Solar cycle 21 was the 21st
solar cycle
The solar cycle, also known as the solar magnetic activity cycle, sunspot cycle, or Schwabe cycle, is a nearly periodic 11-year change in the Sun's activity measured in terms of variations in the number of observed sunspots on the Sun's surfa ...
since 1755, when extensive recording of solar
sunspot
Sunspots are phenomena on the Sun's photosphere that appear as temporary spots that are darker than the surrounding areas. They are regions of reduced surface temperature caused by concentrations of magnetic flux that inhibit convection. Sun ...
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
The Wolf number (also known as the relative sunspot number or Zürich number) is a quantity that measures the number of sunspots and groups of sunspots present on the surface of the Sun.
History
Astronomers have been observing the Sun recordi ...
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 Solar cycles are nearly periodic 11-year changes in the Sun's activity that are based on the number of sunspots present on the Sun's surface. The first solar cycle conventionally is said to start in 1755 when Rudolf Wolf began extensive reporting o ...
References
Solar cycles
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