<P> The discussion is not narrowly limited to astronomical topics, but ranges over much of contemporary science . Some of this is to show what Galileo considered good science, such as the discussion of William Gilbert's work on magnetism . Other parts are important to the debate, answering erroneous arguments against the Earth's motion . </P> <P> A classic argument against earth motion is the lack of speed sensations of the earth surface, though it moves, by the earth's rotation, at about 1700 km / h at the equator . In this category there is a thought experiment in which a man is below decks on a ship and cannot tell whether the ship is docked or is moving smoothly through the water: he observes water dripping from a bottle, fish swimming in a tank, butterflies flying, and so on; and their behavior is just the same whether the ship is moving or not . This is a classic exposition of the Inertial frame of reference and refutes the objection that if we were moving hundreds of kilometres an hour as the Earth rotated, anything that one dropped would rapidly fall behind and drift to the west . </P> <P> The bulk of Galileo's arguments may be divided into three classes: </P> <Ul> <Li> Rebuttals to the objections raised by traditional philosophers; for example, the thought experiment on the ship . </Li> <Li> Observations that are incompatible with the Ptolemaic model: the phases of Venus, for instance, which simply couldn't happen, or the apparent motions of sunspots, which could only be explained in the Ptolemaic or Tychonic systems as resulting from an implausibly complicated precession of the Sun's axis of rotation . </Li> <Li> Arguments showing that the elegant unified theory of the Heavens that the philosophers held, which was believed to prove that the Earth was stationary, was incorrect; for instance, the mountains of the Moon, the moons of Jupiter, and the very existence of sunspots, none of which was part of the old astronomy . </Li> </Ul>

Galileo galilei dialogue concerning the two chief world systems