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Astronomy

As did Avempace and Ibn Tufail, Averroes criticizes the Ptolemaic system using philosophical arguments and rejects the use of eccentrics and epicycles to explain the apparent motions of the Moon, the Sun and the planets. He argued that those objects move uniformly in a strictly circular motion around the Earth, following Aristotelian principles. He postulates that there are three type of planetary motions; those that can be seen with the naked eye, those that requires instruments to observe and those that can only be known by philosophical reasoning. Averroes argues that the occasional opaque colors of the Moon are caused by variations in its thickness; the thicker parts receive more light from the Sun—and therefore emit more light—than the thinner parts. This explanation was used up to the seventeenth century by the European Scholastics to account for Galileo 's observations of spots on the Moon's surface, until the Scholastics such as Antoine Goudin in 1668 conceded that the observation was more likely caused by mountains on the Moon. He and Ibn Bajja observed sunspots, which they thought were transits of Venus and Mercury between the Sun and the Earth. In 1153 he conducted astronomical observations in Marrakesh, where he observed the star Canopus (Arabic: Suhayl) which was invisible in the latitude of his native Spain. He used this observation to support Aristotle's argument for the spherical Earth.

Averroes was aware that Arabic and Andalusian astronomers of his time focused on "mathematical" astronomy, which enabled accurate predictions through calculations but did not provide a detailed physical explanation of how the universe worked. According to him, "the astronomy of our time offers no truth, but only agrees with the calculations and not with what exists." He attempted to reform astronomy to be reconciled with physics, especially the physics of Aristotle. His long commentary of Aristotle's Metaphysics describes the principles of his attempted reform, but later in his life he declared that his attempts had failed. He confessed that he had not enough time or knowledge to reconcile the observed planetary motions with Aristotelian principles. In addition, he did not know the works of Eudoxus and Callippus, and so he missed the context of some of Aristotle's astronomical works. However, his works influenced astronomer Nur ad-Din al-Bitruji (d. 1204) who adopted most of his reform principles and did succeed in proposing an early astronomical system based on Aristotelian physics.