Ibn al-Shatir

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.^[1]

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. Other achievements of Ibn al-Shatir include the separation of natural philosophy from astronomy and 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".^[1]

Testing

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 would usually test the Ptolemaic models, and if any did not match his observations, then he would formulate his own non-Ptolemaic model in its place which did match his observations. For example, it was Ibn al-Shatir's concern for observational accuracy which led him to eliminate the epicycle in the Ptolemaic solar model, as he describes below:^[2]

Ibn al-Shatir was also the earliest astronomer to introduce experiments into planetary theory, in order to test the Ptolemaic models on empirical grounds. For example, while testing the Ptolemaic solar model, Ibn al-Shatir "tested 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.^[3]

Influence

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.^[4] His lunar model was also no different from the lunar model used by Copernicus.^[5] It is thus believed that Ibn al-Shatir's model was adapted by Copernicus into a 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.^[6] 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.^[7]

Engineering

Astrolabic clock

Ibn al-Shatir invented the first astrolabic clock in the early 14th century.^[9]

Polar-axis sundial

Ibn al-Shatir constructed a magnificent sundial for the minaret of the Umayyad Mosque in Damascus.^[9] 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.^[10]^[11]

Compass dial

Compendium

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.^[13]

Universal instrument

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 al-Din from 1577-1580.^[14]

Contents

Astronomy

Theory