Letters on Sunspots

Letters on Sunspots (Istoria e Dimostrazioni intorno alle Macchie Solari) was a pamphlet written by Galileo Galilei in 1612 and published in Rome by the Accademia dei Lincei in 1613. In it Galileo outlined his recent observation of dark spots on the face of the sun. His claims were significant in undermining the traditional Aristotelian view that the sun was both unflawed and unmoving. The third letter contains his first clear public statement of support for Copernicus.

Galileo was not the first person to observe sunspots. The earliest apparent reference to them appears in the I Ching of ancient China, while the earliest recorded observation is also Chinese, dating to 364 BC. Around the same time, the first European mention of sunspots is found, by Theophrastus. There were reports from Islamic and European astronomers of sunspots in the early ninth century; those occurring in 1129 were recorded by both Averroes and John of Worcester, whose drawings of the phenomenon are the earliest surviving today.Johannes Kepler observed a sunspot in 1607 but, like some earlier observers, believed he was watching the transit of Mercury. The sunspot activity of December 1610 was the first to be observed using the newly invented telescope, by Thomas Harriot, who sketched what he saw but did not publish it. In 1611 Johannes Fabricius saw them, and published a pamphlet entitled De Maculis in Sole Observatis, which Galileo was not aware of before he wrote the Letters on Sunspots.

The Jesuit Christoph Scheiner observed sunspots early in 1611. Then, under the pseudonym 'Apelles', he presented his description and conclusions about them in three letters to the Augsburg banker and scholar Mark Welser. Welser published them on his own presses, sent copies to astronomers around Europe, and invited them to reply. It was Welser's invitation which prompted Galileo to reply with two letters, arguing that the sunspots were not satellites, as Scheiner ('Apelles') maintained, but were features either on the sun's surface or just above it.

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