### 4.3 Cycle periods

The period of a sunspot cycle is defined as the elapsed time from the minimum preceding its maximum
to the minimum following its maximum. This does not, of course, account for the fact that each cycle
actually starts well before its preceding minimum and continues long after its following minimum. By this
definition, a cycle’s period is dependent upon the behavior of both the preceding and following cycles. The
measured period of a cycle is also subject to the uncertainties in determining the dates of minimum
as indicated in the previous subsections. Nonetheless, the length of a sunspot cycle is a key
characteristic and variations in cycle periods have been well studied. The average cycle period can be
fairly accurately determined by simply subtracting the date for the minimum preceding cycle 1
from the date for the minimum preceding cycle 23 and dividing by the 22 cycles those dates
encompass. This gives an average period for cycles 1 to 22 of 131.7 months – almost exactly 11
years.
The distribution of cycle periods depends upon the cycles used and the methods used to determine
minima. Eddy (1977) noted that the cycle periods did not appear to be distributed normally.
Wilson (1987) included cycle 8 to 20 and used the dates for minimum from the 13-month mean of the
monthly sunspot numbers and found that a bimodal distribution best fit the data with short period (122
month) cycles and long period (140 month) cycles separated by a gap (the Wilson Gap) surrounding
the mean cycle length of 132.8 months. However, Hathaway et al. (2002) used minima dates
from the 24-month Gaussian smoothing of the International Sunspot number for cycles 1 to
23 and of the Group Sunspot Numbers for cycles –4 to 23 and found distributions that were
consistent with a normal distributions about a mean of 131 months with a standard deviation of 14
months and no evidence of a gap. These cycle periods and their distributions are shown in
Figure 22.