Islam and the measurement of time
Mohammed’s era begins on July 16th 622 CE and is calculated according to lunar cycles. The Islamic calendar year being shorter than the lunar year, and even shorter than the solar year, Muslims were rapidly obliged to adapt its use.
Dominique Fléchon
The development of time measurement in Islam has a particular place in history. At the time of Mohammed, western civilisations had been observing the Julian calendar, based on the 12 lunar months and aligned to the sun by order of Julius Caesar, for almost 600 years. During this same period, the Bedouins who traded with India, Iran, Syria and Egypt borrowed from their neighbours a calendar that sought to combine solar and lunar months by inserting intercalary months. Curiously enough, Mohammed transformed this calendar, which roughly corresponded to the seasons, into a strict lunar calendar.
Mohammed’s era begins with the migration of the prophet from Mecca to Medina (Hijra) on July 16th 622 CE and is calculated according to lunar cycles. The Islamic calendar year has 354 days divided into 12 months of 30 or 29 days, whichever is closest to the time taken for the moon to make one complete revolution of the earth. However, there is a difference of 8 hours, 48 minutes and 36 seconds between the calendar year, thus defined, and the movement of the moon.
Concessions to practical use
It is unlikely that Mohammed imagined the consequences this reform would have on daily life. The calendar year being shorter than the lunar year, and even shorter than the solar year, harvest taxes were sometimes levied twice in one year. On a religious level, the festival of Ramadan no longer fell on the same dates each year, to the point that some years it was celebrated in summer and others in winter. Despite the fact this calendar was consecrated by certain verses of the Koran, Muslims were rapidly obliged to make concessions in both their daily lives and their astronomical observations. The difference between the calendar year and the position of the moon meant that 11 days were added over a period of 30 years so that the calendar and astronomical years would coincide. Cyclically over the course of this period, the 2nd, 5th, 7th, 10th, 13th, 15th, 18th, 21st, 24th, 26th and 29th years were leap years of 365 days.
A system that suffices for everyday life does not necessarily suit business matters, which rapidly adopted the Greek solar year. Meanwhile, Arab astronomers preferred the Persian solar year of 366 days for their observations.
Arab conquests brought them into contact with more civilised peoples which enabled them to rapidly expand their scientific knowledge. This also obliged them to adopt a solar-based system alongside their changing lunar calendar. This is why Oriental calendars give the date in the Egyptian or Syrian solar year in addition to the Arab lunar year.
Mutual influences
Arab invasions of Syria, Mesopotamia, Egypt and Persia began in 632. By 680 they had reached the Atlantic coast. In 710 they conquered Spain from where they continued to central France. The Arab conquest, whose progression north was halted in 732, brought new knowledge both to Arab civilisation and that of the countries they conquered. Always a minority, the Muslims protected their economic interests by maintaining Christians and Jews in important positions in finance and medicine.
They strove to assimilate civilisations more advanced than their own. This enabled them to develop their understanding of astronomy, adding their own knowledge to that of the Chaldeans, the Greeks and the first European astronomers.
Settled in Spain from 747, they developed schools in Cordoba, Granada and Toledo. Gerbert d’Aurillac, elected Christian pope in 999, studied mathematics and science in Catalonia where he was able to draw on Arabic knowledge. In the late tenth century, Tabit Ben Kourra studied the art of sundials. In 1203 Ibn Ridwan wrote a treatise on water clocks and in 1206 described automata and clepsydras, including several that gave astronomical indications.
February 15th 1332 becomes March 1st 1917
In 1308 Ibn al Fahham and Abou’l Hassan Ali ben Ahmed built the Tlemcen water clock with its automata and astronomical indications. The water clock whose remains still stand high in the medina in Fez dates from 1357. As was typical of Iranian clepsydra, automata probably appeared in each of the twelve windows. The hours were also struck on bells. In parallel, the Arabs developed fire clocks which, as with their clepsydras, they decorated with automata.
While the Islamic calendar, because it only partially integrated the sun, remained an archaic system, the Muslim world made considerable progress in its understanding of astronomy and in the development of water clocks and sundials. By allowing western civilisations access to their science, they enabled them to make rapid advances in cosmography and the making of sundials. It was, however, still too early to envisage the first mechanical clocks with weights and escapement, which appeared in Europe from the late thirteenth century.
Despite the difficulties it implied, the Islamic calendar remained in use until recent times. The Turkish financial year (none other than the Julian calendar year) was introduced to the Ottoman Empire in 1840. The Gregorian calendar replaced the Islamic calendar in the early twentieth century: thus February 15th 1332 became March 1st 1917. ■
(1) Water clock by AI Jazari. A mahout sits astride the elephant’s neck, holding a stake in his left hand and brandishing an axe in his right. The elephant is carrying a quadrangular platform covered by a canopy and, on top of this, a turret on which a bird is perched. Fifteen openings around the dome, each the size of a dirham, are covered inside by a silver ring, half black and half white. Modern reproduction Musée d’Horlogerie du Locle, Switzerland. Photo G. Savini, Le Locle.
(2) Gaza clepsydra with automata. Replica by Diels. The water clock at Fez was very probably made to the same Iranian method.
(3) Arab-Andalusian astrolabe. An essential navigational tool, the astrolabe was completed by new geometric projection systems.
