La Theorie Astronomique Selon Jabir IBN Aflah (English Abstract)

Abstract

Ptolemy’s Almagest has been criticized by Islamic astronomers in two different ways: criticisms of Ptolemaic parameters and criticisms, like those of the Maragha school, of the geometrical models used as they contradicted certain basic principles like the principle of uniform motion.Jabir ibn Aflah’s Islah al-Majisti seems to be outside the two aforementioned ways of criticizing the Almagest for he gives an excellent and faithful qualitative account of his kinematical models. He pretends, on one side, to give a complement to the mathematical basis of the Almagest and, on the other, he seems to consider this work as having, in his time, only a theoretical value; due to the modifications of parameters introduced by Islamic astronomers, the Almagest itself has lost, in Jabir’s opinion, all practical value for computation. He also gives a list of mistakes he thinks Ptolemy made, but I will limit myself here to analyse what Jabir seems to consider a methodological lack of which I will give only one example: Jabir criticizes Ptolemy’s determination of the relative positions of the centre of the equant, the centre of the deferent and the centre of the universe in the case of the models for the superior planets, for he considers that Ptolemy did not give any proof of the fact that the centre of the deferent is the midpoint between the two others.