Solar cycle 20

Solar cycle 20 was the twentieth solar cycle since 1755, when extensive recording of solar sunspot activity began. The solar cycle lasted 11.7 years, beginning in October 1964 and ending in June 1976. The maximum smoothed sunspot number (monthly number of sunspots averaged over a twelve-month period) observed during the solar cycle was 110.6 (November 1968), and the minimum was 12.2. There were a total of 272 days with no sunspots during this cycle.

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.