Ala Al-Din Abu'l-Hasan Ali Ibn Ibrahim Ibn al-Shatir (1304 – 1375) (Arabic: ابن الشاطر) was an Arab Muslim astronomer, mathematician, engineer and inventor who worked as muwaqqit (موقت, religious timekeeper) at the Umayyad Mosque in Damascus, Syria.
His most important astronomical treatise was the Kitab nihayat al-sul fi tashih al-usul (The Final Quest Concerning the Rectification of Principles), in which he drastically reformed the Ptolemaic models of the Sun, Moon, and planets, by his introducing his own non-Ptolemaic models which eliminates the epicycle in the solar model, which eliminate the eccentrics and equant by introducing extra epicycles in the planetary models via the Tusi-couple, and which eliminates all eccentrics, epicycles and equant in the lunar model.
While previous Maragha school models were just as accurate as the Ptolemaic model, Ibn al-Shatir's geometrical model was the first that was actually superior to the Ptolemaic model in terms of its better agreement with empirical observations. Another achievement of Ibn al-Shatir was the rejection of the Ptolemaic model on empirical rather than philosophical grounds. Unlike previous astronomers before him, Ibn al-Shatir was not concerned with adhering to the theoretical principles of cosmology or natural philosophy (or Aristotelian physics), but rather to produce a model that was more consistent with empirical observations. His model was thus in better agreement with empirical observations than any previous models produced before him. His work thus marked a turning point in astronomy, which may be considered a "Scientific Revolution before the Renaissance".
Unlike previous astronomers, Ibn al-Shatir generally had no philosophical objections against Ptolemaic astronomy, but was only concerned with how well it matched his own empirical observations. He employed careful observations of the diameters of the Sun and Moon to test the Ptolemaic models on empirical grounds, testing "the Ptolemaic value for the apparent size of the solar disk by using lunar eclipse observations." His work on his experiments and observations, however, has not survived, but there are references to this work in his The Final Quest Concerning the Rectification of Principles.
As a consequence of his observations, he formulated his own modification of the Ptolemaic model. Ibn al-Shatir's concern with the observed solar diameter led him to replace Ptolemy's epicycle and equant solar model with a model using three spheres, a large sphere centered on the Earth which he called the parecliptic, a smaller sphere carried by the pareclptic, which he called the the deferent, and an even smaller sphere carried by the deferent, which he called the director. The Sun was then carried by the director.
Although his system was firmly geocentric, he had eliminated the Ptolemaic equant and eccentrics, and the mathematical details of his system were identical to those in Nicolaus Copernicus' De revolutionibus. His lunar model was also no different from the lunar model used by Copernicus. This suggests that Ibn al-Shatir's model may have influenced Copernicus while constructing the heliocentric model. Though it remains uncertain how this may have happened, it is known that Byzantine Greek manuscripts containing the Tusi-couple which Ibn al-Shatir employed had reached Italy in the 15th century. It is also known that Copernicus' diagrams for his heliocentric model, including the markings of points, was nearly identical to the diagrams and markings used by Ibn al-Shatir for his geocentric model, making it very likely that Copernicus may have been aware of Ibn al-Shatir's work.
Y. M. Faruqi writes:
"Ibn al-Shatir’s theory of lunar motion was very similar to that attributed to Copernicus some 150 years later".
"Whereas Ibn al-Shatir’s concept of planetary motion was conceived in order to play an important role in an earth-centred planetary model, Copernicus used the same concept of motion to present his sun-centred planetary model. Thus the development of alternative models took place that permitted an empirical testing of the models."
Recent evidence has shown that there were Jewish intermediaries who transmitted Ibn al-Shatir's work to Europe, more directly influencing Copernicus. 
Ibn al-Shatir constructed a magnificent sundial for the minaret of the Umayyad Mosque in Damascus. As the ancient sundials were nodus-based with straight hour-lines, they indicated unequal hours—also called temporary hours—that varied with the seasons. Every day was divided into twelve equal segments; thus, hours were shorter in winter and longer in summer. The idea of using hours of equal length throughout the year was the innovation of Abu'l-Hasan Ibn al-Shatir in 1371, based on earlier developments in trigonometry by Muhammad ibn Jābir al-Harrānī al-Battānī (Albategni). He was aware that "using a gnomon that is parallel to the Earth's axis will produce sundials whose hour lines indicate equal hours on any day of the year." His sundial is the oldest polar-axis sundial still in existence. The concept later appeared in Western sundials from at least 1446.
