The earth's rotation and the ship analogy in Islamic astronomy

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. ^{4 1} Anton Heinen traces this debate to explain a variant in one copy of al-Hay'a al-saniyya denying that the earth moves like a ship on the ocean. ^{5 4}

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. ¹ He did not find it in Nasir al-Din al-Tusi's Tadhkira, but did in his contemporary Najm al-Din al-Katibi al-Qazwini, who discussed the earth's motion in Hikmat al-ayn, and found its most thorough treatment in Qutb al-Din al-Shirazi's Nihayat al-idrak fi dirayat al-aflak. ^{1 2}

In his commentary on al-Tusi's Tadhkira al-Sharif al-Jurjani (740–816 AH) repeatedly used the ship analogy, his arguments largely borrowed from Qutb al-Din al-Shirazi. ² He reported the view of some ancients that the daily motion of the stars is due to the earth's rotation from west to east, then examined the two usual refutations — that a stone thrown upward would fall west of its starting point, and that birds and arrows would be affected — and judged them unsatisfactory, since the air might move congruously with the earth and carry with it what it contains. ^{3 6 7} He then reaffirmed the earth's rest, not on those grounds but because the earth is by nature the principle of straight-line attraction. ^{7 8} Heinen compares his falling-stone argument with the examples of "The Second Day" in Galileo's Dialogue. ⁷

A generation after al-Suyuti, Gars al-Din ibn Ahmad al-Halabi (d. 971 AH) devoted a chapter of his Tanbih al-naqqad ala ma fi al-hay'a al-mashhura min al-fasad (958 AH) to the subject, defending the older view of the spheres' motion, while elsewhere in the book rejecting the equant and the eccentric deferent. ^{8 9}

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, al-Suyuti's book reinstated the early views and won popularity. ¹⁰ This is his academic assessment.