Rudolf Wolf to Alfred Wolfer: The Transfer of the Reference Observer in the International Sunspot Number Series (1876–1893)

Abstract In 1876, Alfred Wolfer started observing the Sun and recording properties of sunspots alongside Rudolf Wolf. Their observations became the basis for the construction of the Sunspot Number series. After Wolf’s death in 1893, Wolfer became the primary observer for the Sunspot Number series. Even though the observations of Wolf and Wolfer had an overlap of almost 17 years (1876–1893), this shift of primary observer from Wolf to Wolfer seems to have led to inconsistencies in the Sunspot Number series primarily due to inhomogeneities in Wolf’s observations. To address this issue, we digitise Mittheilungen (Wolf’s Journals) and analyze their tabulated datasets. These journals include the raw sunspot data from various observers that the Zürich Observatory used to compile the Sunspot Number (SNV1). These datasets have been used as source data for the construction of the first version of the Sunspot Number (SNV1) series, but they were not digitally accessible for a recalibration of the Sunspot Number series until recently. We have also acquired external datasets from recent archival investigations for contemporaneous sunspot observations. In this study, we use the Mittheilungen dataset to produce a new recalibration of the sunspot number series covering 1816–1944, using four major observers (Tevel, Schwabe, Weber, and Wolfer) as backbones. The availability of the raw data allows us to identify issues in the determination of the scaling factors or k-factors between the records of different observers, but also the use of modern techniques for cross-calibrations. Our reconstruction for the years 1816–1944 is done with a novel method inspired by Chatzistergos et al.(2017, A&A, 602, A69), allowing to eliminate inconsistencies that resulted from the application of erroneous k-factors.