After long-term observation, people have found that the ebb and flow of sunspot activity on the surface of the sun has a periodicity of about 11 years on average, which is called the 11-year solar activity cycle. During an activity cycle, the number of sunspots on the surface of the sun increases from none to a maximum in the "peak year", and then decreases year by year until they disappear. Sunspots not only show cyclical changes in number, but also have certain regularity in their distribution on the solar disk: at the beginning of the activity cycle, sunspots appear in the regions around 35° north and south of the solar disk; in the year with the maximum number of sunspots, most sunspots appear in the low-latitude regions around 15° north and south of the solar disk; and towards the end of the activity cycle, sunspots appear in the regions around 5° north and south of the solar disk and then disappear. If we plot the average latitude of sunspot groups on the solar disk as the vertical axis and time as the horizontal axis, we can find that the distribution of sunspot groups in different years is like a group of neatly arranged butterflies, repeating every 11 years, just like the sun's face undergoing a recurring "spot removal" beauty treatment.
What exactly causes the 11-year cycle of solar activity remains a mystery to this day. It should be noted that saying that a solar activity cycle lasts about 11 years is just an average result. In fact, the length of the solar activity cycle is not constant and varies greatly, with some lasting less than 9 years and others exceeding 13 years. By analyzing the variation curve of the number of sunspots over more than 300 years, we can see that in addition to the approximately 11-year cycle, there are also some longer cycles superimposed on the 11-year cycle, resulting in large fluctuations in the number of sunspots in the years of maximum activity of different solar activity cycles. This shows that the mechanism causing the cyclical variation of solar activity is quite complex and requires further in-depth research.