Christian Horrebow’s Sunspot Observations – II. Construction of a Record of Sunspot Positions

Abstract

The number of spots on the surface of the Sun is one of the best tracers of solar variability we have. The sunspot number is not only known to change in phase with the 11-year solar cycles, but also to show variability on longer time scales. It is, however, not only the sunspot number that changes in connection with solar variability. The location of the spots on the solar surface is also known to change in phase with the 11-year solar cycle. This has traditionally been visualised in the so-called butterfly diagram, but this is only well constrained from the beginning of the 19th century. This is unfortunate, as knowledge about the butterfly diagram could aid our understanding of the variability and the Sun–Earth connection.

As part of a larger review of the work done on sunspots by the Danish astronomer Christian Horrebow, we here present a reanalysis of Christian Horrebow’s notebooks covering the years 1761 and 1764 – 1777. These notebooks have been analysed in at least three earlier studies by Thiele (Astron. Nachr. 50, 257, 1859), d’Arrest (published in Wolf, Astron. Mitt. Eidgenöss. Sternwarte Zür. 4, 77, 1873) and Hoyt and Schatten (Solar Phys. 160, 387, 1995). In this article, we construct a complete record of sunspot positions covering the years 1761 and 1764 – 1777. The resulting butterfly diagram shows the characteristic structure known from observations in the 19th and 20th century. We do see some indications of equatorial sunspots in the observations we have from Cycle 1. However, in Cycle 2, which has much better coverage, we do not see such indications.