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Galileo Studies: Personality, Tradition, and Revolution

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Sunspots. As the sun rotates on its axis (with a period of less than a month), sunspots trace out latitude lines on its surface. In the course of a year, these curves are seen as alternately happy mouths, straight, sad mouths, straight, etc., which is what one would expect from a Copernican point of view if the sun's axis was inclined to the ecliptic. Galileo was too sloppy an observer to notice this. "On the contrary, he ... described the paths of sunspots as parallel to the ecliptic, notably in his Letters on Sunspots of 1613" (p. 180). Still sixteen years later Galileo was convinced that this was the last word on the matter: "writing ... apropos of recent news that Scheiner would soon publish a thick folio volume on sunspots, he remarked that any such book would surely be filled with irrelevancies, as there was no more to be said on the subject than he had already published in his Letters on Sunspots" (p. 185). Not only was Galileo an unsuccessful and self-important observer, he was also too dense to see the importance of sunspots for the Copernican issue. When he finally took a hint from Sizzi (p. 184) and realised that inclined sunspot paths spoke in favour of Copernicus, he immediately threw all his old observations out the window and rushed the pro-Copernican argument into print without making any new observations. The published argument "displays entire ignorance or complete neglect of the observational data" (p. 186), his vague data being "utterly wrong" (p. 186) and "almost the exact opposite" (p. 187) of the careful data published by Scheiner. Here Drake is eager to point out his hero's missteps since they prove that Galileo did not plagiarise Scheiner, as some people have claimed. But Drake still maintains his pro-Galilean stance as regards the theoretical part of the argument. But, even ignoring Galileo's thoroughly irresponsible handling of data, his argument is also theoretically flawed. To explain the sunspot paths from a geocentric point of view one must ascribe an annual conical motion to the axis of the sun, which Galileo rejects as unreasonable (pp. 194-195). But the earth has precisely such a motion (albeit a much slower one), which is the reason for the procession of the equinoxes, as Copernicus explained. Thus Galileo cannot reject as physically impossible the geocentric account of sunspots without simultaneously rejecting the procession of the equinoxes. (See Topper, Isis, 90(4), for these things ignored by Drake.)

Tides. "Since his death [Galileo's theory of the tides] has generally been considered worthless" (p. 200), having been accused of being based of faulty physics and making false predictions. On physics Drake says: "Galileo's theory depends specifically on the compounding of two circular motions, not of a translation and a rotation [as in Mach's disproof]" (p. 206), and "the tidal effect thus produced would vary considerably for different relative radii and speeds, but reciprocating flows [which Galileo needs] could undoubtedly be produced" (p. 207). But of course the question is not whether tides could result for some radii and speeds, but rather if this is plausible for the well-known specific radii and speeds in the case of the earth, and Mach's argument surely disproves the latter. On predictions Drake's defence is that Galileo's theory does not make any false predictions since it in fact makes no predictions at all: "on Galileo's theory, during the last few minutes before noon, the tide might be rising on the western side of the Gibraltar and falling on the eastern side. Again, it might be rising on both sides, or falling on both sides, depending on the period of reciprocation for each basin." (p. 209). While Galileo was developing this unfalsifiable theory, it was clear to less prejudiced thinkers such as Kepler "that the evident relation of the moon's motion of the tidal periods made it necessary to base any rational explanation of the tides on the action of the moon" (p. 201).

Circular inertia. Drake's "case against 'circular inertia'" (chapter 13) is hardly "against" circular inertia at all for it concludes: "Galileo is pretty consistent in applying the idea of essential circularity to instances in which the motion is a 'natural' one [e.g., objects dropped from towers, ship masts] ... The ideas of essential rectilinearly, on the other hand, he applied most specifically to instances of 'violent' motion---cannonballs and projectiles thrown by slings" (p. 274).