Galileo's ship

Galileo's ship refers to two physics experiments, a thought experiment and an actual experiment, by Galileo Galilei, the 16th and 17th century physicist, astronomer, and philosopher. The experiments were created to argue the idea of a rotating Earth as opposed to a stationary Earth around which rotated the Sun and planets and stars.

An argument that was used at the time was that if the earth were rotating, there would be detectable effects on the trajectories of projectiles or falling bodies.

In 1616, after Galileo had already become concerned that he was a target of suspicion by the Inquisition, he received a letter from Monsignor Francesco Ingoli listing both scientific and theological arguments against Copernicanism. As part of a lengthy 1624 reply, Galileo described the experiment of dropping a rock from the mast of a smoothly moving ship, and observing whether the rock hit at the base of the mast or behind it. Various people had discussed the experiment in theoretical terms, and some claimed to have done it, with conflicting reports as to the result. For example, actual or thought experiments similar to this one had been previously discussed by Jean Buridan,Nicolas Oresme,Nicolaus Cusanus,Clavius and Giordano Bruno.

Galileo told Ingoli (translated by Stillman Drake):

I have been twice as good a philosopher as those others because they, in saying what is the opposite of the effect, have also added the lie of their having seen this by experiment; and I have made the experiment -- before which, physical reasoning had persuaded me that the effect must turn out as it indeed does.

The experiment is also discussed in the Dialogue Concerning the Two Chief World Systems (day two), but without any assertion that it was actually carried out. A similar experiment discussed by Galileo and other authors such as Oresme, Clavius and Bruno is one in which a projectile is launched straight up from the surface of the earth. A common Aristotelian-Scholastic argument was that if the earth's surface were moving to the east, then in this experiment the projectile would land to the west of the launching point, contrary to observation.

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