Updating the Historical Sunspot Record

We review the evidence for the argument that Rudolf Wolf’s calibration of the Sunspot Number is likely to be correct and that Max Waldmeier introduced an upwards jump in the sunspot number in 1945. The combined effect of these adjustments suggests that there has been no secular change in the sunspot number since coming out of the Maunder Minimum ∼1715. 1 The Sunspot Record(s!) The Sunspot Record goes back 400 years and is the basis for many reconstructions of solar parameters (e.g. TSI), but, how good is it? And can we agree on which one (Wolf Number, International Number, ’Boulder’ Number, Group Number, ...)? Are the old values good? Are the new ones? And what is a ’good’ or ’correct’ Sunspot Number anyway? Johann Rudolf Wolf (1859) defined his Relative Sunspot Number, taking into account both individual spots and their appearance in distinct groups (what we today call ’active regions’), as RW = 10Groups + Spots. Wolf started his own observations in 1849 and assembled observations from earlier observers back to 1749 and beyond (Figure 1). Figure 1. Rudolf Wolf and excerpts from his 1861 list of published Relative Numbers compared with his latest list (now the official list from SIDC in Brussels). As is clear, the earlier values were subsequently adjusted (upwards) as Wolf were struggling with the difficulty of bringing different observers onto the same