The recent paucity of sunspots and the delay in the expected start of Solar Cycle 24 have drawn attention to the challenges involved in predicting solar activity .
Traditional models of the solar cycle usually require information about the starting time and rise time as well as the shape and amplitude of the cycle .
With this tutorial , we investigate the variations in the length of the sunspot number cycle and examine whether the variability can be explained in terms of a secular pattern .
We identified long-term cycles in archival data from 1610 – 2000 using median trace analyses of the cycle length and power spectrum analyses of the ( O-C ) residuals of the dates of sunspot minima and maxima .
Median trace analyses of data spanning 385 years indicate a cycle length with a period of 183 - 243 years , and a power spectrum analysis identifies a period of 188 \pm 38 years .
We also find a correspondence between the times of historic minima and the length of the sunspot cycle , such that the cycle length increases during the time when the number of spots is at a minimum .
In particular , the cycle length was growing during the Maunder Minimum when almost no sunspots were visible on the Sun .
Our study suggests that the length of the sunspot number cycle should increase gradually , on average , over the next \sim 75 years , accompanied by a gradual decrease in the number of sunspots .
This information should be considered in cycle prediction models to provide better estimates of the starting time of each cycle .