Solar cycle 21

{{Infobox Solar cycles

|image=Noaa44sunhalpha19800428.png

|caption=The Sun, recorded at the H-alpha wavelength, during solar cycle 21 (28 April 1980).

|cycle_num=21

|start_date=March 1976

|end_date=September 1986

|duration=10.5

|max_count=232.9

|max_count_date=December 1979

|min_count=17.8

|spotless_count=273

|prev_name=Solar cycle 20

|next_name=Solar cycle 22

|prev_dates=1964-1976

|next_dates=1986-1996

}}

Solar cycle 21 was the 21st solar cycle since 1755, when extensive recording of solar sunspot activity began.Kane, R.P. (2002). "[https://doi.org/10.1023%2FA%3A1014296529097 Some Implications Using the Group Sunspot Number Reconstruction]". Solar Physics 205(2), 383-401.{{cite web | title=The Sun: Did You Say the Sun Has Spots? | url=http://www.spacetoday.org/SolSys/Sun/Sunspots.html | publisher=Space Today Online | accessdate=12 August 2010}} 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. "[http://sidc.oma.be/sunspot-data/]" During the minimum transit from solar cycle 21 to 22, there were a total of 273 days with no sunspots.Spotless Days. "[http://spaceweather.com/glossary/spotlessdays.htm?PHPSESSID=dli444kmrjgre0rjq6l86fv144]"What's Wrong with the Sun? (Nothing) more information: Spotless Days. "{{cite web |url=https://science.nasa.gov/headlines/y2008/11jul_solarcycleupdate.htm |title=NASA - What's Wrong with the Sun? |accessdate=2017-07-12 |url-status=dead |archiveurl=https://web.archive.org/web/20080714032353/https://science.nasa.gov/headlines/y2008/11jul_solarcycleupdate.htm |archivedate=2008-07-14 }}"Solaemon's Spotless Days Page. "[http://users.telenet.be/j.janssens/Spotless/Spotless.html]" The largest solar flare of this cycle (X15) occurred on July 11, 1978.{{cite web|url=http://www.spaceweather.com/solarflares/topflares.html|title=The Most Powerful Solar Flares ever Recorded|work=spaceweather.com}}

This solar cycle marked the beginning of systematic monitoring of the total solar irradiance from space. Kopp, G. (2016). "[http://adsabs.harvard.edu/abs/2016JSWSC...6A..30K6/ Magnitudes and timescales of total solar irradiance variability]". JSWSC 6, A30.