There is no reference in the Quran to the pre-Islamic system of anwa_ in which the year is divided into precise periods on the basis of the rising and setting of certain stars. According to tradition, this system was considered anathema in Islam. The most relevant Qur_anic allusion to calendar-related computation is to the stations of the moon (manazil al-qamar, 10:5, 36:39). There are twenty-eight such stations defined on the basis of a combination of the pre-Islamic system of anwa_ with the lunar stations system.
The official Islamic calendar is lunar, with year one coinciding with the year 622 C.E., the date of Muhammad’s migration (hijra) from Mecca to Medina. This calendar was adopted during the reign of the second caliph _Umar. The Hijri lunar calendar is used as the basis for computing the official months (ahilla, new moons), and for determining the dates for important religious activities such as fasting and pilgrimage. The lunar months alternate between twenty-nine and thirty days, and the lunar month retrogrades yearly by about eleven days. Although t e beginning of the lunar month is determined by sight ng the new moon, numerous methods were developed to compute the exact length of the lunar months, to determine the days of the lunar year in relation to the solar year, and to perform calendar conversions between different eras.
Initially, folk astronomy and nonscientific traditions provided handy methods for solving problems related to the regulation of the lunar calendar and the determination of the times of prayer. Folk astronomical methods, such as the observation of the lunar crescent and the use of simple arithmetical shadow schemes, were used even after the introduction and dissemination of sophisticated scientific methods.
A more mathematical approach to timekeeping developed as Muslims acquired and developed skills in mathematical astronomy. Although the computations of astronomers may have initially been appreciated only by a small group of scientists, their methods eventually supplanted the simple methods of folk astronomy. The establishment of the office of a mosque timekeeper (muwaqqit) illustrates the official recognition, by the religious institution, of the authority of the exact-scientific methods of astronomers in the fields of calendar computation and the determination of times of prayer. With the rise of the office of the timekeeper in the thirteenth century, the technical knowledge of the astronomers became more accessible because the compilation of extensive tables made the results of the exact-mathematical methods more readily usable. The science of timekeeping (_ilm al-miqat) was thus an area of investigation where religion and science intersected.
Timekeeping tables, first compiled in Baghdad in the ninth and tenth centuries, were later expanded by timekeepers employed at the major mosques of Syria and Egypt to include hundreds of thousands of entries. In contrast to earlier Greek sources, Islamic astronomical handbooks often started with discussions of calendar computations and conversions between different eras (for example, Persian, Coptic, Syriac, Chinese-Ughur, Jewish, and Hindu calendars). In addition to the basic computational techniques, numerous works also provide additional information covering calendarrelated subjects such as the length of day and night; patterns of weather and wind; dates and descriptions of Christian, Jewish, and Indian festivals; and agricultural practices at various times of the year. Another problem of timekeeping that was addressed in various astronomical treatises is the problem of crescent visibility. The lunar month starts right after sunset with the sighting of the crescent. The visibility of this crescent, however, is itself a function of several variables, including the celestial coordinates of the sun and the moon, the latitude of the place where the crescent is sighted, and the brightness of the sky. Various methods were devised to determine the conditions under which the crescent would be visible.