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THE MERCURY MODELS OF IBN AL-ŠĀṬIR AND COPERNICUS

  • Sajjad Nikfahm-Khubravan (a1) and F. Jamil Ragep (a1)

Abstract

Copernicus' complex Mercury model in De revolutionibus is virtually identical, geometrically, to Ibn al-Šāṭir's (ca. 1305 – ca. 1375). However, the model in his earlier Commentariolus is different and in many ways unworkable. This has led some to claim that the younger Copernicus did not understand his predecessor's model; others have maintained that Copernicus was working totally independently of Ibn al-Šāṭir. We argue that Copernicus did have Ibn al-Šāṭir's models but needed to modify them to conform to a “quasi-homocentricity” in the Commentariolus. This modification, and the move from a geocentric to heliocentric cosmology, was facilitated by the “heliocentric bias” of Ibn al-Šāṭir's models, in which the Earth was the actual center of mean motion, in contrast to Ptolemy and most Islamicate astronomers. We show that: 1) Ibn al-Šāṭir sought to reproduce Ptolemy's critical elongation at the trines(±120°), but changed the Ptolemaic values at 0, ±90, and 180°; 2) in the Commentariolus, Copernicus does not try to produce viable elongations for Mercury; and 3) by the time of writing De revolutionibus, Copernicus is in full control of the Mercury model and is able to faithfully reproduce Ptolemy's elongations at all critical points. We also argue that claims regarding “natural” solutions undermining transmission are belied by historical evidence.

Le modèle complexe de Mercure dans le De revolutionibus de Copernic est virtuellement identique, géométriquement, à celui d'Ibn al-Šāṭir (ca. 1305 – ca. 1375). Cependant, le modèle, antérieur, du Commentariolus est différent et il fonctionne mal. Certains en ont déduit que le jeune Copernic n'avait pas compris le modèle de son pré- décesseur; d'autres ont affirmé que l'œuvre de Copernic était totallement indépendante d'Ibn al-Šāṭir. Nous soutenons que Copernic avait les modèles d'Ibn al-Šāṭir mais qu'il a dû les modifier pour les rendre “quasi-homocentriques” dans le Commentariolus. Cette modification et le passage d'une cosmologie géocentrique à une cosmologie héliocentrique étaient rendus aisés par le “biais héliocentrique” des modèles d'Ibn al-Šāṭir, pour qui la Terre était le centre effectif du mouvement moyen, contrairement à Ptolémée et à la plupart des astronomes islamiques. Nous montrons que : 1) Ibn al-Šāṭir a cherché à reproduire les élongations critiques à ±120° de l'apogée, mais il a changé les valeurs ptoléméennes à 0, ±90 et 180° ; 2) dans le Commentariolus, Copernic n'essaie pas de reproduire des élongations viables pour Mercure;et 3) au moment de la rédaction du De revolutionibus, Copernic contrôle pleinement le modèle de Mercure et il est capable de reproduire les élongations de Ptolémée aux points critiques. Nous soutenons aussi que les arguments concernant des solutions “naturelles” qui excluent la transmission sont niés par l’évidence historique.

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THE MERCURY MODELS OF IBN AL-ŠĀṬIR AND COPERNICUS

  • Sajjad Nikfahm-Khubravan (a1) and F. Jamil Ragep (a1)

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