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The idea of computing sunspot numbers was originated by R. Wolf in 1849 in Zürich, Switzerland and, thus, the procedure he initiated bears his name (or place).

This number has been collected and tabulated by researchers for around 300 years. They have found that sunspot activity is cyclical and reaches its maximum around every 9.5 to 11 years (note: Using data from SIDC for the last 300 years and running a fast FFT function on the data gives an average maximum at 10.4883 years/cycle). This cycle was first noted by Heinrich Schwabe in 1843.

The relative sunspot number is computed using the formula (collected as a daily index of sunspot activity):

,

where R is the relative sunspot number, s is the number of individual spots, g is the number of sunspot groups, and k is a factor that varies with location and instrumentation (also know as the observatory factor).

1 References

• The MathWorks, Inc (the fast FFT was run on MATLAB v6.5R13).
• SIDC, RWC Belgium, World Data Center for the Sunspot Index, Royal Observatory of Belgium, 'year(s)-of-data'.

1 External links

• The Exploratorium's Guide to Sunspots
• RWC Belgium World Data Center for the Sunspot Index