| Sunspot numbers, 1610-1985 : based on "The sunspot activity in the years 1610-1960" - :1344 | National Environmental Satellite and Data Information Service (NESDIS)
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Sunspot numbers, 1610-1985 : based on "The sunspot activity in the years 1610-1960"
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Sunspot numbers, 1610-1985 : based on
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    United States, National Environmental Satellite, Data, and Information Service, ; World Data Center A for Solar-Terrestrial Physics ;
  • Description:
    "In 1848 the Swiss astronomer Johann Rudolph Wolf introduced the sunspot number index--an estimate of the number of spots visible on one hemisphere of the sun. The observation consists of a daily count over the solar disk of the number of spots (regardless of size) and of the number of groups into which the cluster. Rudolph Wolf arbitrarily defined his daily sunspot number as the total spot count added to 10 times the number of spot groups, because neither the number of groups nor the number of spots alone adequately measured the activity. Counting only groups would ignore their great variation in size, since some can contain more than 100 single spots, while others, only a few. Counting only individual spots, on the other hand, would ignore their unequal rank, because spots appearing within an existing group have less significance than ones emerging where none existed before"--Introduction.

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