Sunspot
Chinese astronomers said they could see sunspots. On 17 March 802 the monk Adelmus saw a big sunspot, which he could see for eight days. Adelmus thought that Mercury was going in front of the Sun and making a black spot. When astronomers started using telescopes, most agreed that something was passing in front of the Sun. Galileo Galilei in 1612 guessed that there were actual spots on the Sun and that they showed that the Sun was turning.