Numerous authors have noted multi-cycle periodicities in the sunspot cycle amplitudes.
Gleissberg (1939) noted a periodicity of seven or eight cycles in cycle amplitudes from 1750 to 1928. While
Garcia and Mouradian (1998) suggest that a third period of this cycle can be found in the sunspot data,
others (Hathaway et al., 1999) suggest that the period is changing or (Rozelot, 1994; Ogurtsov
et al., 2002) that it consists of two different components (one with a 90 – 100 year period and
a second with a 50 – 60 year period). A simple sinusoid fit to the residual cycle amplitudes
when the secular trend is removed now gives a 9.1 cycle periodicity. This best fit is shown in
Figure 34: The Gleissberg Cycle. The best fit of cycle amplitudes to a simple sinusoidal function of
cycle number is shown by the solid line (which includes the secular trend).