Early islamic astronomy began to flourish primarily in the Middle East under the patronage of two Abbassid caliphs, Harun-al Rashid and his successor Mamun-al Rashid. The greatest center of learning was Baghdad especially during the reign of Mamun who founded the Darul Hukama or House of Wisdom, one of the greatest libraries and learning centers in the world. It contained Plato, Aristotle and other Greek philosophical texts (including natural philosophy) all translated into arabic. For instance, Ptolemy's Almagest was translated in 827 and revised in the latter part of the 800s. As they expanded into India, the Arabs also came into contact with Indian astronomy and mathematics. Sidhanta a hindu treatise on astronomy was brought back to Baghdad from India and translated into Arabic by al-Farazi, who was also the first to construct an astrolabe. Astronomical tables and manuscripts were also acquired from the conquered Sasanid territories and manuscripts of Greek and Hellenistic origin were acquired not only from cities which had been conquered but some were purchased, even from Byzantium.

al-Khwarazmi (b7??, d.ca.840) was one of the first great astronomers and mathematicians. He translated several mathematical and astronomical works from sanskrit and was the first to introduce the indian numerical system into the islamic world. He wrote the first work on algebra, that, when translated into Latin by Robert of Chester, would start algebra in the West. He wrote a treatise on astronomy, revised Ptolemy's works on geography and also corrected his maps of the world. He went on to calculate his own astronomical tables. He further did work on clocks, sun-dials and astrolabes. Some two centuries later, his tables would be revised by the spaniard al-Majriti and these revised tables were later translated into Latin (along with several of his books) by Adelard of Bath in 1126. The Latin translation of his book on arithmetic  introduces this subject to the West and his books on algebra in fact became texbooks in western universities till the XVIth Cent.

Also during the reign of Mansur al-Rashid, several observatories were constructed. At the one in Jundeshapur, Persia, arabic astronomers recalculated the size of the Earth. Another was erected in Baghdad where they verified the basic elements of the Almagest, and their observations formed the basis for the Tables of Mamun. Another was erected near Damascus, where again they ascertained the size of the earth and verified the various elements of the Almagest.

Al-Farghani (around 860) calculated the diameter of the earth at 6500 miles as well as the sizes and distances to the planets. He wrote a summary of the Almagest and a book on the construction of sundials as well as the Elements. He also supervised the construction of the Nilometer at Fustat in 861. His "Compendium on Astronomy" will be translated into Latin in 1135 by John of Seville and Richard of Cremona. The translation of his works was greatly responsible for spreading Ptolemaic astronomy throughout Europe.

Abul Wafa (940-997) Aside from being one of the last translators of Greek works into Arabic, he studied the movements of the moon and discovered its variation. He contributed extensively to geometry and introduced the use of the tangent, secant and cosecant into his astronomical observations. Much of today's trigonometry can be traced back to his mathematical work.

Al-Battani (known in the West as Albategnius 858-929) calculated the length of the solar year to be 365d, 5h, 46m, 24s. He found that the longitude of the sun's apogee had changed  by 16^o47' since Ptolemy's time. He determined the obliquity of the ecliptic (23^o35'), the length of the seasons and the true and mean solar orbit. He made several corrections to Ptolemy's data especially to his calculations for the orbit of the moon and those of planets. He studied lunar and solar eclipses and proved the possibility of annular eclipses  by measuring the changes in the apparent angular diameter of the sun. He replaced the greek chords with sines and developed the concept of the cotangent. His books and tables were translated into latin in he XIIth Cent. and became the basis of much european astronomy. From his work and from his time on, the aristotelian/ptolemaic model becomes the dominant model in islamic astronomy and much work over the next two centuries will go to refining this model which will come under increasing criticism in the XIIth and XIIIth Cents.

Ibn al-Haitham (known in the West as Alhazen 965-1040) He discovered the laws of refraction, and did the first experiments that show that light is made of constituent colors. He investigated  such issues as shadows, eclipses and the rainbow at length. He is the first to describe accurately the parts of the eye, explains vision, disagreeing with Ptolemy and Euclid by stating that light comes from the object and not from rays emanating from the eyes as they believed. He studied atmospheric refraction and explained why the sun and the moon appear larger near the horizon. He also discussed attraction between masses. He is the first to maintain that a body will move perpetually unless an external force affects it. He invented analytical geometry and had rather modern notions on evolution. Like many other islamic "scientists" his emphasis on observation of natural phenomena leading to theoretical explanations and verification in nature foreshadows the methodology of modern science and created a new notion, namely that "science" discovers aspects of reality, and is not merely an intellectual attempt to save appearances. His works influenced both Bacon and Kepler.

