Astronomical Images : Cannon shots are not proof of the Earth's rest

Galileo Galilei

Astronomical Images

<p style='text-align: justify;'>This image is found in Day 2 of Galileo Galileo's 1632 <i>Dialogue on the Two World Systems</i>. Galileo wrote this text as a dialogue between three interlocutors who debate the merits of the Ptolemaic (geocentric) and Copernican (heliocentric) world systems. Day 2 of the text is devoted to a discussion of arguments about the organization of the heavens based on the behaviour of objects on the Earth's surface. In this excerpt, Galileo's interlocutors discuss the well-known argument regarding the behaviour of projectiles (arrows shot upwards or horizontally, cannon shot vertically, etc.). According to traditional sources, the fact that vertically-thrown projectiles always landed at the same place indicates that the Earth is at rest. Otherwise, as the projectile was in flight, the Earth would turn, and the projectile would land at a different location. Galileo, however, claims that this argument does not hold. Rather, he argues that the Earth's diurnal turning is impressed on the projectile, making it move along with the Earth and thus return to the same place from which it was fired. In the image, Galileo argues that the cannon ball B actually moves along a diagonal as it is shot. For violating the Catholic Church's earlier decree against Copernicanism in 1616, Galileo was condemned by the Church in 1633 and the <i>Dialogue </i>placed on the Index of Forbidden Books.</p>