Science in the Service of Religion: The Case of Islam

by David A King Published on: 28th October 2021

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In Islam, as in no other religion in human history, the performance of various aspects of religious ritual has been assisted by scientific procedures. The organization of the lunar calendar, the regulation of the astronomically defined times of prayer, and the determination of the sacred direction of the Kaaba in Mecca—these are topics of traditional Islamic science still of concern to Muslims today, and each has a history going back close to fourteen hundred years. But the techniques advocated by the scientists of medieval Islam on the one hand and by the scholars of religious law on the other were quite different, and our present knowledge of them is based mainly on research conducted during the past twenty years on one small fraction of the vast literary heritage of the Muslim peoples.



Note of the Editor: This article has been extracted fromScience in the Service of Religion: The Case of Islam.” King, David A., Impact of Science on Society, v40 n3 p245-62 1990, with the permission of the author.


Most historians of Islamic science have concentrated on scientific knowledge which was transmitted to the West; by so doing they have tended to overlook the essence of Islamic science. Indeed, most modern accounts of science in the medieval Islamic world, whether by Western or Muslim writers, have ignored what may well be called the Islamic aspects of Islamic science. These have been researched recently, using the vast amount of relevant medieval Arabic manuscripts available in libraries around the world; most of the results of this research have appeared in scholarly journals not easily obtainable outside academic libraries. The time is therefore ripe for an overview. In fact, this article is the first attempt in the non-scholarly literature to survey the way in which science, particularly astronomy, has been used for purposes relating to Muslim religious life for well over a millennium. Even so, it is not an overview of Islamic astronomy in general, for it deals with only three of the many topics dealt with by the scholars of medieval Islam.

To understand Muslim activity in this domain we must realize that there were two main traditions of astronomy in the Islamic Near East, folk astronomy and mathematical astronomy. Folk astronomy, based on naked-eye observation of celestial phenomena and devoid of theory or computation, has generally been overlooked by historians of science with their predilection for hard-core scientific achievements. Yet, as we shall see, it was far more influential in Islamic society than mathematical astronomy, which as the name indicates was based on systematic observation, theory and mathematical procedures.

A historical investigation of the Islamic aspects of Islamic science provides answers to several questions. First, why is there so much confusion in the modern Islamic world about the determination of the beginning of Ramadan, the sacred month of fasting? Second, why are there five prayers in Islam? These are not specifically prescribed in either the Koran, the ultimate source of Islamic sacred law, or the hadith, the literature dealing with the sayings and practice of the Prophet Muhammad, the second main authority for the sacred law of Islam. Third, why are medieval mosques invariably not oriented properly towards Mecca? This problem has often puzzled those few historians of Islamic architecture who have taken the trouble to measure mosque orientations, but, as we shall see, it is now largely resolved, thanks to the evidence of newly discovered medieval texts. Furthermore, these texts cast new light on the significance of the Kaaba itself and on its original function.

Folk astronomy and mathematical astronomy

The Arabs of the Arabian peninsula before Islam had an intimate acquaintance with the sun, moon and fixed stars, the seasons, the changing night sky and weather patterns throughout the year. Since the sun, moon and stars, as well as the winds and rains, are mentioned in the Koran, a truly Islamic cosmology (quite independent from the tradition adapted from Greek sources by Muslim scientists) developed in the vast corpus of the Koranic commentaries and in separate treatises on the glory of God as revealed by His creation. Since, in addition, the Koran encourages Man to use the stars for guidance, a basic knowledge of the heavens was considered advantageous. Folk astronomy, based on what could actually be seen in the sky throughout the year and innocent of any underlying theory or associated computus, thus became widespread in the Islamic Near East and remained so throughout medieval times. The basics of this subject are outlined in encyclopaedias and a series of special treatises compiled over many centuries, and its application to religious needs is discussed in books dealing with the sacred law of Islam.

The period from the eighth to the fourteenth or fifteenth centuries saw the flourishing in the Near East of a different kind of astronomical knowledge. Muslim astronomers, heirs to the sophisticated astronomical traditions of the Hellenistic world, and also of Iran and India, made new observations, developed new theories, compiled new tables, and invented new instruments. They produced an enormous corpus of scientific literature covering all subjects from cosmology to computational techniques, and they made progress in all branches of their discipline. But the scientists did not have a wide audience. They wrote mainly technical treatises which circulated only within the scientific community, and few of them compiled popular summaries. In particular, the solutions they proposed for problems relating to religious ritual were generally considered to be too complicated or even completely irrelevant.

