What happens to a thrown object?

Look at the expectation: it separates the vertical plane from the horizontal one. In this case, ask what moves up and down, what moves across, and what a source tells you or fails to tell you about either.

Read how one commentator tests a thrown stone. In his commentary on al-Tusi's Tadhkira, al-Sharif al-Jurjani (740–816 AH) repeatedly used the ship analogy 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. He 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 ^3 4 5. He judged them unsatisfactory, since the air might move congruously with the earth and carry with it what it contains ^3 4 5. The record is the ship analogy in Islamic astronomy; it reports what is moving and what carries what, and gives no measurement.

The record does not say why al-Jurjani took up the question. It places his repeated use of the ship in a commentary on al-Tusi's Tadhkira, with arguments largely borrowed from Qutb al-Din al-Shirazi ². It does give the reason he held for the earth's rest: not the refutations he had just set aside, but that the earth is by nature the principle of straight-line attraction ^5 6. That is his reason as Anton Heinen reports it.

Does the earth turn? The record gives two positions, each with its holder. Al-Jurjani reported the view of some ancients that the daily motion of the stars is due to the earth's rotation from west to east ³. He himself reaffirmed the earth's rest ^5 6. The museum states no verdict, and the record reports no measurement on either side.

It would be easy to read al-Jurjani as an early defender of a turning earth, because he found the refutations of it unsatisfactory. The record says he then reaffirmed the earth's rest ^5 6. The comparison with Galileo is Anton Heinen's: he compares al-Jurjani's falling-stone argument with the examples of "The Second Day" in Galileo's Dialogue ⁵.

The stone passage reaches us in layers: al-Jurjani's commentary, and Anton Heinen's study of the debate, written to explain a variant in one copy of al-Suyuti's al-Hay'a al-saniyya ^7 8. Heinen says al-Jurjani's arguments were largely borrowed from al-Shirazi ². Ask whose argument you are reading: al-Jurjani's, al-Shirazi's, or Heinen's account of them.

Al-Jurjani says the usual refutation fails because the air might move with the earth and carry along what it contains. Using your class's description of horizontal and vertical motion, would a stone thrown straight up land west of its starting point if the air and the thrower both moved with the earth, and what does your answer say about the refutation?

• The ship analogy in Islamic astronomy: the whole debate, from Aristotle to al-Halabi.
• Al-Sharif al-Jurjani: the commentator who examined the thrown stone.
• Qutb al-Din al-Shirazi: the author whose treatment of the ship Heinen calls the most thorough.