The Discontinuity Circa 1885 in the Group Sunspot Number

On average, the international sunspot number (R-I) is 44 % higher than the group sunspot number (R-G) from 1885 to the beginning of the R-I series in 1700. This is the principal difference between R-I and R-G. Here we show that this difference is primarily due to an inhomogeneity in the Royal Greenwich Observatory (RGO) record of sunspot groups (1874 - 1976) used to derive observer normalization factors (called k-factors) for R-G. Specifically, annual RGO group counts increase relative to those of Wolfer and other long-term observers from 1876 - 1915. A secondary contributing cause is that the k-factors for observers who began observing before 1884 and overlapped with RGO for any years during 1874 - 1883 were not based on direct comparison with RGO but were calculated using one or more intermediary or additional observers. We introduce R-GC by rectifying the RGO group counts from 1874 - 1915 and basing k-factors on direct comparison with RGO across the 1885 discontinuity, which brings the R-G and R-I series into reasonable agreement for the 1841 - 1885 interval (after correcting R-I for an inhomogeneity from 1849 - 1867 (to give R-IC)). Comparison with an independently derived backbone-based reconstruction of R-G (R-BB) indicates that R-GC overcorrects R-BB by 4 % on average from 1841 - 1925. Our analysis suggests that the maxima of Cycles 10 (in 1860), 12 (1883/1884), and 13 (1893) in the R-IC series are too low by approximate to 10 %.