section_path: Natural sciences/Astronomy
section_name: Astronomy
paragraph_text: Astronomy
  
  Astronomy in Islam was able to grow greatly because of several key factors. One factor was geographical: the Islamic world was close to the ancient lands of the Greeks, which held valuable ancient knowledge of the heavens in Greek manuscripts. During the new Abbasid Dynasty after the movement of the capital in 762 AD to Baghdad, translators were sponsored to translate Greek texts into Arabic. This translation period led to many major scientific works from Galen, Ptolemy, Aristotle, Euclid, Archimedes, and Apollonius being translated into Arabic. From these translations previously lost knowledge of the cosmos was now being used to advance current astrological thinkers. The second key factor of astronomy's growth was the religious observances followed by Muslims which expected them to pray at exact times during the day. These observances in timekeeping led to many questions in previous Greek mathematical astronomy, especially their timekeeping.
  
  The astrolabe was a Greek invention which was an important piece of Arabic astronomy. An astrolabe is a handheld two-dimensional model of the sky which can solve problems of spherical astronomy. It is made up of lines of altitude and azimuth with an index, horizon, hour circle, zenith, Rete, star pointer, and equator to accurately show where the stars are at that given moment. Use of the astrolabe is best expressed in Al-Farghani 's treatise on the astrolabe due to the mathematical way he applied the instrument to astrology, astronomy, and timekeeping. The earliest known Astrolabe in existence today comes from the Islamic period. It was made by Nastulus in 927-28 AD and is now a treasure of the Kuwait National Museum.
  
  In about 964 AD, the Persian astronomer Abd al-Rahman al-Sufi, writing in his Book of Fixed Stars, described a "nebulous spot" in the Andromeda constellation, the first definitive reference to what is now known to be the Andromeda Galaxy, the nearest spiral galaxy to the Milky Way.
  
  The geocentric system developed by Ptolemy placed the sun, moon, and other planets in orbit around the Earth. Ptolemy thought that the planets moved on circles called epicycles and that their centers rode on deferents. The deferents were eccentric, and the angular motion of a planet was uniform around the equant which was a point opposite the deferent center. Simply, Ptolemy's models were a mathematical system for predicting the positions of the planets. One of the first to criticize this model was Ibn al-Haytham, a leader of physics in the 11th century in Cairo. Then in the 13th century Nasir al-Din al-Tusi constructed the Maragha Observatory in what is today Iran. Al-Tusi found the equant dissatisfying and replaced it by adding a geometrical technique called a Tusi-couple, which generates linear motion from the sum of two circular motions. Then, Ibn al-Shatir who was working in Damascus in 1350 AD employed the Tusi-couple to successfully eliminate the equant as well as other objectionable circles that Ptolemy had used. This new model properly aligned the celestial spheres and was mathematically sound. This development by Ibn al-Shatir, as well as the Maragha astronomers remained relatively unknown in medieval Europe.
  
  The names for some of the stars used, including Betelgeuse, Rigel, Vega, Aldebaran, and Fomalhaut are several of the names that come directly from Arabic origins or are the translations of Ptolemy's Greek descriptions which are still in use today.