Christian Horrebow’s Sunspot Observations – I. Life and Published Writings
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Between 1761 and 1776, Christian Horrebow made regular observations of sunspots from Rundetårn in Copenhagen. Based on these observations he writes in 1775 that “it appears that after the course of a certain number of years, the appearance of the Sun repeats itself with respect to the number and size of the spots”. Thus, Horrebow hypothesized the idea of a cyclic Sun several decades before Heinrich Schwabe discovered the solar cycle and estimated its period. This proves the ability of Horrebow as a sunspot observer. In this article, we present a general overview of the work of Christian Horrebow, including a brief biography and a complete bibliography. We also present a translation from Danish to English of his writings on sunspots in the Dansk Historisk Almanak. These writings include tables of daily sunspot measurements of which we discuss the completeness.
KeywordsSolar cycle, observations Sunspots
We would like to thank the referee for thoughtful comments which significantly improved the paper. The translation of Christian Horrebow’s published papers on sunspots was done by Lenore Messick, Events and Communication Support, Aarhus University. The project has been supported by the Villum Foundation. Funding for the Stellar Astrophysics Centre is provided by the Danish National Research Foundation (grant agreement no. DNRF106), funding for VSP is provided by the Deutsche Forschungsgemeinschaft (grant no. AR355/12-1). We are very grateful to Susanne Elisabeth Nørskov, who has been most helpful throughout the project with finding the notebooks and other historical texts. We are also grateful to Kristian Hvidtfelt Nielsen, who helped with interpreting the historical texts.
Disclosure of Potential Conflicts of Interest
The authors declare that they have no conflicts of interest.