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 periodic 11-year change in the Sun's activity measured in terms of Modern Maximum, variations in the number of observed sunspots on the Sun ...
since 1755, when extensive recording of solar
sunspot
Sunspots are temporary spots on the Sun's surface that are darker than the surrounding area. They are one of the most recognizable Solar phenomena and despite the fact that they are mostly visible in the solar photosphere they usually aff ...
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 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
Solar irradiance is the power per unit area ( surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument.
Solar irradiance is measured in watts per square metre ...
from space.
[ Kopp, G. (2016).]
Magnitudes and timescales of total solar irradiance variability
. ''JSWSC'' 6, A30.
See also
*
Solar variation
The Solar cycle, also known as the solar magnetic activity cycle, sunspot cycle, or Schwabe cycle, is a periodic 11-year change in the Sun's activity measured in terms of variations in the number of observed sunspots on the Sun's surface. ...
*
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 have started in 1755. The source data are the revised In ...
References
Solar cycles
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