Discoveries and Opinions of Galileo

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.

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

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