Observed through the telescope, the surface of the Sun shows dark spots, due - as we know today - to intense magnetic fields that block the convective movement of the underlying layers. These areas, receiving a lesser quantity of energy, have a lower temperature and thus appear darker. Thomas Harriot (1560-1621) was the first to observe this phenomenon, which was divulged in a publication by Johann Fabricius (1587-c. 1615) in 1611.
Galileo (1564-1642) had a heated argument on the nature of sunspots with the Jesuit priest Christoph Scheiner (1573-1650), who, in 1612, under the pseudonym of Apelles hidden behind his painting, had published three letters on the subject. To salvage the Aristotelian dogma of the immutability of the celestial bodies, Scheiner had hypothesised the existence of clusters of small planets orbiting around the Sun, which, interposed between the latter and the Earth, appeared as dark spots against the background of the solar disc. Galileo sustained instead that the phenomenon took place on the surface of the Sun or in its immediate vicinity, attributing the motion of the sunspots from east to west to the rotation of the Sun in a period of about one month. To back up this hypothesis, he adduced numerous observational proofs. The spots in fact showed no periodicity, appearing and dissolving continuously, even in proximity to the centre of the solar disk, assuming irregular shapes that changed from day to day. Furthermore, the movement of a planet orbiting at a great distance from the Sun would have proceeded against this background at a practically constant speed. Galileo had observed, on the contrary, a gradual slowing of the spots as they approached the edge of the Sun, in perfect compliance with the hypothesis that they were contiguous to its surface.