Medieval Masters of Astronomy
THROUGHOUT history people have been struck with awe as they gazed at the sun, the moon, and the stars. By studying the positions and movements of those celestial bodies, man has been able to mark the passing of days, months, and years.
The Arabs were one of many peoples who studied the night sky. The golden age of science in the Middle East began in the ninth century C.E., and Arabic-speaking astronomers of that era were regarded as masters of astronomy. Their achievements played a crucial role in the development of this fascinating science. Let us see how.
Pioneers of Astronomy
During the seventh and eighth centuries C.E., Islam expanded west from Arabia across North Africa and into Spain and east as far as Afghanistan. Scholars in this vast area drew on a legacy of scientific research from Persia and Greece, which was largely influenced by Babylon and Egypt.
Then, during the ninth century, important scientific texts were translated into Arabic, including the works of Greek astronomer Ptolemy. * The Abbasid, whose dynasty stretched from Afghanistan to the Atlantic Ocean, acquired Sanskrit texts from India, which contained a wealth of information about mathematics, astronomy, and the other sciences.
Islam prized knowledge of astronomy. Why? One reason was related to their worship. Muslims believe that they should face Mecca when they pray, and astronomers could pinpoint the direction of Mecca from any location. By the 13th century, some mosques even employed a professional astronomer, or muwaqqit, who helped worshippers pray in what they considered to be the proper manner. With their data, astronomers could also determine the dates of religious events and practices, such as the period of fasting during the month of Ramadan. Additionally, they could help pilgrims traveling to Mecca ascertain the length of their journey and plan the most efficient route.
By the early ninth century, in Baghdad, the study of astronomy was part of every scholar’s education. Caliph al-Ma’mūn established an observatory there, and then another near Damascus. His staff of geographers and mathematicians analyzed, compared, and reconciled data received from the Persian, Indian, and Greek astronomical traditions. Observatories were also built in a number of other Middle Eastern cities. *
The scholars who worked in those centers achieved remarkable results for their day. For example, as early as 1031, Abu Rayhan al-Bīrūnī mentioned the possibility that planets revolve in elliptical orbits rather than circular ones.
Measuring the Earth
The expansion of Islam stimulated interest in mapmaking and navigation. Cartographers strove for great accuracy in their measurements, and they often achieved it. With the goal of precision and to establish degrees of latitude for the world map he was producing, caliph al-Ma’mūn sent two teams of surveyors into the Syrian desert. Armed with astrolabes, measuring rods, and cords, the teams walked in opposite directions until they observed a one-degree change in the elevation of the North Star. They reckoned that the distance traveled corresponded to one degree of latitude, or 1/360 of the earth’s circumference. They calculated the polar circumference to be 23,220 miles (37,369 km)—quite close to the true figure of 24,860 miles (40,008 km)!
Middle Eastern observatories possessed an impressive array of sophisticated equipment—astrolabes, quadrants, sextants, sundials, and other instruments that were used to study and track celestial bodies. Some of these devices were huge. The builders reasoned that the greater the size of the instrument, the better the precision.
The Legacy of Medieval Astronomers
The achievements of these medieval masters of astronomy were impressive. They cataloged and illustrated the constellations, named the stars, designed more-accurate calendars, measured eclipses, and continued to refine tables to chart celestial movements. They could pinpoint the positions of the sun, moon, and five visible planets at any time of the day or night—an invaluable aid in navigation. They could also tell time and maintain a calendar by observing the positions of heavenly bodies.
The theories that Arabic-speaking astronomers developed to explain planetary movement came close to resolving the inconsistencies they found in Ptolemy’s model of the universe. What they lacked was the understanding that the sun, not the earth, lay at the center of planetary orbits. Even so, they charted star movements with unprecedented accuracy, and their findings proved invaluable to successive generations of astronomers the world over.
^ par. 6 The Greeks had already determined that the earth is spherical. How else, they reasoned, could the North Star appear lower in the sky as a person traveled south?
^ par. 9 The establishment of those observatories was often prompted by a ruler’s interest in astrology.
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Astronomers kept records of planetary motion in an increasing number of almanacs compiled throughout Islamic lands
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AN ANCIENT “POCKET COMPUTER”
The astrolabe, predecessor of the sextant, has been called “the most important astronomical device before the telescope.” Medieval scientists in the Middle East used this device to solve problems related to time and the positions of heavenly bodies.
The astrolabe consisted of an elegant model of the sky projected onto a polished metal plate. Degrees, or sometimes the hours of the day, were inscribed around the outer rim of the base on which the plate was installed. A swiveling pointer (the alidade) was used to take sightings to determine the elevation of a given star when the device was held aloft and suspended at arm’s length. The results were then read off scaled markings, similar to those on a slide rule.
The versatile astrolabe enabled its users to identify stars, predict the times of sunrise and sunset for any given day, determine the direction of Mecca, survey land, calculate the height of objects, and navigate. It was the “pocket computer” of its day.
A 13th-century astrolabe
A 14th-century astrolabe quadrant
Astrolabe: Erich Lessing/Art Resource, NY; astrolabe quadrant: © New York Public Library/Photo Researchers, Inc.
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A 16th-century illustration depicting Ottoman astronomers using methods established by Arab scholars
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Celestial globe, 1285 C.E.
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Pages from an Arabic manuscript of constellations, written by astronomer ‘Abd al-Raḥmān al-Sufi, about 965 C.E.
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Pages 16 and 17: Art Resource, NY
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Manuscript: By permission of the British Library; globe: © The Bridgeman Art Library