About 350 years ago Galileo made a telescope and looked through it at the sun. What he saw both surprised and frightened him, for he saw dark spots on the sun which at once suggested to him that God had not made the world quite as perfect as he had previously believed. He hesitated to make his discovery known. Meanwhile other scientists noticed the same lack of solar perfection and proclaimed (宣布) the fact.
But Galileo continued his observations and was soon rewarded with another discovery. Fixing his attention on a single sunspot (太阳黑子) group, he noticed that in a few days it had moved in position, just as if the sun itself were turning. Afterwards he found a sunspot group which lived long enough to disappear from view on the western limb (边缘) of the sun, to re-appear on its eastern limb, and finally to regain its old position. This led him to conclude that the sun itself was rotating and that the time it took to make one complete turn was about twenty-five to twenty-seven days. Actually we know from the drawings which Galileo made of sunspots that there must have been quite a lot of them at the time of his observations in the years 1611 and 1612. If he had gone on making his drawings in the years that immediately followed, we know that he would almost certainly have noticed that sunspots were becoming fewer and smaller. But he became interested in other things and so he failed to recognize that there is a kind of long-term cycle in sunspot activity, the sunspots increasing and decreasing as the years go on. Later this discovery of the sunspot activity was made by one of the most patient observers in the history of science, a German chemist, Charles Schwabe.