Scale Transfer in 1849: Heinrich Schwabe to Rudolf Wolf

The focus of this study is to reveal the reason behind a scale problem detected around 1849 in the historical version of the International Sunspot Number Series, i.e. version 1 (Leussu et al. in Astron. Astrophys. 559 , A28, 2013 ; Friedli in Solar Phys. 291 , 2505, 2016 ). From 1826 to 1848 Heinrich Schwabe’s observations were considered primary by Rudolf Wolf, and a shift of primary observer from Schwabe to Wolf in 1849 seems to have led to an inconsistency in the Sunspot Number series. In this study we benefited from various datasets, the most important being Schwabe’s raw counts from the Mittheilungen (Prof. Wolf’s Journals) that have been digitised at the Royal Observatory of Belgium between 2017 and 2019. We provide a robust quantification of the detected problem by using classic algebraic calculations, but also different methods such as a method inspired by Lockwood et al. ( J. Geophys. Res. 119 (7), 5172, 2014 ), hence assigning a modern k-factor to Schwabe’s observations before 1849. We also assess the implications of this 1849 inconsistency on the International Sunspot Number series (Versions 1 and 2) before and after 1849.