Galileo's Letters on Sunspots

Title page of Galileo's Letters on Sunspots, published in Rome in 1613. The Letters were written to the wealthy Augsburg Magistrate Mark Wesler (1558-1614), a well-known patron of the new sciences, in responses to Christoph Scheiner's own Letters on Sunspots, published through Wesler in 1612. The lynx depicted on the Title page indicates Galileo's membership in the Lincean academy (Accademia dei Lincei, for the "sharp-eyed"), the first scientific academy in Europe. The academy was founded 1603 by Prince Federigo Cesi, and was conspicuous in granting membership to a small number of selected intellectuals operating outside the mainstream academic establishment. In April 1611 Galileo became the sixth member elected to the Academy, and to the end of his life would identify himself as a "Lincean" on the title page of all his books.

In his Letters, and unlike Scheiner, Galileo correctly identifies sunspots as markings on the solar surface, as opposed to small intramercurial planets. By studying the position of sunspots on successive days Galileo also inferred that the Sun rotates, and established its rotation period as close to one lunar month. Galileo and Scheiner were later to quarrel bitterly over priority of discovery of sunspots; in fact the first recorded sunspot observation is on 8 December 1610 by the Englishman Thomas Harriott (1560-1621), and the first publication by Johann Fabricius (1587-1616), in the fall of 1611.


Galileo, G. 1613, Letters on Sunspots, trans. S. Drake 1957, in Ideas and Opinions of Galileo, Doubleday.

Mitchell, W.M. 1916, The history of the discovery of the solar spots, in Popular Astronomy, 24, 22-ff.


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