The solar cycle, or the solar magnetic activity cycle, is the main source of periodic solar variation driving variations in space weather. The cycle is observed by counting the frequency and placement of sunspots visible on the Sun. Powered by a hydromagnetic dynamo process driven by the inductive action of internal solar flows, the solar cycle: The solar cycle was discovered in 1843 by Samuel Heinrich Schwabe, who after 17 years of observations noticed a periodic variation in the average number of sunspots seen from year to year on the solar disk.

Rudolf Wolf compiled and studied these and other observations, reconstructing the cycle back to 1745, eventually pushing these reconstructions to the earliest observations of sunspots by Galileo and contemporaries in the early seventeenth century. Starting with Wolf, solar astronomers have found it useful to define a standard sunspot number index, which continues to be used today. The average duration of the sunspot cycle is about 11 years (about 28 cycles in the 309 years between 1699 and 2008), but cycles as short as 9 years and as long as 14 years have been observed.

Significant variations in amplitude also occur. Solar maximum and solar minimum refer respectively to epochs of maximum and minimum sunspot counts. Individual sunspot cycles are partitioned from one minimum to the next.

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