 Galileo was one of the first Europeans to observe sunspots , although Kepler had unwittingly observed one in 1607 , but mistook it for a transit of Mercury . He also reinterpreted a sunspot observation from the time of Charlemagne , which formerly had been attributed ( impossibly ) to a transit of Mercury . The very existence of sunspots showed another difficulty with the unchanging perfection of the heavens as posited in orthodox Aristotelian celestial physics . And the annual variations in sunspots ' motions , discovered by Francesco Sizzi and others in 1612 – 1613 , provided a powerful argument against both the Ptolemaic system and the geoheliocentric system of Tycho Brahe . A dispute over priority in the discovery of sunspots , and in their interpretation , led Galileo to a long and bitter feud with the Jesuit Christoph Scheiner . 
