Sunspot minima dates: A secular forecast

The spectral analysis of sunspot minima date time series demonstrates that a high degree of determinism is peculiar to these data. A simple regression model involving a linear trend (11.083 yr cycle(-1)) and 3 harmonic functions (with periods corresponding to 18.6, 8.8, and 7.0 Schwabe cycles) allows development of an extra-long sunspot-cycle timing forecast.