Moving relative to what?

In each description ask who is moving and who is at rest, and what each observer sees when a stone is dropped or thrown. Then ask which descriptions would change if the ship sped up or turned.

See how a question about a ship changes. Anton Heinen traces the inquiry to Aristotle's analysis of motion in the Physics, where the ship concerns the cause of motion rather than its relativity. Muslim astronomers of the thirteenth to sixteenth centuries shifted it toward relative motion: the ship relative to its passengers or the shores ². In astronomical works connected with the Maragha observatory, the ship sailing on a sea or river, carrying passengers who move about on it as on firm ground, became a standard example in discussing "accidental motion", and then the possibility of the earth's rotation about its axis ^3 2. Heinen's own reason for tracing the debate is a variant in one copy of al-Suyuti's al-Hay'a al-saniyya, which denies that the earth moves like a ship on the ocean ^4 3. Read the record for who is at rest and who moves in each telling.

Were the astronomers around al-Suyuti ready to change their minds? Heinen concludes that al-Suyuti's contemporaries were still open to new ideas, and that common notions of astronomy and geography could be shaken by sharper reasoning and better observation. Against this, he says, al-Suyuti's book reinstated the early views and won popularity ⁵. The record calls this Heinen's academic assessment, and the museum adds no verdict.

Everything here is Heinen's reading. He did not find the ship in al-Tusi's Tadhkira, but did in al-Tusi's contemporary Najm al-Din al-Katibi al-Qazwini, who discussed the earth's motion in Hikmat al-ayn ². He found its most thorough treatment in Qutb al-Din al-Shirazi's Nihayat al-idrak fi dirayat al-aflak ^2 6. A generation after al-Suyuti, Gars al-Din al-Halabi devoted a chapter of his Tanbih al-naqqad to the subject, defending the older view of the spheres' motion, while elsewhere in the book rejecting the equant and the eccentric deferent ^7 8. The record does not report that any of these authors speaks of inertial or non-inertial frames.

Heinen concludes that al-Suyuti's contemporaries were open to new ideas, yet al-Suyuti's book, which reinstated the early views, won popularity. Does a book's popularity show what its readers believed, and how would you answer the objection that the same readers may have valued it for other reasons?

• The ship analogy in Islamic astronomy: the debate from Aristotle's ship to al-Halabi's chapter.
• Al-Hay'a al-saniyya: the book in whose copy the variant appears.
• Qutb al-Din al-Shirazi: the author of the most thorough treatment Heinen found.