Bulletin 33 - November 1987: Ancient Arab Astronomy
Ancient Arab Astronomyby Robin Westgate, Teacher
English Speaking School, Al Ain
Throughout history man has expressed an affinity for knowledge relating to his physical and universal environment. Inevitably such curiosity has also encouraged and given rise to the construction of cosmologies based upon concepts, idiosyncratic or otherwise, extrapolated from observation and deduction.
The historic cultures that have deviated from or omitted such intellectual processes are atypical. Man’s quest to describe the mechanisms and laws governing the cosmos has dictated and inexorable drive towards achieving an understanding of it.
The reasons for this are various but unified: the need to discover underlying truths and reasons for existence are common to the fields of theology, philosophy and metaphysics, not to mention pure scientific enquiry. All these fields of thought have played their part in initiation man’s cognitive research into his environment, with the ultimate aim of reducing the Universe to rational and reasonable principles, sub specie aeternitatis.
In the context of this article I have endeavored to provide a background of astronomical knowledge as a function of time, form the beginnings of recorded history (c.4000 BC) until the time of the ascendancy of Muslim cultures.
Such an approach seems necessary in any discussion involving the relative contributions made by any particular peoples, so that the linear or even exponential progression land development of concepts or inventions can be seen to be integrated into the overall historical continuum.
However, to be realistic, the provision of a full treatment of historical astronomical issues, by each contributing culture, would prove exhaustive an irrelevant to an examination of Muslim values. Accordingly, the descriptions given should be regarded as summaries of earlier achievements, rather than complete and definitive accounts of preceding methods and philosophies.
Apart from its historical interest, the aim of this article is to demonstrate how any one culture may contribute- consciously or not – to the historical progression of scientific and philosophic concepts.
I have tried to indicate this aspect by describing how the Muslims relied heavily upon the literature of preceding civilizations – especially those of classical Greece and India in order to prepare the foundations of their own ideas and requirements. The ensuing developments in the Arab world are then examined in their own right.
Finally, the subsequent link in the historical continuum (specifically the transfer of Arab knowledge into Western Europe at the time of the Crusades) is briefly covered to preserve the chronological sense and structure.
I hope that I have succeeded in showing that the Arab development of astronomical methods and concepts was by no means a negligible one. Rather, theirs was splendid achievement that helped preserve and unify early mediaeval scientific knowledge for posterity.
The time period to which we assign the term ‘history’ covers only the last 6000 years or so, and on this scale astronomical concepts appear very ancient. It is not know, for instance, which cultures first visualized certain groups of stars as shapes of men or beasts, and thus ‘invented’ the constellations. In any case, when the Egyptian and Mesopotamian civilizations arose, mankind already possessed many of our present – day constellations, so it would seem reasonable to assume that their origins lie far back in prehistoric times. Accordingly, the constellations (as imagined to decorate the firmaments) have been called ‘mankind’s oldest picture book’. That they certainly represent some of the more archaic elements of civilization is incontestable, even though they have been subjected to various modifications by successive cultures throughout history.
Yet astronomy is very much more than casually imagining vague shapes in the night sky. Even in prehistoric times people must have noticed the procession of the star patterns along with the passage of their seasons, occurring in a regular cycle over a certain period of time. Once this observation had been established, the progression to the establishment of an annual calendar, relating star patterns to seasons, was inevitable.
The first calendar probably came into use around 4000 BC. The Egyptians had noticed that on one particular day each year the bright star – now know as the ‘Dog Star’ or Sirius – appeared in the eastern sky shortly before sunrise. (This event coincided approximately with the flood – time of the River Nile). By counting the days between successive appearances of this star, they determined that the length of he ‘year’ was about 365 days. By dividing this year into twelve months of thirty days each (with five days left over) they developed a calendar. However, since the year is in fact about 365 days, the error in this calendar eventually became obvious; but it still served the Egyptians well enough for their purposes.
