The Works of Galileo

Sunspots and Floating Bodies

Letters on Sunspots (1613)

Galileo published a description of sunspots in 1613 (Figure 1) . At the top of the frontispiece (Figure 2) , note the geometrical compass and the telescope. Hereafter, Galileo always signed his name Galileo Galilei Linceo. Again, he claimed the title Philosopher or physicist.

In a 1611 book published by none other than the Academy of the Lynx, the Jesuit astronomer Christoph Scheiner argued that sunspots are little planets circling the Sun like Venus. Galileo answered Scheiner with his own Letters on Sunspots, 1613. By tracking the motion of sunspots across the face of the Sun, Galileo proved that they were contiguous with the Sun’s surface, and not little planets.

Browse Galileo’s sunspot observations for 28 days (Galileo’s separate drawings for each day were printed on separate pages; they are superimposed and animated in the movie below). As you watch the movie, follow one or two spots and observe how they move together across the Sun. Watch the movie again, and note the following characteristics of sunspots’ motion: By moving together, and moving slowly (about a month), they cannot be planets. Note how irregular they are in shape, and how they form and disappear with irregular timing. That’s not like planets either. Note the foreshortening of the spots as they approach the edge of the solar disk. That proves they’re contiguous with the surface.

Galileo sunspot observations (2 MB; free QuickTime Player required.)

Or compare still images of 3 of the plates: (Figure 3) (Figure 4) (Figure 5) .

The sunspots also suggest that the Sun and the heavens are corruptible, a tenet contrary to Aristotle but accepted by many Stoics, patristic writers, and theologians, including Cardinal Bellarmine.

Mark Smith shows in a short article noted in your handout how Galileo began to formulate a difficult geometrical argument for Copernicanism based on the motion of sunspots, brilliant and compelling but never published in full (Figure 6) (Figure 7) .

This book also includes diagrams of the periods of Jupiter, and of the phases of Venus, as well as an explicit affirmation of Copernicanism.

Excerpts of Galileo’s Letters on Sunspots are translated in Stillman Drake, Discoveries and Opinions of Galileo (1957).

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Galileo, 1613. Title page.

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Galileo, 1613. Frontispiece.

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Figure 3 Figure 3 - Return to Text

Galileo, 1613.

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Galileo, 1613.

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Galileo, 1613.

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Figure 6 Figure 6 - Return to Text

Galileo, 1613.

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Figure 7 Figure 7 - Return to Text

This work also contains a brilliant analysis of the periods of Jupiter.

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Exhibit credit: Kerry Magruder, with the assistance of , Marilyn B. Ogilvie, Duane H. D. Roller.

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