[2][3] The solar cycle lasted 11.4 years, beginning in October 1964 and ending in March 1976.

The maximum smoothed sunspot number observed during the solar cycle was 156.6 (November 1968), and the starting minimum was 14.3.

[4] During the minimum transit from solar cycle 20 to 21, there were a total of 272 days with no sunspots.

[8] Heavy solar activity was a factor in causing the earlier-than-expected atmospheric reentry of Skylab in 1979.

One CME traveled to Earth in a record low of 14.6 hours and produced a strong geomagnetic storm that caused widespread electrical and communications grid disturbances and the accidental detonation of numerous U.S. Navy magnetic sea mines in North Vietnam.