Around 3500 BC, in Mesopotamia, the Sumerians independently invented a calendrical system, this time based on the movements of the moon. Since the latter orbits the earth in approximately one ‘month’ of 28 days (and goes through an observable series of phases in doing so) this was also a logical way of dividing up he year, except that they ended up with 13 months instead of 12.
The Sumerians are also credited with the division of a circle into different parts – based on their hexadecimal numeral system. Each ‘ degree’ was further divided into 60 ‘minutes’ and each of these into 60 ‘seconds’. These geometrical divisions led ultimately to the establishment of accurate time measuring intervals and methods, culminating in the invention of clocks and the science of horology.
The preceding conditions of calendrical science survived, virtually unchallenged, for the ensuing three millennia. Then, around 600 BC, the Chaldeans, under the rule of Nebuchadnezzar (605 – 561 BC), extended the work of earlier peoples and conducted an intensive study of the motions of the sun, moon, planets and stars.
Without any telescopes or accurate time – recording instruments the Chaldeans used their advanced mathematics – derived from the Sumerian numerical system – to calculate detailed tables concerning and predicting the motions of these bodies. In so doing they made notable contributions to the science of these bodies. In so doing they made notable contributions to the science of astronomy and, coincidentally, improved the accuracy of the existing calendar.
The Chaldeans also named the days of the week after heavenly bodies, a practice further elaborated by many later cultures. However, they mistakenly believed that a study of the stars enabled them to foretell the future. From this type of belief arose the pseudo-science of astrology, which has survived till modern times.
Independently, and only a little later than the Chaldeans, the Maya of Yucatan (About 400-200 BC) developed a calendar as accurate as the Gregorian one used today. These people were also skilled in mathematics and possessed an advanced number system. By 300 BC at the latest they had invented the concept of a zero, which Indian mathematicians developed only hundreds of years later. (Of course the early Arabs had never heard of the Maya of their achievements, but this note is included in the general background of ancient Arab astronomy).
The greatest historic contributions to the astronomic sciences indubitably came form the Greeks. Unlike the Egyptians and Mesopotamians, who imagined celestial gods and demons as the cause of changes in nature and the elements, the Greeks rejected magic and sought rational and logical explanations of events. They believed that man had the intelligence to understand nature. In particular, they wanted to know how the universe began, what its constituents were, and what made it work. Theirs was the first culture to suggest that the universe was regulated by invariant physical laws and constants.
The achievement and progress of Greek astronomy are impressive. Perhaps the first true astronomer was Anaximander (610-546 BC), who realized that the earth’s surface is curved, not flat. To Pythagoras (ca.530 BC) the earth was no longer merely a curved surface with the sky-vault of the firmament overhead, but a sphere suspended in a void. Philolaus, a disciple of Pythagoras, taught that this sphere rotated, and that the motion of the stars – only apparent – was caused thereby. Archytas (ca.400 BC) calculated the size of the earth to well within an order of magnitude, an astounding feat of mathematics for his time.
However, as far as cosmology was concerned Eudoxus (400-355 BC), like most of these early Greek astronomers, believed that the earth was situated at the center of the universe. It was not remarkable strides were made. During this ‘Hellenistic’ age (323-BC) many more scientific discoveries and technological advances distinguished the Greeks.
Aristotle (384-322 BC), Archimedes (287-212 BC), Euclid (c.200 BC) and Eratosthenes (276-195 BC), all left their respective marks on mathematics and astronomy. Aristarchus (ca.200 BC) correctly maintained that the earth and other planets orbited the sun. Unfortunately his contemporaries considered this concept too revolutionary to be acceptable. At least Aristarchus was not burnt at the stake as Giordano Bruno was in 1600 AD for teaching the same theory. Eratosthenes and Aristarchus between them established that the earth rotated and orbited the sun; estimated the circumference of the earth to an accuracy of within one percent; and calculated the distance from the earth to the moon and sun.
It is astonishing that the Greeks know all these facts more than seventeen centuries before Columbus set sail to prove to Mediaeval Europe that the earth was round and navigable. Even after this, Greek astronomers still had significant contributions to make.
