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Solar cycle 21
The Sun, recorded at the H-alpha wavelength, during solar cycle 21 (28 April 1980).
Sunspot data
Start date	March 1976
End date	September 1986
Duration (years)	10.5
Max count	232.9
Max count month	December 1979
Min count	17.8
Spotless days	273
Cycle chronology
Previous cycle	Solar cycle 20 (1964-1976)
Next cycle	Solar cycle 22 (1986-1996)

Solar cycle 21 was the 21st solar cycle since 1755, when extensive recording of solar sunspot activity began. 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. During the minimum transit from solar cycle 21 to 22, there were a total of 273 days with no sunspots. 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.

See also

• Solar variation
• List of solar cycles

References

1. Kane, R.P. (2002). "Some Implications Using the Group Sunspot Number Reconstruction". Solar Physics 205(2), 383-401.
2. "The Sun: Did You Say the Sun Has Spots?". Space Today Online. Retrieved 12 August 2010.
3. SIDC Monthly Smoothed Sunspot Number. ""
4. Spotless Days. ""
5. What's Wrong with the Sun? (Nothing) more information: Spotless Days. ""NASA - What's Wrong with the Sun?". Archived from the original on 2008-07-14. Retrieved 2017-07-12."
6. Solaemon's Spotless Days Page. ""
7. "The Most Powerful Solar Flares ever Recorded". spaceweather.com.
8. Kopp, G. (2016). "Magnitudes and timescales of total solar irradiance variability". JSWSC 6, A30.

v t e Solar cycles
18th century	1740–1759 Solar cycle 1 (1755 Feb – 1766 Jun) 1760–1779 Solar cycle 2 (1766 Jun – 1775 Jun) Solar cycle 3 (1775 Jun – 1784 Sep) 1780–1799 Solar cycle 4 (1784 Sep – 1798 Apr) Solar cycle 5 (1798 Apr – 1810 Aug)
19th century	1800–1819 Solar cycle 6 (1810 Aug – 1823 May) 1820–1839 Solar cycle 7 (1823 May – 1833 Nov) Solar cycle 8 (1833 Nov – 1843 Jul) 1840–1859 Solar cycle 9 (1843 Jul – 1855 Dec) Solar cycle 10 (1855 Dec – 1867 Mar) 1860–1879 Solar cycle 11 (1867 Mar – 1878 Dec) Solar cycle 12 (1878 Dec – 1890 Mar) 1880–1899 Solar cycle 13 (1890 Mar – 1902 Jan)
20th century	1900–1919 Solar cycle 14 (1902 Jan – 1913 Jul) Solar cycle 15 (1913 Jul – 1923 Aug) 1920–1939 Solar cycle 16 (1923 Aug – 1933 Sep) Solar cycle 17 (1933 Sep – 1944 Feb) 1940–1959 Solar cycle 18 (1944 Feb – 1954 Apr) Solar cycle 19 (1954 Apr – 1964 Oct) 1960–1979 Solar cycle 20 (1964 Oct – 1976 Mar) Solar cycle 21 (1976 Mar – 1986 Sep) 1980–1999 Solar cycle 22 (1986 Sep – 1996 Aug) Solar cycle 23 (1996 Aug – 2008 Dec)
21st century	2000–2019 Solar cycle 24 (2008 Dec – 2019 Dec) Solar cycle 25 (2019 Dec – present)
List of solar cycles List of solar storms List of solar maxima and minima

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