We now consider three facets of Islamic religious practice involving astronomy. As we shall see, the simple techniques of folk astronomy were applied to these practical problems by the legal scholars, and the complicated techniques of mathematical astronomy were applied to the same problems by astronomers. The former, generally disinclined to listen to the opinions of scientists, had far greater control over the practice of the people than had the astronomers. On the other hand, the solutions developed by Muslim scientists, invariable too complicated for widespread application in the medieval milieu, are impressive indeed from a scientific point of view.

The regulation of the lunar calendar

The Islamic calendar is strictly lunar. The beginnings and ends of the lunar months, in particular of the holy month of Ramadan, and various festivals throughout the twelve-month ‘year’, are regulated by the first appearance of the lunar crescent.

Since twelve lunar months add up to about 354 days, the twelve-month-cycles of the Islamic calendar occur some eleven days earlier each year, and the individual months move forward through the seasons. To keep the lunar months in line with the seasons of the solar year it was the custom in pre-Islamic times to insert an additional intercalary’ month in the lunar calendar every few years. This practice the Prophet Muhammad abandoned. The Koran expressly forbids such intercalation, and the exegetes explain that the proscription was necessary because intercalation caused months that God had intended to be holy to be confused with other months.

For scholars of the sacred law, the month began with the first sighting of the crescent moon. This observation is a relatively simple affair, provided that one knows roughly where and when to look and the western sky is clear. Witnesses with exceptional eyesight were sent to locations that offered a clear view of the western horizon, and their sighting of the crescent determined the beginning of the month; otherwise, they would repeat the process the next day. If the sky was cloudy, the calendar would be regulated by assuming a fixed number of days for the month just completed. Also, the crescent might be seen in one locality and not in another. Unfortunately, the historical sources contain very little information on the actual practice of regulating the calendar.

Astronomers, on the other hand, knew that the determination of the possibility of sighting on a given day was a complicated mathematical problem, involving knowledge of the positions of the sun and moon and the mathematical investigation of the positions of the both celestial bodies relative to each other and to the local horizon (see figure 1). In short, the lunar crescent will be seen after sunset on a given evening at the beginning of a lunar month if it is far enough away from the sun, and if it is high enough above the horizon not to be overpowered by the background sky glow. Conditions required to assure crescent visibility on most occasions can be determined by observation, but the formulation of a definitive set of conditions has defied even modern astronomers. The positions of the sun and moon must be investigated to see whether the assumed visibility conditions are satisfied, but, even if they are, the most ardent astronomer can be denied the excitement of sighting the crescent at the predicted time if clouds or haze on the western horizon restrict his view.

The earliest Muslim astronomers adopted a lunar visibility condition which they found in Indian sources. It was necessary to calculate the positions of the sun and moon from tables and then to calculate the difference in setting times over the local horizon. If the latter was 48 minutes or more, the crescent would be seen, if it was less the crescent would not be seen. Based on this condition and computed specifically for the latitude of Baghdad, the astronomer al-Khwarizmi in the early ninth century compiled a table showing the minimum distances between the sun and moon (measured on the ecliptic) to ensure crescent visibility throughout the year.

Figure 1. The western horizon at sunset on the evening of first visibility of the crescent moon. For predicting visibility Muslim astronomers devised sets of conditions on such quantities as the apparent distance between the sun and moon (d), the altitude of the moon above the horizon at sunset (h), and the difference in setting times of the sun and moon (s).

During the following centuries, Muslim astronomers not only derived far more complicated conditions for visibility determination but also compiled highly sophisticated tables to facilitate their computations. Some of the leading Muslim astronomers proposed conditions involving three different quantities, such as the apparent angular separation of the sun and moon, the difference in their setting times over the local horizon, and the apparent lunar velocity. Annual ephemerides or almanacs gave information about the possibility of sighting at the beginning of each month (see figure 2). In brief, the achievements of the Muslim astronomers in this area were impressive.

