solar cycle 20

{{Infobox Solar cycles

|image=Solar-magnetogram-from-skylab-era.png

|caption=Solar magnetogram from solar cycle 20 (1974).

|cycle_num=20

|start_date=October 1964

|end_date=March 1976

|duration=11.4

|max_count=156.6

|max_count_date=November 1968

|min_count=14.3

|spotless_count=272

|prev_name=Solar cycle 19

|next_name=Solar cycle 21

|prev_dates=1954–1964

|next_dates=1976–1986

}}

File:Skylab Solar flare.jpgs ever recorded, from solar cycle 20 (19 December 1973).{{cite web |last1=Martin |first1=James |title=NASA celebrates the 40th anniversary of Skylab (pictures) |url=https://www.cnet.com/pictures/nasa-celebrates-the-40th-anniversary-of-skylab-pictures/ |website=CNET |access-date=19 March 2022 |language=en}}]]

Solar cycle 20 was the twentieth solar cycle since 1755, when extensive recording of solar sunspot activity began.{{citation|doi=10.1023/A:1014296529097|year=2002|last1=Kane|first1=R.P.|title=Some Implications Using the Group Sunspot Number Reconstruction|journal=Solar Physics|volume=205|issue=2|pages=383–401|bibcode=2002SoPh..205..383K}}{{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 11.4 years, beginning in October 1964 and ending in March 1976. The maximum smoothed sunspot number observed during the solar cycle was 156.6 (November 1968), and the starting minimum was 14.3.SIDC Monthly Smoothed Sunspot Number. "[http://sidc.oma.be/sunspot-data/]" During the minimum transit from solar cycle 20 to 21, there were a total of 272 days with no sunspots.Spotless Days. "[http://spaceweather.com/glossary/spotlessdays.htm?PHPSESSID=dli444kmrjgre0rjq6l86fv144]"{{cite web |url=https://science.nasa.gov/headlines/y2008/11jul_solarcycleupdate.htm|archive-url=https://web.archive.org/web/20080714032353/https://science.nasa.gov/headlines/y2008/11jul_solarcycleupdate.htm|url-status=dead|archive-date=14 July 2008|title=What's Wrong with the Sun? (Nothing)|date=11 July 2008|author=Dr. Tony Phillips|publisher=NASA}}Solaemon's Spotless Days Page. "[http://users.telenet.be/j.janssens/Spotless/Spotless.html]"

Comparison with other cycles shows that geomagnetic activity during the declining phase of cycle 20 (1973–1975) was unusually high.{{cite journal |last1=Gosling |first1=J. T. |last2=Asbridge |first2=J. R. |last3=Bame |first3=S. J. |date=1 August 1977 |title=An unusual aspect of solar wind speed variations during solar cycle 20 |journal=Journal of Geophysical Research |volume=82 |issue=22 |pages=3311–3314 |doi=10.1029/JA082i022p03311 |bibcode = 1977JGR....82.3311G }} Heavy solar activity was a factor in causing the earlier-than-expected atmospheric reentry of Skylab in 1979.{{cite book|last1=Benson|first1=Charles Dunlap|first2=William David|last2=Compton|name-list-style=amp |year=1983 |url=https://history.nasa.gov/SP-4208/contents.htm|title=Living and Working in Space: A History of Skylab |publisher=NASA Scientific and Technical Information Office|id=SP-4208|oclc=8114293|pages=362–363}}

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.{{cite book |last=Miroshnichenko |first=Leonty |date=2001 |title=Solar Cosmic Rays |url=https://books.google.com/books?id=tE604-gWX34C&pg=PA395 |publisher=Springer |page=395 |isbn=0792369289 }}