RésuméNous avons tenté de restituer ici les mathématiques qui président aux théories planétaires exposées par l'astronome syrien Ibn al-Šāṭir dans son ouvrage Nihāyat al-Sūl. Dans la lignée des astronomes de l’école de Marāgha, en composant des mouvements de rotation à vitesse angulaire constante, Ibn al-Šāṭir atteint deux objectifs. Non seulement il élimine tout recours aux excentriques et aux points équants; mais il décrit aussi longitudes et latitudes planétaires par une méthode unique, sans adjoindre aucun orbe en sus des orbes nécessaires (...) à la description des seuls mouvements en longitude. Une meilleure compréhension des rotations comme transformations spatiales lui permet cette grande économie de moyens. Dans notre commentaire, nous prenons pour exemple la planète Vénus dont les latitudes posent un problème intéressant. C'est aussi l'occasion d'offrir l’édition critique d'un chapitre de la Nihāyat al-Sūl consacré aux mouvements en latitude de Mercure et Vénus. (shrink)

ArgumentIn theAlmagest, Ptolemy finds that the apogee of Mercury moves progressively at a speed equal to his value for the rate of precession, namely one degree per century, in the tropical reference system of the ecliptic coordinates. He generalizes this to the other planets, so that the motions of the apogees of all five planets are assumed to be equal, while the solar apsidal line is taken to be fixed. In medieval Islamic astronomy, one change in this general proposition took (...) place because of the discovery of the motion of the solar apogee in the ninth century, which gave rise to lengthy discussions on the speed of its motion. Initially Bīrūnī and later Ibn al-Zarqālluh assigned a proper motion to it, although at different rates. Nevertheless, appealing to the Ptolemaic generalization and interpreting it as a methodological axiom, the dominant idea became to extend it in order to include the motion of the solar apogee as well. Another change occurred after correctly making a distinction between the motion of the apogees and the rate of precession. Some Western Islamic astronomers generalized Ibn al-Zarqālluh's proper motion of the solar apogee to the apogees of the planets. Analogously, Ibn al-Shāṭir maintained that the motion of the apogees is faster than precession. Nevertheless, the Ptolemaic generalization in the case of the equality of the motions of the apogees remained untouchable, despite the notable development of planetary astronomy, in both theoretical and observational aspects, in the late Islamic period. (shrink)

This paper analyses a kinematic model for the solar motion by Quṭb al-Dīn al-Shīrāzī, a thirteenth-century Iranian astronomer at the Marāgha observatory in northwestern Iran. The purpose of this model is to account for the continuous decrease of the obliquity of the ecliptic and the solar eccentricity since the time of Ptolemy. Shīrāzī puts forward different versions of the model in his three major cosmographical works. In the final version, in his Tuḥfa, the mean ecliptic is defined by an eccentric (...) of fixed mean eccentricity and a mean obliquity fixed with respect to the celestial equator, and the center of the epicycle, which is inclined to the eccentric, moves on the eccentric with an annual period. By an additional slow motion of the sun on the epicycle, the true eccentricity of the solar deferent, defined by the annual motion of the sun, and the sun’s extreme declination from the equator change, accounting for the reduction of the eccentricity and the obliquity of the ecliptic since the time of Ptolemy. (shrink)