What happens to a thrown object?

This lesson asks you to describe the characteristics of a projectile's motion in the vertical and horizontal planes and to give examples. The museum offers one historical case of a thrown object: a stone that scholars argued about. The descriptions and measurements of projectile motion are for your class to bring. ¹

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.

Read the record of the earth's rotation and the ship analogy. Sketch the path of the thrown stone and mark what the record says and what you would have to measure to say more.