Ibn al-Shatir, The "Father" of Copernicus' Theory

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.

Ibn al-Shatir conducted extensive observations which led to some of his theoretical contributions, designed and constructed new instruments, and made advanced contributions to Islamic astronomy in the field of planetary theory.

His most important astronomical treatise was the Kitāb Nihāyat al-Suʾāl fī Taṣḥīḥ al-ʾUṣūl (كتاب نهاية السؤال في تصحيح الأصول - The Final Quest Concerning the Rectification of Principles), in which he drastically reformed the Ptolemaic models of the Sun, Moon, and planets. 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 both contemporary theory and empirical observations. 

Experimentally Ibn al-Shatir employed careful eclipse observations to measure the apparent size of the Sun and Moon and found that they disagreed with Ptolemaic expectations. His work on his experiments and observations (e.g. Ta'liq al-arsad, or Accounting for Observations) has not survived, but there are references to it in his Final Quest Concerning the Rectification of Principles.

Theoretically, Ibn al-Shatir objected to Aristotle's ether, in its eternal uniformity, and argued that if one grants that the heavens must allow for a variation in composition then there's no reason to reject epicycles, while agreeing that equants and eccentrics, which violated Aristotelian principles of uniform circular motion and gravity, were impossible. He then built a model that by adding new epicycles utilizing the Tusi-couple eliminated entirely the epicycle in the solar model, the eccentrics and equants in the planetary models, and the eccentric, epicycles and equant in the lunar model. The resulting model was one in which the Earth was at the exact center of the universe around which all heavenly bodies moved in uniform circular motions, remained as accurate as Ptolemy in predicting the paths of heavenly bodies, and improved on Ptolemy by accurately predicting the apparent size and distance of the Sun and Moon.

By creating the first model of the cosmos in which physical theory, mathematical model, and empirical observation were in agreement, Ibn al-Shatir marked a turning point in astronomy which may be considered a "Scientific Revolution before the Renaissance".

Although his system was firmly geocentric — he had eliminated the Ptolemaic equant and eccentrics  — the mathematical details of his system encompassed those in Nicolaus Copernicus' De revolutionibus, which had retained the Ptolemaic eccentric.  Copernicus' lunar model was identical to the lunar model of al-Shatir.  It is noted that in Copernicus' Commentariolus his model of Mercury is mistaken, and that snce it is Ibn al-Shatir's model, this is further evidence, and perhaps the best evidence, that Copernicus was in fact copying without full understanding from some other source. All this suggests that Ibn al-Shatir's model may have influenced, if indirectly, Copernicus while constructing the latter's heliocentric model. How Copernicus would have come across al-Shatir's work, exactly, remains an open question, but there are some number of possible routes for first or secondhand transmission.

Ibn al-Shatir constructed a magnificent sundial for the minaret of the Umayyad Mosque in Damascus which gave both seasonal and equinoctial hours. The fragments of this sundial in a Damascus museum make this the oldest polar-axis sundial still in existence.

Ibn al-Shatir made a timekeeping device incorporating both a universal sundial and a magnetic compass. 

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-ʾashiʿʿa al-lāmiʿa fī al-ʿamal bi-l-āla al-jāmiʿa). A commentary on this work entitled Book of Ripe Fruits from Clusters of Universal Instrument (Kitāb al-thimār al-yāni'a ʿan qutāf al-āla al-jāmiʿa) 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.

In the case of lunar motion, Ibn al-Shatir corrected Ptolemy, whose imagined Moon approached far closer to the Earth than did the actual Moon.

Many believe that astronomy died with the Greeks, and was brought back to life again by Copernicus, the 15th century Polish astronomer who is famous for introducing the Sun-centered (heliocentric) theory of the solar system, which marked the beginning of modern astronomy.

However, many historians now think it is not a coincidence that his models of planetary theory are mathematically identical to those prepared by Ibn al-Shatir over a century before him.  It is known that Copernicus relied heavily on the comprehensive astronomical treatise by al-Battani, which included star catalogues and planetary tables.

The mathematical devices discovered by Muslims before Copernicus, referred to in modern terms as linkages of constant length vectors rotating at constant angular velocities, are exactly the same as those used by Copernicus.  The only, but important, differences between the two was that the Muslims' Earth was fixed in space, whereas Copernicus had it orbiting around the Sun.  Copernicus also used instruments which were particular to astronomy in the East, like the parallactic ruler, which had previously only been used in Samarkand and Maragha Observatories.