Is God a Mathematician_ - Mario Livio [33]
One of the distinguishing characteristics of the individuals who had a significant impact on the history of science was their ability to grasp immediately which discoveries were truly likely to make a difference. Another trait of many influential scientists was their skill in making the discoveries intelligible to others. Galileo was a master in both of these departments. Concerned that someone else might also discover the Jovian satellites, Galileo rushed to publish his results, and by the spring of 1610 his treatise Sidereus Nuncius (The Sidereal Messenger) appeared in Venice. Still politically astute at that point in his life, Galileo dedicated the book to the grand duke of Tuscany, Cosimo II de Medici, and he named the satellites the “Medicean Stars.” Two years later, following what he referred to as an “Atlantic labor,” Galileo was able to determine the orbital periods—the time it took each of the four satellites to revolve around Jupiter—to within an accuracy of a few minutes. The Sidereal Messenger became an instant best seller—its original five hundred copies quickly sold out—making Galileo famous across the continent.
The importance of the discovery of the Jovian satellites cannot be overemphasized. Not only were these the first bodies to be added to the solar system since the observations of the ancient Greeks, but the mere existence of these satellites removed in a single stroke one of the most serious objections to Copernicanism. The Aristotelians argued that it was impossible for the Earth to orbit the Sun, since the Earth itself had the Moon orbiting it. How could the universe have two separate centers of rotation, the Sun and the Earth? Galileo’s discovery unambiguously demonstrated that a planet could have satellites orbiting it while the planet itself was following its own course around the Sun.
Another important discovery that Galileo made in 1610 was that of the phases of the planet Venus. In the geocentric doctrine, Venus was assumed to move in a small circle (an epicycle) superimposed on its orbit around the Earth. The center of the epicycle was supposed to always lie on the line joining the Earth and the Sun (as in figure 17a; not drawn to scale). In this case, when observed from Earth, one would expect Venus always to appear as a crescent of somewhat varying width. In the Copernican system, on the other hand, Venus’s appearance should change from a small bright disk when the planet is on the other side of the Sun (as seen from Earth), to a large and almost dark disk when Venus is on the same side as Earth (figure 17b). Between those two positions Venus should pass through an entire sequence of phases similar to that of the Moon. Galileo corresponded about this important difference between the predictions of the two doctrines with his former student Benedetto Castelli (1578–1643), and he conducted the crucial observations between October and December of 1610. The verdict was clear. The observations confirmed conclusively the Copernican prediction, proving that Venus indeed orbits the Sun. On December 11, a playful Galileo sent Kepler the obscure anagram “Haec immatura a me iam frustra leguntur oy” (“This was already tried by me in vain too early”). Kepler tried unsuccessfully to decipher the hidden message and eventually gave up. In his following letter, of January 1, 1611, Galileo finally transposed the letters in the anagram to read: “Cynthiae figuras aemulatur mater amorum” (“The mother of love [Venus] emulates the figures of Cynthia [the Moon]”).
Figure 17
All the findings I have described so far concerned either planets in the solar system—celestial bodies that orbit the Sun and reflect its light—or satellites revolving around these planets. Galileo also made two very significant discoveries related to stars—heavenly objects that generate their own light, such as the Sun. First,