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How did scientists discover the 11-year solar cycle?

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Space weather Earth's full North Polar auroral oval, in an image taken in ultraviolet light by the U.S. Polar spacecraft over northern Canada, April 6, 1996. In the colour-coded image, which simultaneously shows dayside and nightside auroral activity, the most intense levels of activity are red, and the lowest levels are blue. Polar, launched in February 1996, was designed to further scientists' understanding of how plasma energy contained in the solar wind interacts with Earth's magnetosphere.

Although sunspots had been observed since the early 1600s, the periodic nature of solar activity remained unrecognized for over two centuries. The 11-year solar cycle was officially discovered in 1843 by the German amateur astronomer Samuel Heinrich Schwabe, who identified the pattern through long-term observations of the rise and fall in the number of sunspots.12

Following this discovery, the Swiss astronomer Rudolf Wolf analyzed historical records to establish a standardized numbering system for these cycles, beginning with solar cycle 1 in 1755. While the 11-year cycle describes the fluctuation in sunspot numbers, it is part of a broader 22-year magnetic cycle, which was later identified in 1925 by the American astronomer George Ellery Hale.12