Period in which several important kinds of solar activity repeat, discovered in 1843 by Samuel Heinrich Schwabe (1789–1875). Lasting about 22 years on average, it includes two 11-year cycles of sunspots, whose magnetic polarities alternate between the Sun's northern and southern hemispheres, and two peaks and two declines in the phenomena (e.g., solar prominences, auroras) that vary in the same period. Attempts have been made to connect the solar cycle to various other phenomena, including possible slight variations in the diameter of the Sun, sequences of annual growth rings in trees, and even the stock market's rise and fall.

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