms of "AH," the Anno Hegirae.

Criteria for Lunar Crescent Visibility

The main area of concern regarding the implementation of the Islamic Calendar is the Lunar Crescent Visibility or Hilal. This has been the topic of various debates and there hasn't been a commonly agreed global consensus regarding the accurate sighting of the Hilal across the globe. S.K. Shaukat defined certain criteria for the lunar crescent visibility, which are:

   Lunar altitude > 4.1 degrees
   Arc of light, i.e., the elongation of the Moon from the Sun or the apparent angular distance > 10.6 degrees
   Age of the moon should be between New Moon and First Quarter

This criterion has been used in a lot of astronomical functions and have made calculations approximate at best for many reasons.

MABIMS have defined another criteria for lunar crescent visibility in Brunei, Indonesia, Malaysia and Singapore; which is:

   Lunar altitude > 2 degrees
   Arc of light > 3 degrees
   Age of the moon > 8 hours

MUIS further improvised on this criteria for Singapore and modified the values as:

   Lunar altitude > 6 degrees
   Arc of light > 7 degrees
   Age of the moon > 16 hours

Apart from the aforementioned lunar criteria, the subject of the crescent visibility has led to the concept of the International Lunar Date Line(ILDL). The ILDL is a curved line on a world map which separates areas (west of the line) where the crescent is likely to be seen at the start of the lunar month from areas (east of the line) where the crescent is unlikely to be seen. The position of the ILDL moves from month to month.