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To Galileo this was a natural motion. He rejected Copernicus' three motions for the Earth, instead arguing that the Earth's axis would tend naturally to remain fixed with respect to its orbit and, hence, he took the more modern view that (apart from precession) the motion of the Earth was made up of just two rotations.

Described previously (Figure 7.1). Salviati states that 'it is impossible to explain the movements perceived in the waters and at the same time maintain the immovability of the vessel which contains them'. In the Dialogue, Galileo expanded on his earlier theory, attempting to account for the monthly and annual inequalities exhibited by the tides. The English mathematician John Wallis tried to improve on Galileo's theory in 1666 by treating the Earth-Moon system as a single body, the centre of gravity of which described an orbit round the Sun. This idea was a good one, but Wallis' theory was still based on the same fundamental misconceptions as Galileo's (Aiton (1954)). The question of whether the tides could possibly provide a proof of the Earth's motion, even in the context of Newtonian physics, was addressed by Burstyn (1962) and Aiton (1965), the former believing that tides do offer such a proof, and the latter arguing to the contrary. There is no simple answer to this question (see Palmieri (1998)).

great force of reasoning, and with the charms of the most lively eloquence. They are written, indeed, with such singular felicity, that one reads them at the present day, when the truths contained in them are known and admitted, with all the delight of novelty, and feels one's self carried back to the period when the telescope was first directed to the heavens, and when the earth's motion, with all its train of

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