The Length of the Solar Cycle
Most people think of the solar cycle as having a fixed length of 11 years. This is not strictly true as cycles vary considerably in length from as little as 9 years to almost 14 years.
The figure is a histogram showing the length of cycles, in bins of 0.5 years, for Cycle 1 (peak in 1761) to Cycle 23 (peak in 2000). Records of earlier cycles are available but are generally regarded as not being reliable and so have not been included.
The figure demonstrates that there is no single value for the cycle length with values spread across the graph. The average cycle length is 11 years. Some people believe that solar cycles typically come in two varieties with long and short periods. The long ones have a period of around 12 years and the short ones a period of around 10 years. In this view, the traditional cycle length of 11 years is just the average of these two types and is not representative of either variety!