Planetary motion. Demonstration that an earthy substance and water will travel in a curved path together (left). They will still travel together when carried round by the universe. This was one of the arguments used by the Italian astronomer and mathematician Galileo (1564-1642) to prove that the Moon is a body similar to the Earth. Engraving from Utriusque cosmi historia by Robert Fludd (Oppenheim 1617-1619).