The inclination of the Sun's rotation axis
One of a great many sunspot drawings in Scheiner's Rosa Ursina, reproduced from The history of the discovery of the solarspots, in Popular Astronomy, 24, W.M. Mitchell, 1916. This drawing illustrates the apparent paths of sunspots across the solar disk, for two sets of observations taken six months apart. Based on such observations, Scheiner correctly concluded that the Sun's equatorial plane isinclined by 7° with respect to the ecliptic. This observation was taken up as his own by Galileo in his Dialogues as a further argument for the heliocentric hypothesis, which was to further provoke Scheiner into accusations of plagiarism.