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Ibn al-Shatir (full name: 'Ala' al-Din 'Ali ibn Ibrahim ibn Muhammad ibn al-Shatir al-Ansari al-Miqati; born c. 1304, Damascus; died c. 1375, Damascus) was a medieval Arab astronomer, mathematician, and engineer who served as the chief muwaqqit (timekeeper) at the Umayyad Mosque in Damascus. He is most widely known for constructing a geocentric planetary model that eliminated the equant and other Ptolemaic irregularities by using combinations of uniform circular motions - and for the striking mathematical correspondence between that model and the heliocentric models later published by Nicolaus Copernicus in 1543.

Ibn al-Shatir worked within the tradition of hay'a (theoretical astronomy) that had developed across the Islamic world from the 11th century onward. His major astronomical work, the Nihayat al-Sul fi Tashih al-Usul (“A Final Inquiry Concerning the Rectification of Planetary Theory”), presented a complete revision of Ptolemaic planetary theory. For each planet, he replaced the Ptolemaic equant and eccentric with a system of secondary epicycles, producing models in which all motions were uniform about their own centers. His lunar model was a significant improvement over Ptolemy's in that it eliminated the large variation in the apparent size of the Moon that the Ptolemaic model incorrectly predicted. He also produced a notable zij (astronomical handbook with tables), the Al-Zij al-Jadid (“The New Astronomical Handbook”), and designed and built several sophisticated instruments, including a polar-axis sundial.

The relationship between Ibn al-Shatir's work and Copernicus has been a significant topic in the history of science since the 1950s and 1960s, when historians including E. S. Kennedy and Victor Roberts identified the close mathematical parallels. For the Moon and Mercury in particular, the mathematical structures of the two astronomers' models are identical. Whether Copernicus had access to Ibn al-Shatir's work - and if so, through what channel - remains an open scholarly question. Some historians, including George Saliba and Noel Swerdlow, have argued for transmission through Byzantine or Italian intermediaries in the 15th century; others hold that independent rediscovery cannot be ruled out. For the debate over transmission and its implications, see Heliocentrism - History and Islamic Science - History.

Ibn al-Shatir is also associated with the broader Maragha School tradition, which includes astronomers such as Nasir al-Din al-Tusi (inventor of the Tusi Couple) and Ibn al-Shatir's possible predecessor Mu'ayyad al-Din al-'Urdi. He is generally regarded as the culminating figure of that tradition.

There is broad scholarly consensus that the mathematical structures of Ibn al-Shatir's planetary models and those of Copernicus are closely - and in some cases exactly - parallel. The question of whether and how Copernicus's models were influenced by or derived from Islamic predecessors remains contested. See Heliocentrism - Transmission Consensus for the state of scholarly opinion.

• Independent parallel development - Some scholars hold that the mathematical similarities between Ibn al-Shatir and Copernicus resulted from both astronomers independently confronting the same defects in Ptolemaic theory, without requiring direct transmission. See Heliocentrism - Copernicus Independent Development Viewpoint.
• Transmission from Islamic sources - Other scholars argue that the correspondences are too precise to be coincidental and that Copernicus must have had access, directly or indirectly, to Arabic manuscripts. See Heliocentrism - Islamic Transmission Viewpoint.
• Islamic priority in the Copernican revolution - A broader interpretive position, associated with scholars such as George Saliba and Noel Swerdlow, holds that the mathematical core of the Copernican revolution was largely developed by Islamic astronomers, with Copernicus providing primarily the heliocentric reframing. See Islamic Science - European Renaissance Viewpoint.

• Heliocentrism
• Heliocentrism - History
• Maragha School
• Nasir al-Din al-Tusi
• Tusi Couple
• Ptolemaic Model
• Nicolaus Copernicus
• Islamic Science - History
• George Saliba

1. Roberts, Victor. “The Solar and Lunar Theory of Ibn ash-Shatir: A Pre-Copernican Copernican Model.” Isis 48, no. 4 (1957): 428-432.
2. Kennedy, E. S., and Victor Roberts. “The Planetary Theory of Ibn al-Shatir.” Isis 50, no. 3 (1959): 227-235.
3. Swerdlow, N. M. “The Derivation and First Draft of Copernicus's Planetary Theory: A Translation of the Commentariolus with Commentary.” Proceedings of the American Philosophical Society 117, no. 6 (1973): 423-512.
4. Saliba, George. Islamic Science and the Making of the European Renaissance. Cambridge, MA: MIT Press, 2007.
5. Neugebauer, Otto, and N. M. Swerdlow. Mathematical Astronomy in Copernicus's De Revolutionibus. New York: Springer, 1984.
6. Hartner, Willy. “Copernicus, the Man, the Work, and Its History.” Proceedings of the American Philosophical Society 117, no. 6 (1973): 413-422.
7. King, David A. In Synchrony with the Heavens: Studies in Astronomical Timekeeping and Instrumentation in Medieval Islamic Civilization. 2 vols. Leiden: Brill, 2004-2005.