太陽活動周期
Solar cycle 21 was the 21st solar cycle since 1755, when extensive recording of solar sunspot activity began. The solar cycle lasted 10.5 years, beginning in March 1976 and ending in September 1986. The maximum smoothed sunspot number (SIDC formula) observed during the solar cycle was 232.9, in December 1979, and the starting minimum was 17.8. During the minimum transit from solar cycle 21 to 22, there were a total of 273 days with no sunspots. The largest solar flare of this cycle (X15) occurred on July 11, 1978.