Christian Horrebow’s Sunspot Observations – I. Life and Published Writings

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.