The paper brings into light and discusses a concentric solar model briefly described in Chapter 5 of Section III of ‘Abd al-Raḥmān al-Khāzinī’s On experimental astronomy, a treatise embedded in the prolegomenon of his comprehensive Mu‘tabar zīj, completed about 1121 c.e. In it, the Sun is assumed to rotate on the circumference of a circle concentric with the Earth and coplanar with the ecliptic, but the motion of the vector joining the Earth and Sun is monitored by a small eccentric (...) hypocycle. The ratio between the distance of the hypocycle’s center from the Earth and the hypocycle’s radius is equal to the solar eccentricity in the eccentric model. The model is to account for the constancy of the apparent diameter of the solar disk as held by Ptolemy. The source of the model is unknown. Since the mechanism employed in it clearly resembles the pin-and-slot device, whose use in mechanical astronomical instruments has a long history from the Antikythera Mechanism to the medieval solar, lunar, and planetary equatoria and dials, we argue that the solar model can be positioned within this long-standing tradition and considered the result of the correct understanding of some Byzantine prototype and thus a typical example of the transmission of astronomical ideas via media of the material culture. (shrink)

From Antiquity through the early modern period, the apparent motion of the Sun in longitude was simulated by the eccentric model set forth in Ptolemy’s Almagest III, with the fundamental parameters including the two orbital elements, the eccentricity e and the longitude of the apogee λ A, the mean motion ω, and the radix of the mean longitude $$ \bar{\lambda }_{0} $$ λ¯0. In this article we investigate the accuracy of 11 solar theories established across the Middle East from 800 (...) to 1600 as well as Ptolemy’s and Tycho Brahe’s, with respect to the precision of the parameter values and of the solar longitudes λ that they produce. The theoretical deviation due to the mismatch between the eccentric model with uniform motion and the elliptical model with Keplerian motion is taken into account in order to determine the precision of e and λ A in the theories whose observational basis is available. The smallest errors in the eccentricity are found in these theories: the Mumtaḥan : − 0.1 × 10−4, Bīrūnī : + 0.4 × 10−4, Ulugh Beg : − 0.9 × 10−4, and Taqī al-Dīn : − 1.1 × 10−4. Except for al-Khāzinī, the errors in the medieval determinations of the solar eccentricity do not exceed 7.7 × 10−4 in absolute value, with a mean error μ = + 2.57 × 10−4 and standard deviation σ = 3.02 × 10−4. Their precision is remarkable not only in comparison with the errors of Copernicus and Tycho, but also with the seventeenth-century measurements by Cassini–Flamsteed and Riccioli. The absolute error in λ A varies from 0.1° to 1.9° with the mean absolute error MAE = 0.87°, μ = −0.71° and σ = 0.65°. The errors in λ for the 13,000-day ephemerides show MAE < 6′ and the periodic variations mostly remaining within ± 10′, closely correlated with the accuracy of e and λ A. (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)

Abraham ibn Ezra d'Espagne (m. 1167) fut l'un des plus importants savants ayant contribué à la transmission de la science arabe à l'Occident. Ses ouvrages en astrologie et en astronomie, rédigés en hébreu puis traduits en latin, étaient considéréd comme faisant autorité par de nombreux savants juifs et Chrétiens. Parmi les ouvrages qu'il a traduits de l'arabe en hébreu, certains sont perdus dans leur langue originale et ses propres ouvrages renferment certaines informations concernant des sources anciennes mal ou pas du (...) tout connues par ailleurs. Ibn Ezra semble être le premier a avoir consigne l'une des sept méthodes pour dresser les maisons astrologiques. Cette méthode avait par la suite été utilisée par Lévi ben Gershom (m. 1344) dans le Midi de la France. (shrink)

L'Abrégé de l'Almagested'Averroès, conservé uniquement en traduction hébraïque, reste inédit et peu étudié. Cet article a pour but de le faire connaître. Après avoir retracé l'histoire de l'Abrégé: date de rédaction, traduction, transmission de cette traduction, diffusion et audience, nous procédons à une première étude du texte: aperçu commenté du contenu, identification des sources et examen de leur exploitation critique par Averroès. Nous donnons également une traduction d'extraits significatifs du Prologue de l'Abrégé, avec une brève analyse. Avec l'Abrégé, nous disposons (...) d'un témoignage direct et relativement précoce, dû à l'un des plus importants artisans du mouvement de contestation et de réforme de l'astronomie ptoléméenne, animé au XIIe siécle par des penseurs andalous, philosophes aristotéliciens pour la plupart. (shrink)

A characteristic hallmark of medieval astronomy is the replacement of Ptolemy’s linear precession with so-called models of trepidation, which were deemed necessary to account for divergences between parameters and data transmitted by Ptolemy and those found by later astronomers. Trepidation is commonly thought to have dominated European astronomy from the twelfth century to the Copernican Revolution, meeting its demise only in the last quarter of the sixteenth century thanks to the observational work of Tycho Brahe. The present article seeks to (...) challenge this picture by surveying the extent to which Latin astronomers of the late Middle Ages expressed criticisms of trepidation models or rejected their validity in favour of linear precession. It argues that a readiness to abandon trepidation was more widespread prior to Brahe than hitherto realized and that it frequently came as the result of empirical considerations. This critical attitude towards trepidation reached an early culmination point with the work of Agostino Ricci, who demonstrated the theory’s redundancy with a penetrating analysis of the role of observational error in Ptolemy’s Almagest. (shrink)

El manuscrito 2461 de la Biblioteca General de Rabat conserva un tercer zi)3 del astrónomo Muhammad ibn al-Raqqám al-Andalusí (m. 1315): al-Ziy al-Mustawfa. Los cánones (capitulo 18) y la tabla del movimiento del apogeo de al-Mustawfa están relacionados con el trabajo de Ibm Isháq al-Túnisí (1193-1222), quien compiló, probablemente, un conjunto de tablas carentes de cánones. Ibn al-Raqqám conocía bien el ziy de Ibn Isháq del que realizó dos recensiones conocidas hasta ahora: al-Ziy al- amil y al-Ziy al-Qawim. En al-Mustawfó, (...) tercera recensión de la misma obra, Ibn al-Raqqám aclara cuál era la opinión de Ibn Isl3áq sobre el movimiento de los apogeos planetarios y la división que existía, a este respecto, entre los astrónomos de la escuela andaluso-magrebí. (shrink)

The undoubted truth is that there exist for the planetary motions true and constant configurations from which no impossibilities or contradictions follow; they are not the same as the configurations asserted by Ptolemy; and Ptolemy neither grasped them nor did his understanding get to imagine what they truly are.