Aristotle had placed a stationary Earth at the centre of the Universe with the celestial bodies carried on spheres centred on Earth and rotating at constant velocity. Ptolemy had departed from Aristotle’s cosmology only when the need was to match the observations of Hipparchus, but the Arab astronomers had accepted Aristotle’s cosmology completely.
In the ninth century Al-Sabi Thabit ibn Qurra al-Harrani (Thebit; 826-901) drew attention to the inconsistencies: the equant caused the speed of a planet to vary and thereby violated the fundamental principle of Aristotelian astronomy that all motions must be uniform; eccentres involved circular motions about centres other than that of Earth; and even epicycles were objected to by the Aristotelian fundamentalists.
Abu Ali al-Hasan ibn al-Haytham (Alhazen; c.965-c.1040) in his On the Configuration of the World attempted to give physical reality to the geocentric model, presenting a detailed description of the physical structure of the concentric spheres. Muhammad ibn Rushd (Averroes; 1126-1198) maintained that the eccentric and epicyclic spheres were contrary to nature and that there was no deeper truth in the Ptolemaic system but that it was simply a mathematical tool to describe celestial movements.
Nasir al-Din al-Tusi (1201-1274), in his Tadhkira (‘Memorandum’), replaced the equant by the Tusi-couple, a device that generates linear motion from the sum of two circular motions. Ibn al-Shatir (1304-1375) made the best attempt to remove the ‘imperfections’ from the Ptolemaic models. His own geocentric models eliminated the epicycle from the solar model, eliminated the eccentrics and the equant from all the planetary models, and avoided the gross variation in the diameter of the Moon implied in Ptolemy’s model.
Leave a Reply