The compass dial, a timekeeping device incorporating both a universal sundial and a magnetic compass, was invented by Ibn al-Shatir in the early 14th century.
The compendium, a multi-purpose astronomical instrument, was first constructed by Ibn al-Shatir. His compendium featured an alhidade and polar sundial among other things. These compendia later became popular in Renaissance Europe.
Ibn al-Shatir described another astronomical instrument which he called the "universal instrument" in his Rays of light on operations with the universal instrument (Al-Ashi'a al-lāmi'a fī 'l-'amal bi-'l-āla al jāmi'a). A commentary on this work entitled Book of Ripe Fruits from Clusters of Universal Instrument (Kitab al-thimār al-yāni'a ‘an qutāf al-āla al-jāmi'a) was later written by the Ottoman astronomer and engineer Taqi al-Din, who employed the instrument at the Istanbul observatory of Taqi al-Din from 1577-1580.
- List of Arab scientists and scholars
- Islamic scholars
- George Saliba (1994), A History of Arabic Astronomy: Planetary Theories During the Golden Age of Islam, p. 233-234 & 240, New York University Press, ISBN 0814780237.
- George Saliba (1994), A History of Arabic Astronomy: Planetary Theories During the Golden Age of Islam, p. 238, New York University Press, ISBN 0814780237.
- George Saliba (1994), A History of Arabic Astronomy: Planetary Theories During the Golden Age of Islam, p. 239, New York University Press, ISBN 0814780237.
- The model Copernicus used in his earlier Commentariolus differs in minor detail from that of Ibn al-Shatir. V. Roberts and E. S. Kennedy, "The Planetary Theory of Ibn al-Shatir", Isis, 50(1959):232-234.
- George Saliba (1994), A History of Arabic Astronomy: Planetary Theories During the Golden Age of Islam, p. 236, New York University Press, ISBN 0814780237.
- Guessoum, N. (June 2008). "Copernicus and Ibn Al-Shatir: does the Copernican revolution have Islamic roots?". The Observatory 128: 231-239. Bibcode: 2008Obs...128..231G.
- A. I. Sabra (1998). "Configuring the Universe: Aporetic, Problem Solving, and Kinematic Modeling as Themes of Arabic Astronomy", Perspectives on Science 6 (3), p. 288-330.
- George Saliba (2007), Lecture at SOAS, London - Part 4/7 and Lecture at SOAS, London - Part 5/7
- Y. M. Faruqi (2006). "Contributions of Islamic scholars to the scientific enterprise", International Education Journal 7 (4), p. 395-396.
- David A. King (1983), "The Astronomy of the Mamluks", Isis 74 (4), pp. 531-555 [545-6]
- "History of the sundial". National Maritime Museum. Retrieved on 2008-07-02.
- Jones, Lawrence (December 2005), "The Sundial And Geometry", North American Sundial Society 12 (4)
- David A. King (1983). "The Astronomy of the Mamluks", Isis 74 (4), p. 531-555 [547-548].
- King, David A., "Astronomy and Islamic society", pp. 163–8, in (Rashed & Morelon 1996, pp. 128-184)
- Dr. Salim Ayduz (26 June 2008). "Taqi al-Din Ibn Ma’ruf: A Bio-Bibliographical Essay". Retrieved on 2008-07-04.)
- Fernini, Ilias. A Bibliography of Scholars in Medieval Islam. Abu Dhabi (UAE) Cultural Foundation, 1998
- Kennedy, Edward S. "Late Medieval Planetary Theory." Isis 57 (1966):365-378.
- Kennedy, Edward S. and Ghanem, Imad. The Life and Work of Ibn al-Shatir, an Arab Astronomer of the Fourteenth Century. Aleppo: History of Arabic Science Institute, University of Aleppo, 1976.
- Roberts, Victor. "The Solar and Lunar Theory of Ibn ash-Shatir: A Pre-Copernican Copernican Model". Isis, 48(1957):428-432.
- Roberts, Victor and Edward S. Kennedy. "The Planetary Theory of Ibn al-Shatir". Isis, 50(1959):227-235.
- Saliba, George. "Theory and Observation in Islamic Astronomy: The Work of Ibn al-Shatir of Damascus". Journal for the History of Astronomy, 18(1987):35-43.
- Turner, Howard R. Science in Medieval Islam, an illustrated introduction. University of Texas Press, Austin, 1995. ISBN 0-292-78149-0 (pb) ISBN 0-292-78147-4 (hc)