been noisily denied and impugned, obliging me to hide in silence every new idea of mine until I have more than proved it.” Nonetheless, Galileo expounded on the essence and substance of sunspots for many pages, initiating an ongoing correspondence with Welser—and through him “the masked Apelles”—that sounded the full thunder of the new debate. Indeed, Galileo’s letters on sunspots speak almost as much about the system of the world as they do about the solar spots.
“With absolute necessity we shall conclude,” Galileo wrote early in the first of his three letters to Welser, “in agreement with the theories of the Pythagoreans and of Copernicus, that Venus revolves about the Sun just as do all the other planets. . . . No longer need we employ arguments that allow any answer, however feeble, from persons whose philosophy is badly upset by this new arrangement of the universe.”
Apelles upheld the idea that the dark spots must be many small stars circling the Sun. Galileo saw nothing starlike about them. To his mind, they more closely resembled clouds: “Sunspots are generated and decay in longer and shorter periods; some condense and others greatly expand from day to day; they change their shapes, and some of these are most irregular; here their obscurity is greater and there less. They must be simply enormous in bulk, being either on the Sun or very close to it. By their uneven opacity they are capable of impeding the sunlight in differing degrees; and sometimes many spots are produced, sometimes few, sometimes none at all.”
But he quickly added: “I do not assert on this account that the spots are clouds of the same material as ours, or aqueous vapors raised from the Earth and attracted by the Sun. I merely say that we have no knowledge of anything that more closely resembles them. Let them be vapors or exhalations then, or clouds, or fumes sent out from the Sun’s globe or attracted there from other places; I do not decide on this—and they may be any of a thousand other things not perceived by us.” (He could never have imagined, despite his long-standing interest in magnets, that the spots marked the sites of the Sun’s most potent magnetic fields.)
“If I may give my own opinion to a friend and patron,” Galileo continued, “I shall say that the solar spots are produced and dissolve upon the surface of the Sun and are contiguous to it, while the Sun, rotating upon its axis in about one lunar month, carries them along, perhaps bringing back some of those that are of longer duration than a month, but so changed in shape and pattern that it is not easy for us to recognize them.”
In closing this first letter, Galileo begged Welser’s indulgence:
And forgive me my indecision, because of the novelty and difficulty of the subject, in which various thoughts have passed through my mind and met now with assent and again with rejection, leaving me abashed and perplexed, for I do not like to open my mouth without declaring anything whatever. Nevertheless, I shall not abandon the task in despair. Indeed, I hope that this new thing will turn out to be of admirable service in tuning for me some reed in this great discordant organ of our philosophy—an instrument on which I think I see many organists wearing themselves out trying vainly to get the whole thing into perfect harmony. Vainly, because they leave (or rather preserve) three or four of the principal reeds in discord, making it quite impossible for the others to respond in perfect tune.
These off-key reeds that Galileo decried sounded several flat notes, including the immutability of the heavens, the farrago of the celestial spheres, and the immobility of the Earth.
Welser wrote back gratefully to say, “You have paid a high rate of interest for the favor of a little time, sending me so copious and diffuse a treatise in reply to a few lines.” The thrill of witnessing new philosophy grow up around the astronomical anomaly of the sunspots
“With absolute necessity we shall conclude,” Galileo wrote early in the first of his three letters to Welser, “in agreement with the theories of the Pythagoreans and of Copernicus, that Venus revolves about the Sun just as do all the other planets. . . . No longer need we employ arguments that allow any answer, however feeble, from persons whose philosophy is badly upset by this new arrangement of the universe.”
Apelles upheld the idea that the dark spots must be many small stars circling the Sun. Galileo saw nothing starlike about them. To his mind, they more closely resembled clouds: “Sunspots are generated and decay in longer and shorter periods; some condense and others greatly expand from day to day; they change their shapes, and some of these are most irregular; here their obscurity is greater and there less. They must be simply enormous in bulk, being either on the Sun or very close to it. By their uneven opacity they are capable of impeding the sunlight in differing degrees; and sometimes many spots are produced, sometimes few, sometimes none at all.”
But he quickly added: “I do not assert on this account that the spots are clouds of the same material as ours, or aqueous vapors raised from the Earth and attracted by the Sun. I merely say that we have no knowledge of anything that more closely resembles them. Let them be vapors or exhalations then, or clouds, or fumes sent out from the Sun’s globe or attracted there from other places; I do not decide on this—and they may be any of a thousand other things not perceived by us.” (He could never have imagined, despite his long-standing interest in magnets, that the spots marked the sites of the Sun’s most potent magnetic fields.)
“If I may give my own opinion to a friend and patron,” Galileo continued, “I shall say that the solar spots are produced and dissolve upon the surface of the Sun and are contiguous to it, while the Sun, rotating upon its axis in about one lunar month, carries them along, perhaps bringing back some of those that are of longer duration than a month, but so changed in shape and pattern that it is not easy for us to recognize them.”
In closing this first letter, Galileo begged Welser’s indulgence:
And forgive me my indecision, because of the novelty and difficulty of the subject, in which various thoughts have passed through my mind and met now with assent and again with rejection, leaving me abashed and perplexed, for I do not like to open my mouth without declaring anything whatever. Nevertheless, I shall not abandon the task in despair. Indeed, I hope that this new thing will turn out to be of admirable service in tuning for me some reed in this great discordant organ of our philosophy—an instrument on which I think I see many organists wearing themselves out trying vainly to get the whole thing into perfect harmony. Vainly, because they leave (or rather preserve) three or four of the principal reeds in discord, making it quite impossible for the others to respond in perfect tune.
These off-key reeds that Galileo decried sounded several flat notes, including the immutability of the heavens, the farrago of the celestial spheres, and the immobility of the Earth.
Welser wrote back gratefully to say, “You have paid a high rate of interest for the favor of a little time, sending me so copious and diffuse a treatise in reply to a few lines.” The thrill of witnessing new philosophy grow up around the astronomical anomaly of the sunspots
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