Hipparchus (ca.100 BC), by consensus of opinion considered to have been the greatest Greek astronomer, carefully recorded the positions of the sun, moon, planets and stars. Indeed, his work was so accurate and complete that later astronomers used it to predict eclipses of the sun and moon. He also introduced the idea of comparative stellar magnitudes – used by modern astronomers.
Then, along with increasing Roman dominance and imperialism Greek science, with its civilization, declined and atrophied. Claudius Ptolemy (c.100-178 AD), a Greek astronomer working in Egypt, actually published a book (based upon the work of Hipparchus) now known as the ‘Almagest’ (an Arabic name). This would have been an excellent publication if Ptolemy had not, unfortunately, supported and modified the Eudoxian concept of a geocentric universe. Nonetheless his book, for reasons more religious than scientific, became highly respected and firmly established his theory amongst surviving Greek, Roman and later Arab astronomers. In fact, it was the Arabs that provided the essential link between the classical Greek and the mediaeval European scholars.
Surprisingly perhaps, in some respects Ptolemy’s theories went virtually unquestioned for another thirteen centuries until a succession of brilliant scientists: Copernicus (1473-1543), Brahe (1546-1601), Kepler (1571-1630), Galileo (1564-1642) and Newton (1642-1727) challenged and eventually destroyed them during the ‘Renaissance’.
To return to the chronological narrative, however, after the fall of the Roman Empire (and certainly during the decline of ancient Middle-Eastern science as a whole), science began to thrive under the ‘Gupta’ cultures (c.320 AD onwards) in India. This transfer of science in the geographical sense can be recognized today as a saving grace and a fostering of knowledge during the so-called; dark age; of barbarism that afflicted Europe and the Middle East after the final collapse of the Roman Empire, even though it was to later re-establish itself in Byzantium. In India, then, the contributions made by Gupta mathematicians and astronomers towards world scientific achievement were second only to those of the Greeks.
The astronomer and mathematician Aryabhata, for instance, discussed amongst other things the value of ‘Pi’ and its relationship to the rotation land spherical shape of the earth. Other Indian astronomers calculated the diameter of the moon by observing and measuring the curved shadow cast by the earth during eclipses, and wrote treatises upon the prediction of eclipses and theories of gravitation.
Lastly, but not least in importance, the Indians developed the number symbols which served as a basis for our own ‘Arabic’ numerals’. Actually, these were only called ‘Arabic’ numerals by Western mediaeval scholars; they were really Hindi in origin and were adopted and carried westwards by Arab traders and invaders. Guptan scholars also pioneered the decimal system of notation land (apart from the Maya Indians) were probably the first Eurasians to employ a symbol for zero, which the Arabs later developed the use of.
Development of Arab Scientific Interest
The Arabs once ruled an empire that stretched from central Asia to Spain. This empire reached its zenith between the eighth and thirteenth centuries AD. Arab scholars of that period knew more about science and the arts than any other contemporary peoples. They also translated many classical (Greco-Roman) works of literature and science.
In fact generally speaking the Muslims were very interested in books and learning. Rhazes (al Razi-C. ninth century), Vienna (Ibn Sina-c. tenth century) and Averroes (1126-1198) were among the best known of Muslim philosophers. They studied the great Greek writers, particularly Plato and Aristotle. Their goal seems to have been (perhaps rather surprisingly) to try to find ways to reconcile the ideas of the ancient Greeks with the teachings of Islam.
Consequently, universities were established in the leading Muslim cities of Baghdad, Damascus, Jerusalem, Alexandria, Cairo and Cordoba. Further, the book business flourished. In universities, palaces and the homes of wealthy merchants could be found large and impressive libraries. By 1250 AD the most valuable material in Islamic libraries had become available to European scholars in translation.
The latter point was of great historical importance; eventually much of the scientific knowledge that the Muslims had gathered from the ancient classical world and from India was passed to the west, through Spain and southern Italy, around the time of the Crusades. At that time most European scholars realized the tremendous scientific and technological superiority of the Islamic world. And eagerly sought translations of Muslim works.
