What did Galileo Galilei argue about sunspots?

Galileo Galilei was an independent discoverer of sunspots, which he studied systematically starting around 1611.12

In his 1613 work, Istoria e dimostrazioni intorno alle macchie solari e loro accidenti ("Letters on Sunspots"), Galileo engaged in a debate with the German Jesuit Christoph Scheiner regarding the nature of these phenomena. While Scheiner argued that sunspots were satellites orbiting the Sun—a theory intended to preserve the traditional Aristotelian view that the heavens were perfect and unchanging—Galileo countered that the spots were located on or near the surface of the Sun itself.1

Galileo supported his conclusion by providing a series of detailed engravings based on his telescopic observations, which demonstrated that the spots were not independent celestial bodies but features associated with the Sun's surface.1