Solar cycle 20

Solar cycle 20
Solar magnetogram from solar cycle 20 (1974).
Sunspot data
Start date	October 1964
End date	March 1976
Duration (years)	11.4
Max count	156.6
Max count month	November 1968
Min count	14.3
Spotless days	272
Cycle chronology
Previous cycle	Solar cycle 19 (1954-1964)
Next cycle	Solar cycle 21 (1976-1986)

Solar cycle 20 was the twentieth solar cycle since 1755, when extensive recording of solar sunspot activity began. The solar cycle lasted 11.4 years, beginning in October 1964 and ending in March 1976. The maximum smoothed sunspot number (SIDC formula) observed during the solar cycle was 156.6 (November 1968), and the starting minimum was 14.3. During the minimum transit from solar cycle 20 to 21, there were a total of 272 days with no sunspots.

Comparison with other cycles shows that geomagnetic activity during the declining phase of cycle 20 (1973–1975) was unusually high.

Data from solar cycle 20 was used to build the K-1974 solar proton fluence model, used for planning space missions during solar cycle 21.