Thus these Greek and Arab writings that flowed into Europe, especially after the beginning of the twelfth century, made up a rich legacy of scientific and philosophical knowledge. For example, Arabic (Hindi) numerals and the zero symbol made possible a decimal system of computation. Also, Euclidean geometry, together with algebra and trigonometry from the Arab world, greatly increased the scope and accuracy of mathematics, especially useful for later astronomical applications during and after the Renaissance.
This transfer of information occurred just in time, for from about 1350 onwards, the Mongols in the east and the Christians in Spain began systematically to destroy Islamic books in a wholesale manner, as part of their anti-Muslim wave of feeling at that time
Fortunately, a large number of Islamic books survived in Egypt, Persia and India, from where most of our knowledge of Muslim civilization has originated. As a result, every present-day intellectual disciple owes a debt to the scientific heritage of Islam
Arab Astronomy and Navigation
Muslim scholars made significant contributions towards the development of many ‘modern’ sciences, such as physics, chemistry, medicine, mathematics and astronomy. They were particularly interested in the latter.
Using the work of the second century Greek astronomer Ptolemy as a basis, Muslim thinkers greatly increased man’s knowledge of astronomy. Indeed, during the Middle Ages, when European science underwent a decline, it was the Arabs who preserved the astronomical heritage.
In some respects this achievement may have been inevitable, since knowledge of the stars was essential for navigational purposes and for telling the times of prayers and religious festivals. In other words, being adventurous traders and mariners and temporally precise worshippers, the Arabs needed to study astronomy other than for its purely scientific interest, though undoubtedly such an interest existed. Consequently they constructed many observatories and improved certain measuring instruments such as the astrolabe for determining and recording the positions and movements of celestial bodies.
Foremost amongst early Arab scientists was al Khawarzimi, who lived in Baghdad during the ninth century. His work was mainly concerned with astronomy and mathematics. In fact, his mathematical treatise was the first to employ what westerners term ‘Arabic numerals’ (which were really borrowed from the Indians, as explained earlier. Although it seems most likely that the Indians invented the zero symbol or cipher (‘sifr’ means empty in Arabic), al Khwarzimi is attributed with greatly developing its use n mathematics to simplify multiplication and division. He also gave a systematic account of algebra and geometry, for use in solving practical astronomical and navigational problems.
Other notable Arab astronomers were al Battam (d. 929), al Zarquli (d. 1087) and Omar al Khayyami (d. 1123). The latter was a Persian mathematician who devised a very accurate calendar based on astronomical observations. It was reputed to have been more accurate than the Gregorian one we use today, with an error factor of only one day in 3770, rather than the Gregorian’s one in 33303.
Incidentally, the mathematics used for astronomical calculations by the Arabs involved the use of degrees and minutes of arc-first developed by the Sumerians, and later developed extensively by the Babylonians, millennia earlier. The direct expansion and inclusion of this system into Euclidean geometry is the main reason why we measure angles in degrees, minutes and seconds nowadays (not to mention basing our system of measuring time upon it also).
Arab interest in astronomy was also continued in Moghul India, where massive observatories were built in Jaipur, for example. It is interesting to consider that some modern historians think that the writings of the great Copernicus (who was the first westerner to propose an heliocentric planetary system) show much that could be attributable to these early Muslim astronomers.
When the first Portuguese navigators, like Vasco da Gama, sailed along the East African coast and around the Arabian Peninsula they encountered a well-established Arab seafaring tradition, utilizing an advanced navigational science dating from the eighth century. Techniques used were basically simple, but never the less by the eleventh century Arab mariners had adapted the Chinese discovery of the magnetic properties of lodestone for use as a compass at sea. Earlier methods had relied on steering by Polaris, the ‘North’ star, and ‘Kamal’, a kind of simple astrolabe used to reckon relative latitude. In fact, it is believed that Europeans first acquired a knowledge of the magnetic compass and the astrolabe (later to become the sextant) from Muslim sailors.
The renowned Arab navigator Shihab al Ahmed bin Majid al Najdi (c.1500), at the height of Arab navigational prowess, wrote a masterpiece entitled “the Book of Profitable Things concerning the First Principles and Rules of Navigation” which featured much astronomical observational data, amongst other things nautical.