In Egypt, Ibn Yunus (950-1009) is famed for his observations on planets and the moon later used by Simon Newcomb in his lunar theory. First study of the oscillations of a pendulum

Abu Raihan al-Biruni (973-1048) was a contemporary of Ibn Sina (Avicenna) with whom he corresponded. These two men are justly considered two of the greatest islamic "scientists". During his extensive travels to India he recorded what he saw and thereby made contributions to both physical and economic geography. He correctly noted that the Indus Valley is an old sea basin filled with river sediments. He also correctly explained artesian systems and wells. He believed in the rotation of the earth and accurately calculated latitude and longitude. He developed a method for trisecting an angle. He has the wirst work mon sperical trigonometry.  He maintained that the speed of light is immense compared to the speed of sound.

 In Neshapur in 1074, Omar Khayam the great mathematician (1044-1123) and al-Hazini elaborated a solar calendar actually slightly more accurate than the Gregorian calendar.

Much work was also done in Spain by such famous astronomers as al-Majriti of Cordova who revised al-Khwarizmi's tables, al-Zarqali (known in the West as Azarquiel-1029-1087) of Toledo who edited the Toledan Tables, ibn Afiah of Seville(died in1140 or1150), who built  the Tower of Seville, the first observatory in Europe; and al-Bitruji (Alpetragius) critical of ptolemaic astronomy, and much in favor of aristotelian astronomy of spheres.  Great attempts are made to remove the notion of epicycles and deferrents from the models. Athough never successful other than on a philosophical level, the criticisms will again surface during the western renaissance.

Ibn Afiah wrote the Book of Astronomy (Kitab al Hayat) critical of Ptolemy and noted that Mercury and Venus have no appreciable parallax. He was translated into Latin by Richard of Cremona.

Al-Zarqali (1029-1087) was an instrument maker who improved the astrolabe. He also calculated the length of the mediterranean at 42^o whereas Ptolemy has placed it at 62^o and al-Kwarizmi at 52^o.  He was well known in the West and is quoted by Copernicus.

Al-Bitruji's (known in the West as Alpetragius) work, also strongly critical of Ptolemy, was translated into Latin by Michael Scot in 1217.

The Mongol invasions in the mid 1200s did to Islam what the barbarian invasions did to the West centuries before and dealt islamic learning a blow from which it never really recovered to its former brilliance, although it did produce some eminent late astronomers such as al-Tusi (1201-1274) who established a famous observatory in Maraghah (1259) and whose books and tables became popular among astronomers for the following two centuries. al-Tusi, severely criticising Ptolemy's epicycles and deferrents, was also able to show that planetary motion could besolved by only using  uniform circular motion. It is this model of planetary motion that was used by Copernicus in his heliocentric model.  His observatory in Maraghah also became the model for later obervatories in Samarqand and Istanbul and these three observatories became the models for Brahe and Kepler's observatories that were outfitted with the same instruments.
 

I have but briefly mentioned a few of the islamic astronomers. Certainly, I have not even come close to do them justice, and I have failed to mention any islamic contribution to other subject matters. When one surveys the breadth and quality of the Islamic contribution to human knowledge, it is uncontestable that islamic sources and works were as important to Western  (re)learning as the Greek and Hellenistic sources. Not only did they pass on to us much of the Greek and Hellenistic body of knowledge, but they added their own tremendous body of observations (far more than the Greeks and Hellenes ever did) and understanding that had been generated by the Islamic culture, especially the application of mathematics (including the Islamic inventions of algebra and trigonometry) to astronomy. This allowed much greater precision.  Moreover, they transformed the astronomical models from rational exercises (the greek notion that these models were created only to "save the phenomena") to descriptions of physical realities. This concept, inherited by the West from Islam, left no question or doubt in the western mind that the goal of "science" was to discover aspects of reality, not to "explain appearances". More than that, they also taught Western thinkers to base their theories on observations and to verify and test them in nature, not test their truth by argument. Still, despite its enormous achievements islamic knowledge, it never separated the universe from its philosophical underpinning and remained a natural philosophy throughout.  It should be emphasized however that it is the inheritance by the West, not only of the Greek and Hellenistic tradition but also (and perhaps more importantly) of the Islamic approach to knowledge and development of new bodies of knowledge, tools and techniques that was to culminate in the Renaissance and modern science in the Late Renaissance.

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