Solar cycle 21 is the 21st solar cycle

Solar cycle

The solar cycle, or the solar magnetic activity cycle, is a periodic change in the amount of irradiation from the Sun that is experienced on Earth. It has a period of about 11 years, and is one component of solar variation, the other being aperiodic fluctuations. Solar variation causes changes in...

 since 1755, when recording of solar sunspot

Sunspot

Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They are caused by intense magnetic activity, which inhibits convection by an effect comparable to the eddy current brake, forming areas of reduced surface temperature....

activity began. The solar cycle lasted 10.3 years, beginning in June 1976 and ending in September 1986. The maximum smoothed sunspot number (monthly number of sunspots averaged over a twelve month period) observed during the solar cycle was 164.5, and the minimum was 12.3. There were a total of 273 days with no sunspots during this cycle. The largest solar flare of this cycle (X15) occurred on July 11, 1978.