The Works of Galileo

A New Pope and a New Dialogue

Dialogue, the Tides

On the last day of the dialogue, Galileo presented a causal, physical proof of the dual motion of the Earth based on the tides. The axial rotation of the Earth would cause waters to move regularly from east to west, accounting for equatorial ocean currents. But this movement can never reach equilibrium, because it is continually compounded with the motion of the Earth around the Sun. Just as water sloshes around within a container constantly moved in two directions, so the tides result from the combination of the Earth’s two daily and annual motions (Figure 1) . On the last page of the Dialogue, Simplicio remarks that God in his omnipotence could have produced the tides in a different way, if he wanted to. In conversation with Galileo in 1623, Urban VIII had insisted upon this limit to human certainty based on divine omnipotence. And for those who were inclined toward Kepler’s theory that the tides are caused by an invisible attraction of the Moon and the Sun, Galileo’s argument didn’t hold water. In any case, Galileo’s argument from physical causes was a calculated risk: because Galileo wanted a causal proof of Copernicanism, he invaded the territory of the physicists in what was supposed to be a mathematical, and therefore hypothetical, work. This time Galileo’s legendary confidence spectacularly failed him.

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Dialogue (1632), pages 384-385.

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Exhibit credit: Kerry Magruder, with the assistance of , Marilyn B. Ogilvie, Duane H. D. Roller.

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