Galileo observed that sunspots move across the face of the sun, disappear and reappear in a period of about 30 days, and from this he deduced that the sun rotates about its own axis; and modern astronomers have discovered that the sun's surface activity increases and decreases in an 11-year cycle.
從伽利略觀測太陽黑子約三十天移動一周，推論出太陽會自轉，進而現代天文學家發現太陽表面活動，按著十一年週期強弱交替。