Born : 1304, Damascus
Died : 1375 (aged 71)
ʿAbu al-Ḥasan Alāʾ al‐Dīn ʿAlī ibn Ibrāhīm al-Ansari known as Ibn
al-Shatir or Ibn ash-Shatir (Arabic: ابن الشاطر; 1304–1375) was a Syrian Arab astronomer, mathematician and
engineer.
He worked as muwaqqit (موقت,
religious timekeeper) in the Umayyad Mosque in Damascus and constructed a
sundial for its minaret in 1371/72.
Astronomy
Ibn al-Shatir most important astronomical treatise was kitab nihayat al-sul
fi tashih al-usul ("The Final Quest Concerning the Rectification of
Principles"). In it he drastically reformed the Ptolemaic models of the
Sun, Moon and planets. His model incorporated the Urdi lemma, and eliminated
the need for an equant by introducing an extra epicycle (the Tusi-couple),
departing from the Ptolemaic system in a way that was mathematically identical
to what Nicolaus Copernicus did in the 16th century.
Ibn al-Shatir's model for the
appearances of Mercury,
showing the multiplication of
epicycles in a Ptolemaic enterprise
Unlike previous astronomers before him, Ibn al-Shatir was not concerned
with adhering to the theoretical principles of natural philosophy or
Aristotelian cosmology, but rather to produce a model that was more consistent
with empirical 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 and all the eccentrics, epicycles and equant in the Ptolemaic lunar
model. His model was thus in better agreement with empirical observations than
any previous model, and was also the first that permitted empirical testing.
His work thus marked a turning point in astronomy, which may be considered a
"Scientific Revolution before the Renaissance".
Ibn al-Shatir's model for the appearances of Mercury, showing the
multiplication of epicycles in a Ptolemaic enterprise
Ibn al-Shatir's model for the appearances of Mercury, showing the
multiplication of epicycles in a Ptolemaic enterprise
Drawing on the observation that the distance to the Moon did not change as
drastically as required by Ptolemy's lunar model, Ibn al-Shatir produced a new
lunar model that replaced Ptolemy's crank mechanism with a double epicycle
model that reduced the computed range of distances of the Moon from the Earth.
This was the first accurate lunar model which matched physical observations.
Possible influence on Nicolaus Copernicus
Although Ibn al-Shatir's system was firmly geocentric (he had eliminated
the Ptolemaic eccentrics), the mathematical details of his system were
identical to those in Copernicus's De revolutionibus. Furthermore, the exact
replacement of the equant by two epicycles used by Copernicus in the
Commentariolus paralleled the work of Ibn al-Shatir one century earlier. Ibn
al-Shatir's lunar and Mercury models are also identical to those of Copernicus.
This has led some scholars to argue that Copernicus must have had access to
some yet to be identified work on the ideas of ibn al-Shatir. It is unknown
whether Copernicus read ibn al-Shatir and the argument is still debated.
Instruments
The idea of using hours of equal time length throughout the year was the
innovation of Ibn al-Shatir in 1371, based on earlier developments in
trigonometry by al-Battānī. Ibn al-Shatir 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.
Ibn al-Shatir also invented a timekeeping device called "ṣandūq
al‐yawāqīt" (jewel box), which incorporates both a universal sundial and a
magnetic compass. He invented it for the purpose of finding the times of
prayers. Other notable instruments invented by him include a reversed astrolabe
and an astrolabic clock.
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