Solar cycle 21

Solar cycle 21 was the 21st solar cycle since 1755, when extensive recording of solar sunspot activity began.^[1][2] The solar cycle lasted 10.5 years, beginning in March 1976 and ending in September 1986. The maximum smoothed sunspot number observed during the solar cycle was 232.9, in December 1979, and the starting minimum was 17.8.^[3] During the minimum transit from solar cycle 21 to 22, there were a total of 273 days with no sunspots.^[4][5][6] The largest solar flare of this cycle (X15) occurred on July 11, 1978.^[7]

Solar cycle 21
The Sun, recorded at the H-alpha wavelength, during solar cycle 21 (28 April 1980).
Sunspot data
Start date	March 1976
End date	September 1986
Duration (years)	10.5
Max count	232.9
Max count month	December 1979
Min count	17.8
Spotless days	273
Cycle chronology
Previous cycle	Solar cycle 20 (1964-1976)
Next cycle	Solar cycle 22 (1986-1996)

This solar cycle marked the beginning of systematic monitoring of the total solar irradiance from space.^[8]

See also

• Solar variation
• List of solar cycles