Spots on the sun, 1612-13

Galileo's observations of mountains and depressions on the surface of the Moon raised urgent questions regarding the other heavenly bodies. Were the Sun and the other planets the perfect spheres required by traditional cosmology? Or would closer observation reveal imperfections on their surfaces as well?

From 1612 onward, a controversy erupted between Galileo and the Jesuit astronomer, Christoph Scheiner (1575-1650), regarding the nature of sunspots. In order to preserve the Aristotelian conception of the Sun's geometrical perfection, Scheiner argued that these spots were not actually on the surface of the sun: instead they were formed by clusters of small planets orbiting the Sun.

On successive days during the summer of 1612, Galileo observed the sun through his helioscope.  These drawings were published the following year in his Istoria e Dimostrazioni Intorno Alle Macchie Solari e Loro Accidenti Rome (History and Demonstrations Concerning Sunspots and their Properties), commonly known in English as the Letters on Sunspots. Individually, these drawing may seem unimpressive. Rather like the observations of the moons of Jupiter published three years before, their significance begins to become apparent when studied in series. 

Galileo sunspots single series.png

More revealing still is to display this series in animated fashion, in the manner of the brief video embedded below. Because these observations were made at appoximately the same time of day on successive days, their series displays the motion of the spots across the face of the Sun. Galileo's observations and calculations demonstrated that these spots were in fact to be found on the surface of the Sun, which rotates continuously on its axis, roughly once a month. This controversy is recounted in a second brief video.

Source. High resolution versions of the 36 sunspot drawing on which this video (and those in the series above) derive from The Galileo Project, where they are published, with permission, from Owen Gingrich's copy of the first edition of Galileo's Istoria e Dimostrazioni of 1613.

Additional resources. A detailed summary of the Letters on Sunspots is available on Wikipedia.