Fourier transform of the sunspot cycle

The duration of individual sunspot cycles is known to vary over wide limits. These limits depend also on the value of the selected sunspot number mean whose appropriate recurrence interval is used to specify a duration of a cycle [ Davies , 1965]. Because of the inherent variability of this technique it is of interest to determine periods of solar cycles by means of the Fourier transform method. This method, however, suffers from truncation effects that can be severe if the data sample is short containing only one cycle, or less, of the period of interest.