The Islamic calendar of the Hijra: twelve lunar months without corrections
The Hijri calendar: a 354-day year, the twelve months from Muharram to Dhu al-Hijja, the sacred months, the Hijra era of 622 and sighting the hilal.
Among the world's great calendars, the Islamic one is the only widely used calendar that is purely lunar. It adds no months to chase the Sun: its twelve months last 29 or 30 days and the year has 354 or 355 days. As a result every date slips back by about eleven days relative to the Gregorian calendar, and in roughly thirty-three years it passes through all the seasons. It is a precise religious choice, rooted in the Qur'an.
The Qur'anic roots
Verse 36 of the ninth sura states that «the number of months with God is twelve» and that four of them are sacred. Tradition identifies the sacred months as Muharram, Rajab, Dhu al-Qa'da and Dhu al-Hijja, periods in which hostilities were suspended in pre-Islamic Arabia. The following verse condemns the nasi', a term that referred to the shifting or adding of months: its exact meaning is debated, but the outcome was the abandonment of all intercalation.
Many historians believe that pre-Islamic Arabia used a lunar calendar corrected from time to time by an added month, which kept fairs and pilgrimage in relatively fixed seasons. Once that practice was abolished, the pilgrimage to Mecca, the Hajj, and the fast of Ramadan began to rotate freely through the whole solar year.
The era of the Hijra
The count of years was introduced some years after the Prophet's death, under the caliph Umar, around AD 638. The starting point chosen was the year of the Hijra, the migration of Muhammad and his companions from Mecca to Medina, which took place in 622. Year 1 begins on the first day of Muharram, conventionally equated with 16 July 622 in the Julian calendar; dates are marked AH, from the Latin anno Hegirae.
The twelve months are Muharram, Safar, Rabi' al-awwal, Rabi' al-thani, Jumada al-ula, Jumada al-akhira, Rajab, Sha'ban, Ramadan, Shawwal, Dhu al-Qa'da and Dhu al-Hijja. Some names preserve traces of the seasons of the old Arabian calendar: Rabi' recalls the spring of the pastures and Ramadan, according to a widespread etymology, intense heat. Today those references no longer bear any relation to the climate.
Observation and calculation
The start of the month is traditionally tied to the sighting of the hilal, the first crescent after the new Moon. Since visibility depends on place, atmosphere and the Moon's height above the horizon, different countries may begin the same month a day apart. Alongside observation there are calculated calendars: Saudi Arabia uses the Umm al-Qura calendar, based on astronomical criteria, for civil purposes.
Medieval astronomers also developed a tabular calendar, with a thirty-year cycle in which eleven years have 355 days. It served administrative and historical purposes, and it still allows Islamic dates to be converted into Christian ones with good approximation. It does not, however, replace the tradition of observation, which remains decisive for the start of Ramadan and of the two great festivals of the year.
For Islamic astronomy, the need to predict the visibility of the crescent was a powerful stimulus. Scholars such as al-Battani and al-Biruni tackled the problem with refined mathematical tools, and astronomical tables, the zij, often included chapters devoted precisely to the new Moon. A calendar born of naked-eye observation thus came to nourish one of the richest scientific traditions of the Middle Ages.