Solar Physics

, Volume 291, Issue 9–10, pp 2763–2784 | Cite as

The Discontinuity Circa 1885 in the Group Sunspot Number

  • E. W. Cliver
  • A. G. Ling
Sunspot Number Recalibration


On average, the international sunspot number (\(R_{\mathrm{I}}\)) is 44 % higher than the group sunspot number (\(R_{\mathrm{G}}\)) from 1885 to the beginning of the \(R_{\mathrm{I}}\) series in 1700. This is the principal difference between \(R_{\mathrm{I}}\) and \(R_{\mathrm{G}}\). 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 \(R_{\mathrm{G}}\). 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 \(R_{\mathrm{GC}}\) 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 \(R_{\mathrm{G}}\) and \(R_{\mathrm{I}}\) series into reasonable agreement for the 1841 – 1885 interval (after correcting \(R_{\mathrm{I}}\) for an inhomogeneity from 1849 – 1867 (to give \(R_{\mathrm{IC}}\))). Comparison with an independently derived backbone-based reconstruction of \(R_{\mathrm{G}}\) (\(R_{\mathrm{BB}}\)) indicates that \(R_{\mathrm{GC}}\) over-corrects \(R_{\mathrm{BB}}\) 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 \(R_{\mathrm{IC}}\) series are too low by ≈ 10 %.


Sun Sunspots Sunspot number 



Ed Cliver acknowledges helpful discussions with Leif Svalgaard, Frédéric Clette, and Laure Lefèvre. We thank David Willis and Matthew Wild and an unidentified referee for helpful comments. Alan Ling acknowledges support from AFRL contract FA9453-12-C-0231.


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Copyright information

© Springer Science+Business Media Dordrecht 2016

Authors and Affiliations

  1. 1.National Solar ObservatorySunspotUSA
  2. 2.Space Vehicles DirectorateAir Force Research LaboratoryKirtland AFBUSA
  3. 3.Atmospheric Environmental ResearchAlbuquerqueUSA

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