Discoveries and Opinions of Galileo

Nonfiction | Book | Adult | Published in 1957

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Part 1

Part 2

Part 3

Part 4

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Part 2Chapter Summaries & Analyses

Part 2: “Letters on Sunspots”

First Letter of Mark Welser

Praising Galileo’s leadership in scientific discovery, Mark Welser asks him what he thinks about a report he recently received from his friend Apelles about solar spots.

First Letter of Galileo

Galileo replies that he has been ill, which caused him to reply three months late. He writes that the question of sunspots is a difficult one; for this reason, combined with his inability to make many observations due to his illness and the vehement attacks of his enemies, he is cautious in making judgments on the subject.

In response to Apelles’s report, Galileo makes the following points:

- Sunspots are real objects, not optical illusions.

- They do not remain stationary but appear to move in relation to the sun from west to east.

- The fact that the spots have no parallax (see Index of Terms) suggests that they are not in the earth’s atmosphere but in the vicinity of the sun.

- The sunspots are not blacker that the moon’s spots but are on the contrary “at least as bright as the brightest part of the moon” (92).

- Observing the sun and its spots shows us that Venus revolves around the sun, as do the other planets.

- He faults Apelles for adhering to the outmoded Ptolemaic system instead of embracing new evidence from sense experience.

- He leaves open the possibility that the spots are in the space where the moon, Venus, and Mercury revolve.

- Galileo is not certain what the spots are made of; but, judging from their appearance, they are closer to clouds than to stars in composition. This is especially suggested by the fact that the spots appear, dissolve, condense, expand, and change shape, much as clouds do. Stars, on the other hand, do not change shape as sunspots clearly do.

Galileo apologizes for the length of his letter and for the uncertain and tentative nature of his ideas on sunspots, a subject filled with “novelty and difficulty” (103). He affirms his friendly intentions and respect for Apelles and promises to send him exact diagrams of sunspots.

Second Letter of Mark Welser

Welser thanks Galileo for writing such an extensive reply to his letter, saying that it has brought cheer to him during his illness. He promises to have the letter translated for Apelles, who does not understand Italian well. In addition, he states his desire to publish Galileo’s letter to keep the public abreast of new discoveries.

Second Letter of Galileo

Galileo declares that his continued observations have confirmed his previously stated convictions about sunspots. He reviews his opinion that sunspots are located on or near the surface of the sun and continually change in shape, degree of darkness, and density. From these movements, Galileo deduces that the sun is spherical and rotates, completing “an entire revolution in about one lunar month” (106). The spots all fall in a particular zone of the sun’s body, near its equator.

Third Letter of Mark Welser

Welser says that his illness continues, preventing him from writing long letters, and that he has heard of Galileo’s second letter but has not yet received it.

Fourth Letter of Mark Welser

Welser says that Galileo’s second letter arrived and that he received it “like manna from heaven” (121). He reiterates his plans to have the letters printed and urges Galileo to keep up his good work of dispelling ignorance through his discoveries.

Third Letter of Galileo

Galileo’s third letter is sociable and philosophical in tone, reiterating some of his previously stated ideas. He muses about the difficulty of acquiring certain knowledge about our world. Although it is impossible in this present life to penetrate to the true essences of things (such as the material which composes sunspots), it is still possible to find out something about their properties through observation. Galileo complains of contemporary philosophers’ overreliance on the works of Aristotle instead of using direct reasoning and observation.

Galileo directly addresses some of Apelles’s points. Apelles claimed to demonstrate that based on sunspots, Venus rotates around the sun. Galileo argues that Apelles’s demonstrations are inadequate and that the fact can be well proved by Copernicus’s theories and, experientially, by the fact that Venus changes in shape like the moon (133). The discovery of sunspots has caused scientists to modify their long-held opinion that the sun is “hard and unchangeable” (135); on the contrary, it is highly malleable. Galileo reiterates his view that sunspots are “neither stars nor permanent materials” (143) but are instead like clouds that generate and dissolve on or near the surface of the sun.

Part 2 Analysis

As known to modern science, a sunspot is a “vortex of gas on the surface of the sun associated with strong local magnetic activity” (Encyclopedia Britannica). Sunspots are cooler than the surrounding area and thus appear darker. Galileo was the first scientist to study sunspots in depth, although they were noted in ancient times. Like others of Galileo’s discoveries, this one encountered a certain amount of resistance from his contemporaries, since it implied that the sun was “blemished” and “imperfect.”

Galileo laid out his thoughts on the subject in his “Letters on Sunspots,” addressed to Mark Welser of Augsburg, Germany. Both Galileo and Welser were members of a scientific society called Accademia dei Lincei, or Academy of the Lynx-eyed; members of this group were known as Linceans, and both Galileo and Welser sign themselves this way in their correspondence.

The letters were prompted by investigations into sunspots carried out by a German priest and astronomer named Christopher Scheiner, a friend of Welser. Scheiner had been forbidden by church authorities to publish his findings and thus went under the pseudonym “Apelles.” In the letters, Galileo frequently references Apelles and critiques his findings and claims.

Especially significant is Galileo’s statement of what would become known as the law of inertia on Page 113. According to this law, “a body will preserve a state of uniform motion or of rest unless acted upon by some force” (footnote, 113). Galileo discovered that physical bodies have inclinations to certain motions that are intrinsic to them and do not depend on an external mover. This became one of the foundational rules of modern physics.

Throughout the letters, Galileo reiterates many of his familiar themes and ideas. He affirms the need to revise or reject the authority of Aristotle if necessary and to rely on direct observation instead. Galileo shows his humility as a scientist by apologizing to Welser for his “procrastination,” his “poor style,” and his hesitance to make positive judgments about sunspots (126). Galileo is reluctant to make definite statements about scientific matters that are uncertain, because of a fear of being proven wrong later. In addition, Galileo reaffirms the role of religious faith in his scientific activities. He states his hope that God—“the true, pure, and immaculate Sun” (123)—will reveal the truth of obscure and uncertain scientific matters.

The letters were written in Italian—specifically, the Florentine dialect, then considered the most prestigious form of Italian. Welser was fluent in the language, and Galileo praises his command of it (126). This marks a departure in the history of science toward the democratization of scientific ideas. Whereas Galileo wrote “The Starry Messenger” in Latin to reach an international audience of scholars, he wrote his subsequent works in Italian to reach a wider, non-scholarly readership. Galileo wanted ordinary, intelligent people to realize that “just as nature has given to them, as well as to philosophers, eyes with which to see her works, so she has also given them brains capable of penetrating and understanding them” (Introduction: Second Part, Page 84).

At the time the letters were written, Galileo’s fame had reached a peak, thanks to his publication of “The Starry Messenger” and his promotion and sale of the telescope throughout Europe. However, the most difficult controversies of Galileo’s career still lay ahead.