sometimes with 29 days and sometimes with 30.” (Reported by Al Bukhari, vol. The months are sometimes like this and sometimes like that, i.e. If you are asked about the phases of the moon, tell them: ‘They are marks in time, intended for men and for fixing the pilgrimage.'” (Koran, Surat 2, verse 189).Īs stipulated by the Prophet (saws) in several Hadiths, the Hegirian month begins with the visibility of the new moon. The celestial clock is thus fashioned, thanks be to God. It is known that, over a cycle of 30 years of the Islamic calendar, 19 years have 354 days and 11 years have 355 days (years of abundance). Calculation allows us to predict whether a given month will have 29 or 30 days. Consequently, the months alternate between 29 and 30 days. The synodic month is a fraction of the number of days. The sidereal month is of no importance for the Islamic calendar. Thus the moon reverts earlier to its position with respect to the background of fixed stars. The sidereal month is shorter than the synodic month because the earth drags the moon along with it in its orbit around the sun. We note that the sidereal month, that is, the duration after which the moon reverts to the same position with respect to the celestial sphere (the background of stars) is 27.31 days. The phases of the moon and thus the Islamic calendar are linked to the synodic month. The synodic month, that is, the duration after which the moon reverts to the same position with respect to the sun is 29.53 days. This means that it follows the revolution of the moon around the earth and that of the earth around the sun. In order to elaborate an Islamic calendar both scientifically exact and faithful to the Sharia it is necessary to take simultaneously into account certain astronomical facts and the teaching of the Prophet (saws).

I The problem: Specificities of the Islamic Calendar

Islamic calendar months in order