Following the discovery of the solar cycle by S. H. Schwabe, Wolf set about collecting all available data on sunspot numbers. He established the dates of all sunspot maxima and minima going back to the year 1610, and calculated the length of the solar cycle to be 11.1 years. He devised the system of measuring solar activity by counting sunspots (see relative sunspot number). Wolf was one of several astronomers to note that variations in terrestrial magnetism and auroral activity mirrored the solar cycle.