Part of the legacy of this period include the fact that many of the brightest stars still bear Arabic names, allocated to them by Arab astronomers and navigators, for example Betelgeuse, Deneb, Aldebaran and Altair. These names, along with numerous other facets of Arab scientific nomenclature and mathematics, passed into Europe during the Renaissance.
However, this period, marked by the ‘collision’ of two great maritime powers – the European and the Arabian – was the beginning of the era of European ascendancy and the decline of Arab commercial dominance in Middle Eastern and Oriental seas.
Our modern world of easily accessible information and almost instantaneous communication is often taken for granted. Indeed, it is hard to imagine any society operating without such facilities. However, these systems represent recent phenomena – the by-products of an exponential scientific revolution with which we are inextricably linked. Even 50 years ago, let alone 500 or 5 000, such systems were comparatively inferior, primitive or non-existent.
During the post Greco-Roman ‘dark’ ages (c. AD 400-900), whatever advances had been achieved in science and literature by classical cultures were arrested or destroyed by decadent barbaric peoples throughout Europe and the Near East.
Fortunately, the Byzantine and Gupta empires served during this period as repositories of classical information. The Indians did much to develop science and astronomy, whilst Byzantium contained while libraries of ancient texts until conquered by Muslim expansionism in the 15th Century.
The Arabs, although a fierce, conquering race in the historical context, were also intellectually curious. Unlike the illiterate and destructive Aryans who besieged the ailing Roman Empire, Muslim scholars preserved, translated and developed the philosophical and scientific legacy of the ancient classical world.
Later the salvation of Arab literature, especially of philosophic and scientific works, by Renaissance scholars during the decline of Islam in Europe and the East, was remarkably fortuitous, though perhaps inevitable. The ’rebirth’ of European scientific perception and application rescued much relevant literature from the rapidly retreating boundaries of the perceived Muslim empire, whose libraries in satellite university cities, such as Cordova in Spain, were being pillaged (or ignominiously burnt) by advancing Christians in the west and Mongols in the east.
Subsequently, from the ‘ashes’ of medieval Arab scientific dominance, there arose a new tree of knowledge, spreading its roots and branches throughout Renaissance Europe. A wind of change then blew through its literary leaves, an element conceived within the mind of enlightenment, despite repression and persecution by fanatical theological dogmatism.
Thus the era of medieval natural philosophy pioneered by Islam yielded to the incipience of Renaissance European thought, at which point in the historical continuum I conclude.
It remains only for me to suggest, subjectively at least, that the scientific contributions made by medieval Islam were undoubtedly significant ones, and that the inclusion of astronomical investigation within that wide spectrum was, in retrospect, something more than just a useful by-product of practical requirements. Rather, Muslim intellectualism provided an essential link between Classical and Renaissance achievement, which led ultimately to the development of lucid contemporary theories.
Bibliography and References
Franklin, K.L. “Astronomy-History”, The World Book Encyclopaedia (1978), 812-813
Gingerich, Owen. “The Origin of the Zodiac”, Sky and Telescope Vol. 67, No. 3 (March 1984), 218-220
Howard, Sethanne. “Clocks and Calendars in Astronomy”, Astronomy, Vol. 12, No. 1 (Jan. 1984) 51-54
Kownslar, Allan and Terry Smart. People and Our World. New York: Holt, Rinehart & Winston, 1981
Perry, Marvin. Man’s Unfinished Journey. Boston: Houghton Mifflin Co., 1974.
Rey, H.A. The Stars. London: Chatto & Windus, 1975.
Ridpath, Ian. Stars and Planets. London: Hamlyn Ltd., 1979.
Ronan, Colin A. The Practical Astronomer. London: Pan Books Ltd., 1981.
Salah, Basem. The Arabian Heritage. Dhahran: Said Salah Ltd., 1984.
Wallbank, T. Walter, and Arnold Schrier. Living World History. Illinois: Scott, Foresman & Co., 1974.
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