Editing Ibn al-Haytham (section)

== Astronomical works ==

=== ''On the Configuration of the World'' ===

In his ''On the Configuration of the World'' Alhazen presented a detailed description of the physical structure of the earth:{{blockquote|The earth as a whole is a round sphere whose center is the center of the world. It is stationary in its [the world's] middle, fixed in it and not moving in any direction nor moving with any of the varieties of motion, but always at rest.<ref>{{harvnb|Langermann|1990}}, chap. 2, sect. 22, p. 61</ref>}}

The book is a non-technical explanation of Ptolemy's ''[[Almagest]]'', which was eventually translated into [[Hebrew language|Hebrew]] and [[Latin]] in the 13th and 14th centuries and subsequently had an influence on astronomers such as [[Georg von Peuerbach]]<ref name="{{harvnb|lorch|2008}}.">{{harvnb|Lorch|2008}}.</ref> during the European [[Middle Ages]] and [[Renaissance]].<ref>{{harvnb|Langermann|1990|pp=34–41}}; {{harvnb|Gondhalekar|2001|p=21}}.</ref>

=== ''Doubts Concerning Ptolemy'' ===

In his ''Al-Shukūk ‛alā Batlamyūs'', variously translated as ''Doubts Concerning Ptolemy'' or ''Aporias against Ptolemy'', published at some time between 1025 and 1028, Alhazen criticized [[Ptolemy]]'s ''Almagest'', ''Planetary Hypotheses'', and ''Optics'', pointing out various contradictions he found in these works, particularly in astronomy. Ptolemy's ''Almagest'' concerned mathematical theories regarding the motion of the planets, whereas the ''Hypotheses'' concerned what Ptolemy thought was the actual configuration of the planets. Ptolemy himself acknowledged that his theories and configurations did not always agree with each other, arguing that this was not a problem provided it did not result in noticeable error, but Alhazen was particularly scathing in his criticism of the inherent contradictions in Ptolemy's works.<ref name="{{harvnb|sabra|1998}}.">{{harvnb|Sabra|1998}}.</ref> He considered that some of the mathematical devices Ptolemy introduced into astronomy, especially the [[equant]], failed to satisfy the physical requirement of uniform circular motion, and noted the absurdity of relating actual physical motions to imaginary mathematical points, lines and circles:<ref>{{harvnb|Langermann|1990|pp=8–10}}</ref>

{{blockquote|Ptolemy assumed an arrangement (''hay'a'') that cannot exist, and the fact that this arrangement produces in his imagination the motions that belong to the planets does not free him from the error he committed in his assumed arrangement, for the existing motions of the planets cannot be the result of an arrangement that is impossible to exist...  [F]or a man to imagine a circle in the heavens, and to imagine the planet moving in it does not bring about the planet's motion.<ref>{{harvnb|Sabra|1978b|p=121, n. 13}}</ref>}}

Having pointed out the problems, Alhazen appears to have intended to resolve the contradictions he pointed out in Ptolemy in a later work. Alhazen believed there was a "true configuration" of the planets that Ptolemy had failed to grasp. He intended to complete and repair Ptolemy's system, not to replace it completely.<ref name="{{harvnb|sabra|1998}}." /> In the ''Doubts Concerning Ptolemy'' Alhazen set out his views on the difficulty of attaining scientific knowledge and the need to question existing authorities and theories:

{{blockquote|Truth is sought for itself [but] the truths, [he warns] are immersed in uncertainties [and the scientific authorities (such as Ptolemy, whom he greatly respected) are] not immune from error...<ref name="{{harvnb|sabra|1989}}." />}}

He held that the criticism of existing theories{{snd}}which dominated this book{{snd}}holds a special place in the growth of scientific knowledge.

=== ''Model of the Motions of Each of the Seven Planets'' ===

Alhazen's ''The Model of the Motions of Each of the Seven Planets'' was written {{circa}} 1038. Only one damaged manuscript has been found, with only the introduction and the first section, on the theory of planetary motion, surviving. (There was also a second section on astronomical calculation, and a third section, on astronomical instruments.) Following on from his ''Doubts on Ptolemy'', Alhazen described a new, geometry-based planetary model, describing the motions of the planets in terms of spherical geometry, infinitesimal geometry and trigonometry. He kept a geocentric universe and assumed that celestial motions are uniformly circular, which required the inclusion of [[epicycles]] to explain observed motion, but he managed to eliminate Ptolemy's [[equant]]. In general, his model didn't try to provide a causal explanation of the motions, but concentrated on providing a complete, geometric description that could explain observed motions without the contradictions inherent in Ptolemy's model.<ref>{{harvnb|Rashed|2007}}.</ref>

=== Other astronomical works ===

Alhazen wrote a total of twenty-five astronomical works, some concerning technical issues such as ''Exact Determination of the Meridian'', a second group concerning accurate astronomical observation, a third group concerning various astronomical problems and questions such as the location of the [[Milky Way]]; Alhazen made the first systematic effort of evaluating the Milky Way's parallax, combining Ptolemy's data and his own. He concluded that the parallax is (probably very much) smaller than Lunar parallax, and the Milky way should be a celestial object. Though he was not the first who argued that the Milky Way does not belong to the atmosphere, he is the first who did quantitative analysis for the claim.<ref>{{harvnb|Eckart|2018}}</ref>
The fourth group consists of ten works on astronomical theory, including the ''Doubts'' and ''Model of the Motions'' discussed above.<ref>{{harvnb|Rashed|2007|pp=8–9}}.</ref>