In modern times the regulation of the calendar has led to controversy between religious authorities and scientists. The main problems are the difficulty of making sure predictions for a multiplicity of locations and the unwillingness of the religious authorities to listen to the scientists. For example, Ramadan has sometimes been announced one, or even two, days early in some Islamic countries. (see, for example, Al-Ahram, Cairo, 26 and 27 September 1973). This occurrence, unthinkable in medieval times, resulted not only from the enthusiasm to begin the fast, but also from the ineptitude of the responsible authorities in matters scientific. Modern communications and divergent political interests also played a role. An international commission has recently been formed to handle problems associated with the Islamic calendar, happily under the enlightened leadership of an astronomer, Dr. Mohammed Ilyas of Malaysia.

Figure 2. A table showing visibility predictions for the first evening in each month of the civil calendar during the year 1129 Hijra (=1716/17). Calculations of the positions of the sun and moon relative to each other and relative to the local horizon lead to pronouncements such as the crescent ‘will be seen clearly’, ‘will be seen with difficulty’, or—in the case of Ramadan that year—’will not be seen at all’. In the last case, the religious authorities would have announced the beginning of Ramadan the following evening Numbers in these and other medieval tables were written in the standard alphanumerical system (called abjad in Arabic) and are expressed sexagesimally, that is to base 60-as in the modern notation for angles in degrees and minutes or time in hours and minutes. (Courtesy of the Egyptian National Library.)

The regulation of the five daily prayers

Figure 3. The five standard prayers of Islam and their timings. The three prayers at night are regulated by horizon and twilight phenomena and the two daylight prayers by means of shadow lengths.

The times of the five daily prayers in Islam are defined in terms of astronomical phenomena dependent upon the position of the sun in the sky. More specifically, the times of daylight prayers arc defined in terms of shadows, and those of night prayers in terms of twilight phenomena. They, therefore, vary with terrestrial latitude, and unless measured with respect to a local meridian, also with terrestrial longitude.

Because the months begin when the new moon is seen for the first time shortly after sunset the Islamic day is considered to begin at sunset. Each of the five prayers in the Islamic day (see figure 3) may be performed during a specified interval of time, and the earlier during the interval that the prayer is performed, the better.

The day begins with the maghrib or sunset prayer. The second prayer is the isha or evening prayer, which begins at nightfall. The third is the fajr or dawn prayer, which begins at daybreak. The fourth is the zuhr or noon prayer, which begins shortly after astronomical midday when the sun has crossed the meridian. The fifth is the asr or afternoon prayer, which begins when the shadow of any object has increased beyond its mid-day minimum by an amount equal to the length of the object casting the shadow. In some medieval circles, the zuhr prayer began when the shadow increase was one-quarter of the length of the object, and the asr prayer continued until the shadow increase was twice the length of the object—see figure 4. In other communities, a prayer at midmorning, called the duha, began at the same time before midday as the asr began after midday. This prayer is mentioned in the hadith, and there are varying accounts about it. In some, the Prophet is said to have performed it himself. In others, he is said to have declared it a heretical innovation. This is a clear indication that later authorities were undecided about whether or not to include it in the daily ritual.

The five prayers adopted by the Muslim community are not specifically mentioned in the Koran. In the hadith literature, more than five prayers are mentioned but no specific definitions are given in their times. The duha prayer at midmorning was clearly practised by some in the early Muslim community, but later it was generally, although not completely, abandoned. Also, there is a reference to the night-vigil called tahajjud; this was later made optional. The standard definitions of the times for the daytime prayers in terms of shadow increases rather than shadow lengths (as mentioned in the Prophetic hadith quoted above) first appear in the eighth century.

The reason why five prayers were adopted by the early Muslim community is clear from the definitions of their times. The definitions of the duha, zuhr, and asr prayers in terms of shadow increases provide simple and practical means for regulating them at the ends of the third, sixth and ninth hours of daylight, the hours being seasonal hours, that is, one-twelfth divisions of the length of daylight. Seasonal hours, which vary in duration throughout the year, were in standard use in the Near East in Antiquity. The relationship between these and the shadow increases is provided by a simple, approximate formula for timekeeping of Indian origin known to the Muslims in the eighth century. Even the names of the prayers in Islam are the same as those of the corresponding seasonal hours recorded by some of the Arab lexicographers. Their times correspond to the times of the seven prayers of early Syrian Christianity, but with the omission of the prayer at sunrise because it was expressly forbidden by the Prophet and the dropping of the prayer at midmorning in all but a few communities.

