Galileo is credited with many inventions. Among them was a telescope that allowed an observer to see the moon, certain planets, a supernova, and, of course, sunspots—dark areas of irregular shape on the surface of the sun. Scheiner had postulated these were shadows of small planets, but Galileo explained them as solar activity. Through his study of their position on successive days through his telescope, and through mathematical diagrams as shown here, he deduced that the sun (here represented by the semicircle CE) rotates on a fixed axis.