Editing Sunspot number (section)

==Alternative series==
The accuracy of the compilation of the group sunspot number series has been questioned, motivating the development of several alternative series<ref>{{cite journal|last1=Lockwood|display-authors=etal|title=Centennial variations in sunspot number, open solar flux, and streamer belt width: 1. Correction of the sunspot number record since 1874.|journal=J. Geophys. Res. Space Phys.|date=2014|volume=119|issue=7|pages=5172–5182|doi=10.1002/2014JA019970|bibcode=2014JGRA..119.5172L|url=http://centaur.reading.ac.uk/36853/1/308_OSF1_jgra51127.pdf|doi-access=free}}</ref><ref>{{cite journal|last1=Svalgaard|first1=Schatten|title=Reconstruction of the sunspot group number: the backbone method.|journal=Solar Physics|date=2016|volume=291|issue=9–10|page=2653|doi=10.1007/s11207-015-0815-8|arxiv=1506.00755|bibcode=2016SoPh..291.2653S|s2cid=119221326}}</ref><ref>{{cite journal|last1=Usoskin|display-authors=etal|title=A new calibrated sunspot group series since 1749: statistics of active day fractions|journal=Solar Physics|date=2016|volume=291|issue=9–10|pages=2685–2708|doi=10.1007/s11207-015-0838-1|arxiv=1512.06421|bibcode=2016SoPh..291.2685U|s2cid=32791225}}</ref><ref name=":1">{{cite journal|last1=Chatzistergos|first1=T.|display-authors=etal|title=New reconstruction of the sunspot group numbers since 1739 using direct calibration and "backbone" methods|journal=Astron. Astrophys.|date=2017|volume=602|page=A69|doi=10.1051/0004-6361/201630045|arxiv=1702.06183|bibcode=2017A&A...602A..69C|s2cid=55139041}}</ref> suggesting different behavior of sunspot group activity before the 20th century.<ref>{{cite journal|last1=Usoskin|first1=I.|title=A history of solar activity over millennia|journal=Living Reviews in Solar Physics|date=2017|volume=14|issue=1|page=3|doi=10.1007/s41116-017-0006-9|bibcode=2017LRSP...14....3U|arxiv=0810.3972|s2cid=195340740}}</ref>
However, indirect indices of solar activity<ref>{{cite journal|last1=Asvestari|first1=E.|display-authors=etal|title=Assessment of different sunspot number series using the cosmogenic isotope 44Ti in meteorites|journal=Monthly Notices of the Royal Astronomical Society|date=2017|volume=467|issue=2|pages=1608–1613|doi=10.1093/mnras/stx190|doi-access=free |bibcode=2017MNRAS.467.1608A|url=http://centaur.reading.ac.uk/69290/1/2017%20Avestari%20et%20al.%2C%20MNRAS%20stx190.pdf}}</ref><ref>{{cite journal|last1=Wu|first1=C.-J.|display-authors=etal|title=Solar total and spectral irradiance reconstruction over the last 9000 years|journal=Astron. Astrophys.|date=2018|volume=620|page=A120|doi=10.1051/0004-6361/201832956|bibcode=2018A&A...620A.120W|arxiv=1811.03464|s2cid=118843780}}</ref><ref>{{cite journal|last1=Petrovay|first1=K.|title=Solar cycle prediction|journal=Living Reviews in Solar Physics|volume=7|pages=6|date=2019|issue=1 |arxiv=1012.5513|bibcode=2020LRSP...17....2P|doi=10.12942/lrsp-2010-6|doi-access=free |pmid=27194963|pmc=4841181}}</ref> favor the group sunspot number series by Chatzistergos T. et al.<ref name=":1" />

A different index of sunspot activity was introduced in 1998 in the form of the number of groups apparent on the solar disc.<ref>{{cite journal|last1=Hoyt|first1=D.|last2=Schatten|first2=K. H.|title=Group Sunspot Numbers: A New Solar Activity Reconstruction|journal=Solar Physics|date=1998|volume=179|issue=1|pages=189–219|doi=10.1023/A:1005007527816|bibcode=1998SoPh..179..189H|s2cid=189822917}}</ref>
With this index it was made possible to include sunspot data acquired since 1609, being the date of the invention of the telescope.