Figure 4. The times of the zuhr and the asr prayers are defined in terms of the increase of the shadow of a vertical object over its midday minimum. Thanks to medieval texts on folk astronomy these curious definitions have now been explained.

In the first few decades of Islam, the times of prayer were regulated by observation of shadow lengths by day and of twilight phenomena in the evening and early morning. Precisely how either the daylight or the night-time prayers were regulated is unfortunately not clear from the available historical sources. Muezzins who performed the call to prayer from the minarets of mosques were chosen for their piety and the excellence of their voices, but their technical knowledge was limited to the basics of folk astronomy.

On the other hand, the determination of the precise moments (expressed in hours and minutes, local time) when the prayers should begin, according to the standard definitions, required complicated mathematical procedures in spherical astronomy, that is the study of problems associated with the apparent daily rotation of the celestial sphere. Accurate as well as approximate formulae for reckoning time of day or night from solar or stellar altitudes were available to Muslim scholars from Indian sources and these were improved and simplified by Muslim astronomers. Certain individual astronomers from the ninth century onwards applied themselves to the calculation of tables for facilitating the determination of the prayer-times. The earliest prayer-tables were prepared by al-Khwarizmi for the latitude of Baghdad. The first tables for finding the time of day from the solar altitude or the time of night from the altitudes of certain prominent fixed stars appeared in Baghdad in the ninth and tenth centuries. The extent to which these tables deriving from mathematical procedures were used before the thirteenth century is unknown; the earliest examples are contained in technical works which must have had fairly limited circulation. The muezzins certainly had no need of them. One had to be an astronomer, for they had to be used together with some kind of observational instrument for measuring the sun’s altitude and reckoning the passage of time.

It was not until the thirteenth century that the institution of the muwaqqit appeared in mosques and madrasas. These professional astronomers associated with a religious institution not only regulated the prayer times, but constructed instruments, wrote treatises on spherical astronomy, and gave instruction to students. In thirteenth-century Cairo, new tables were compiled which set the tone for astronomical timekeeping all over the Islamic world in the centuries that followed. In medieval Cairo there was a corpus of some 200 pages of tables available for time-keeping by the sun and for regulating the times of prayer (see figure 5).

Figure 5. An extract from the main corpus of astronomical tables for regulating the times of the prayers in medieval Cairo. These tables, discovered only in 1970, cast considerable light on contemporary religious practice. This extract displays for each degree of solar longitude, corresponding roughly to each day of the year, the time from sunset until the moment when the muezzin should extinguish the candles on the minarets during the month of Ramadan. (Courtesy of the Egyptian National Library.)

Impressive innovations in astronomical timekeeping were made in other medieval cities, especially Damascus, Tunis and Taiz, although by the sixteenth century Istanbul had become the main centre of this activity. We may mention, for example, highly sophisticated tables of special trigonometric functions especially designed to solve problems of spherical astronomy for any latitude. Tables for finding the time of day from the solar altitude at any time of year were compiled for Cairo, as we have mentioned, and also for Damascus, Tunis, Taiz, Jerusalem, Maragha, Mecca, Edirne and Istanbul. Medieval tables for regulating the times of prayer have been found for a series of localities between Fez in Morocco and Yarkand in China. Such tables have a history spanning the Millenium from the ninth century to the nineteenth.

As noted above astronomical tables for regulating the prayer-times had to be used together with instruments. Only in this way could one ascertain that the time advocated in the table had actually arrived. The most popular of these instruments was the astrolabe and the quadrant. Hundreds of Islamic astrolabes and several dozen quadrants are preserved in the museums of the world, a small fraction of the instruments actually made by Muslim astronomers. An alternative means of regulating the daytime prayers was available to the Muslims in the form of the sundial. Many mosque sundials from the later period of Islamic astronomy survive to this day, though most are non-functional.

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