Although sunspot data has been recorded in one form or another for over 400 years, there have been few changes in the counting system since the introduction of the 'Wolf Number' in 1849. Recording of the sunspot 'Group Number' came into existence in 1998. It seems there were some strong differences in the two parallel series of systems and in 2011 a group of 40 experts undertook a full examination and revision of both systems in order to identify and fix the defects.
The new system, which became effective on July 1st, has brought both systems into alignment, with the most notable correction being in the lowering, by about 18%, of all numbers after 1947. The new Group Number has been corrected for a large underestimate of all values before the 20th century and has resulted in a fully reconstructed series of Group Numbers.
The graphic of Group Numbers shows significant increases for cycles up until the 1900's.
The new system brings the correlation of Group Numbers and Sunspot Numbers into harmony, unlike before.
Although these changes became effective on July 1st, the work is far from complete. According to the folks at SILSO (Sunspot Index and Long-term Solar Observations):
Still, as can be seen, significant deviations remain, mainly before 1825, when the observations become scarce and in periods of activity minima (low counts). So, more work definitely remains to be done for many years to come, but given the major improvements harvested at this stage, the WDC - SILSO is now going to proceed with the public release of this new version.
The preparation of this major operation is now almost completed. It required a huge organizational and programming work from the small SILSO team. Indeed, the release of the new past Sunspot series is just a starting point for the WDC-SILSO. Indeed, it requires a deep reworking of the operational software that will process the data from our worldwide network on July 1st and in the future. Indeed, various data products must be made seamlessly compatible with the new base total Sunspot Number series: the hemispheric Sunspot Numbers, the daily Estimated Sunspot Number, the 12-month solar-cycle forecasts, all data plots and the derivation of personal k coefficients for all stations of the network.
After a rather uneventful life over the past 166 years, the Sunspot Number will thus be reborn in a new incarnation on Wednesday July 1st. We hope that the science community will welcome this revived data set and will appreciate the considerable community effort accomplished over the past four years to produce a better reference for long-term solar and Sun-Earth studies.