The Arabic Rescue

After 632 CE, Islam expands in the Middle East and by 661 the arabic expansion has conquered most of the world originally influenced by the hellenistic culture (including Syria, Persia, Egypt and Libya). As happens to many conquerors, the Arabs become hellenized under Persian and Nestorian influence. From 750 on, translation of Greek and Syriac works into Arabic become increasingly common. Also in the VIIIth Cent. the main works of Indian astronomy are translated and incorporated into the body of Islamic astronomy.  By the IXth Cent., the main Hellenistic works (Ptolemy, Hipparchus, Aristarchus to name but a few) are translated into Arabic.  This is also whenArabic numerals enter the Western world.  By the year 1000, virtually the entire body of Greek and Hellenistic medicine, natural philosophy and mathematics has been translated. From the IXth Cent. on, Islamic thinkers also add much detail and original contributions to Hellenistic natural philosophy and mathematics (spherical trigonometry for instance). And much of this body of work becomes widely disseminated in the Islamic world, including the western parts of the Islamic world, Sicily and Spain.
More about some islamic astronomers

Relearning in the West

One of the pioneers of western relearning is Gerbert (945-1003), who goes to Spain to study arabic mathematics. Gradually, through increased contact between the Christian and Islamic world, it becomes clear to the western world that Islam and Byzantium are repositories of knowledge from which the West could profit. Between 1065 and 1085 the first translations of medicine texts appear. In the 1100s, after the fall of Toledo, the works of this rich arabic library fall into christian hands and are translated back. One rather important figure is Gherard of Cremona (1114-1187). He originally went to Toledo, Spain so he could learn Arabic and thereby be able to read Ptolemy since there were no existing translations. He never left Toledo and wound up translating Ptolemy and Euclid into latin. Another seminal figure was Adelard of Bath (1075-1160) who made a famous translation of Euclid from Arabic into Latin, translated al-Khwarizmi's tables and wrote on the abacus and astrolabe (an astronomical instrument). 

Similar translation work takes place in Italy and Sicily during this time, with new schools intent on recovering and mastering the classical texts. This period of time also sees the formation of universities, great centers of learning, where instruction includes the newly recovered “classics”. Thus, by 1200, some eighty original Greek texts have been translated back from arabic. In 1202 Fibonacci (Leonardo Pisano,1170-1250) publishes Liber abaci where he introduces the decimal system and arabic numbers to the West. Robert Grosseteste (1168-1253) makes many translations of Greek and Arabic writings. Thus, as major portions of the Greek and Arabic knowledge enter the western world, Aristotelian philosophy and logic slowly come to dominate academic discourse and learning with the scholastics.

With the transfer of the astrolabe (a more precise astronomical measuring/observation instrument) from the Arabs, western astronomy becomes more quantitative. This shift, from philosophical to quantitative, is also helped by the translation of arabic observational tables. From 1250 on, the main aspect of western astronomy is one of growing mastery of the ancient knowledge and its dissemination through the universities. And it is these currents that lead us to the Renaissance natural philosophers, Copernicus and Brahe.