Sunspot
A sunspot cycle of eleven years has been found, with changes in activity. Every 11 years the number of sunspots increases, and later declines. Sunspots on one cycle are magnetically different from the next, so the total sunspot cycle is 22 years. This cycle has been seen since the 18th century. Before that time, for over a hundred years, there were very few sunspots. Astronomers do not know what caused this "Maunder Minimum".