Circumsolar Models for Venus and Mercury in Islamic Astronomy

24.08.2026 | 24.08.2026

Dr Sajjad Nikfahm-Khubravan, University of Maryland

 

In Almagest IX, Ptolemy addresses the order of the planets and inherits two ancient hypotheses on the position of Venus and Mercury relative to the Sun: one placing them below the Sun, the other above it. Weighing considerations of parallax, transits, and the “nature” of the bodies involved, Ptolemy settles for the traditional order, with Venus and Mercury below the Sun and above the Moon. Notably absent from his discussion, however, is a third ancient possibility: a circumsolar model in which Venus and Mercury revolve around the Sun itself. Such a model eventually surfaces in the Latin astronomical literature of late antiquity, in the writings of Theon of Smyrna, Macrobius, and Martianus Capella, where it is presented as an alternative to the Ptolemaic arrangement. Less well known is the fact that circumsolar models were also discussed, independently, in ancient and medieval Eastern astronomy. This talk examines these Eastern discussions, focusing on two major astronomers of the Islamic world, Naṣīr al-Dīn al-Ṭūsī (d. 1274) and Abū al-Rayḥān al-Bīrūnī (d. after 1050). I will first present Ṭūsī’s brief but pointed discussion of a circumsolar model for Venus and Mercury, transmitted through the otherwise obscure Almagest of Ṣāliḥ ibn Muḥammad al-Zaynabī al-Baghdādī, together with Ṭūsī’s observational argument against it based on reported transits of Venus. I will then turn back to al-Bīrūnī, whose Treatise on Transits preserves two distinct circumsolar models that he attributes to Persian sources, offering our richest evidence for such theories in the Islamic world. Finally, I will consider the possible background of these Persian models within Sasanian and Indian astronomy, situating the Islamic discussions within a longer, cross-cultural history of circumsolar thinking.

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