Fourier Analysis of the Solar Sunspot Cycle

The most obvious periodic cyclicity as far as Earth is concerned must be the solar cycle, which is commonly (but incorrectly) assumed to be fixed at 11 years but in reality is twice as long (the Hale cycle) and varies in length from one cycle to the next.

Here is the solar cycle plotted as a time series

And here is the Fourier spectrum of the 18th and 20th Centuries compared:

As can be seen, the 18th Century showed solar cycles that had a lower frequency (and therefore longer period) than the 20th Century

The Fourier Spectrum for the entire data set is

which shows quite a bit more spread around the supposed “average” 11 year cycle.

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