Solar cycle 21 was the 21st solar cycle since 1755, when recording of solar sunspot 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.