Solar cycle 21 was the 21st solar cycle since 1755, when extensive recording of solar sunspot activity began.Kane, R.P. (2002). "Some Implications Using the Group Sunspot Number Reconstruction". Solar Physics 205(2), 383-401. 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.SIDC Monthly Smoothed Sunspot Number. "" During the minimum transit from solar cycle 21 to 22, there were a total of 273 days with no sunspots.Spotless Days. ""What's Wrong with the Sun? (Nothing) more information: Spotless Days. ""Solaemon's Spotless Days Page. "" The largest solar flare of this cycle (X15) occurred on July 11, 1978.
This solar cycle marked the beginning of systematic monitoring of the total solar irradiance from space. Kopp, G. (2016). "Magnitudes and timescales of total solar irradiance variability". JSWSC 6, A30.
See also Solar variation List of solar cycles
References