Revisiting the Sunspot Number

@article{Clette2014RevisitingTS,
  title={Revisiting the Sunspot Number},
  author={Fr{\'e}d{\'e}ric Clette and Leif Svalgaard and Jos{\'e} Manuel Vaquero and Edward W. Cliver},
  journal={Space Science Reviews},
  year={2014},
  volume={186},
  pages={35-103}
}
Our knowledge of the long-term evolution of solar activity and of its primary modulation, the 11-year cycle, largely depends on a single direct observational record: the visual sunspot counts that retrace the last 4 centuries, since the invention of the astronomical telescope. Currently, this activity index is available in two main forms: the International Sunspot Number initiated by R. Wolf in 1849 and the Group Number constructed more recently by Hoyt and Schatten (Sol. Phys. 179:189–219… 
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