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Homogeneity is considered as the most important property of the Wolf series of sunspot relative numbers, or Wolf numbers, since without a stable scale no valid conclusions about variations in the long-term progress of solar activity can be drawn. However, the homogeneity testing of the Wolf series is a difficult task, since the raw data entering the series and the methods of data-reduction and interpolation used to compile the series are largely unknown. In this article we reconstruct the data-reduction algorithms based on hitherto unpublished original sources from the archives of the former Swiss Federal Observatory in Zürich and discuss their impact on the homogeneity of the Wolf series. Based on Alfred Wolfer as reference, we recalculate the progress of the Wolf series from 1877 to 1893, correcting for the widely disregarded diminishing of Wolf’s eyesight, for the change of Wolf’s main instrument from the 40/700 mm Parisian refractor to the 42/800 mm Fraunhofer refractor, and for the inhomogeneities in the data-reduction procedure during the same time period. The maxima of Cycle 12 in 1884 and of Cycle 13 in 1893 are roughly 10% higher in the recalculated and corrected Wolf series than in the original Wolf series as provided by WDC-SILSO version 1.0. From 1877 to 1883 the smoothed monthly means of the recalculated and corrected Wolf series are up to a factor of 0.76 lower than the original values.

Recalculation of the Wolf Series from 1877 to 1893