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# 2.22 Arab Astronomy

Arab scholars kept alive some of the Greek knowledge and preserved a few copies of Greek books. Pre-Islamic Arabs relied entirely on empirical observations of the sky, primarily the rising and setting of particular stars. A century after the time of the Muslim prophet Muhammad, around 760 A.D., Islamic leaders in the new capital of Baghdad began to sponsor translation and distribution of surviving Greek texts. This started an Islamic flowering of science. Astronomy began to incorporate mathematical methods. There was a religious obligation to follow set prayer times and observe particular festival days, and this spurred better observations of the sky. Ptolemy’s work came down to us in an Arabic translation called Al-Magiste or "The Greatest." Although Arabic astronomer accepted the Ptolemaic system, they were aware of its limitation and they made a number of improvements. Astronomers had status and prestige in the caliphate of the early Islamic countries, and they flourished.

?Islamic Celestial Globe, 1630 A.D. This brass globe served both as a map of the heavens, as viewed from outside the starry sphere, and as a precision tool for making astronomical calculations. Engraved on its surface are various coordinate lines, constellation figures, and Arabic inscriptions. The stars are made of embedded bits of silver. The globe is hollow and was cast in one seamless piece. It was originally set in a cradle of rings, which depicted the horizon and other astronomical circles. Click here for original source URL

Arab astronomers did not just revive and transmit Greek knowledge; they made real advances. Islamic scholars refined the measurement of Earth's circumference in 820 A.D., coming within 4% of the actual circumference. In 850 A.D., Al-Farghani wrote an astronomy compendium that corrected a number of errors made by Pyolemy. He also improved measurements of the Earth's orbital tilt, the precession of the greatest distances of the Sun and Moon from the Earth, and the Earth's circumference. The book was translated into Latin so became influential in Western Europe. Arab naturalists also advanced the field of optics. They recognized that the eye is an optical device, and studied the ways in which light reflected. They also invented the pinhole camera and used it for accurate observation of stellar positions. They build a number of major observatories. They fashioned a number of exquisite measuring devices: Armillary spheres, astrolabes, and sundails. No new cosmological models were developed because the Koran forbids pictorial representations of the heavens.

An illustration from an astronomer al-Biruni's astronomical works, explains the different phases of the moon. Click here for original source URL.

The influence of Arab astronomy remains in the night sky. We still use Arabic names for many mathematical and chemical terms and names for many stars, as indicated by the Arab prefix, "al-." Examples are algebra, algorithm, alkali, alcohol, and the stars Algol, Alcor, and others. Travel in hot desert countries was best done at night, so Arabs were intimately acquainted with the constellations and other patterns of stars. By 1000, the Islamic empire had spread to Spain, carrying the old Greek knowledge and the new Arab discoveries into Europe. Spanish astronomers published astronomical tables using a longitude system with 0° in Cordoba (rather than Greenwich, England, as in the modern longitude system, introduced when Britannia ruled the waves). We have an incomplete understanding of Arab astronomy, since over 10,000 manuscripts have been found, most of which hasve not been read or even cataloged.