10.24 The sunspot cycle. Some plots against time show cycles of up-and-down movements. Figure 10.19 is a line graph of the average number of sunspots on the sun’s visible face for each month from 1900 to 2011. What is the approximate length of the sunspot cycle? That is, how many years are there between the successive valleys in the graph? Is there any overall trend in the number of sunspots?