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  • Monthly mean total sunspot number and 13-month smoothed numbers

    Monthly time series (Jan 1749–May 2015). It was last modified on 1 Nov 2016 at 17:20.

    • Units: Sunspots
    • Dimensions:
    • Description:

      Monthly mean total sunspot number obtained by taking a simple arithmetic mean of the daily total sunspot number over all days of each calendar month.Monthly means are available only since 1749 because the original observations compiled by Rudolph Wolf were too sparse before that year. (Only yearly means are available back to 1700)The 13-month smoothed monthly sunspot number is derived by a 'tapered-boxcar' running mean of monthly sunspot numbers over 13 months centered on the corresponding month (Smoothing function: equal weights = 1, except for first and last elements (-6 and +6 months) = 0.5, Normalization by 1/12 factor). There are no smoothed values for the first 6 months and last 6 months of the data series. NB: Although many other smoothing functions can be used, this method introduced by the Zurich observatory in the early 20th century still serves as a base standard (e.g. for the conventional definition of the times of minima and maxima of solar cycles).

    • Last updated: 1 Nov 2016
    • Citing: WDC-SILSO, Royal Observatory of Belgium, Brussels
    • Access: Public
    • Price: Free