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Solar Physics

, 294:78 | Cite as

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

  • Christoffer KaroffEmail author
  • Carsten Sønderskov Jørgensen
  • V. Senthamizh Pavai
  • Rainer Arlt
Article

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.

Notes

Acknowledgements

We would like to thank the referee for thoughtful comments, which significantly improved the paper. 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). We are very thankful to librarian Susanne Elisabeth Nørskov, who has been very helpful throughout the projects, digging out the notebooks and other historical texts. We are also thankful to Kristian Hvidtfelt Nielsen for help with interpreting the historical texts.

Disclosure of Potential Conflicts of Interest

The authors declare that they have no conflicts of interest.

Supplementary material

11207_2019_1466_MOESM1_ESM.txt (844 kb)
(TXT 844 kB)

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Copyright information

© Springer Nature B.V. 2019

Authors and Affiliations

  1. 1.Stellar Astrophysics Centre, Department of Physics and AstronomyAarhus UniversityAarhus CDenmark
  2. 2.Department of GeoscienceAarhus UniversityAarhus CDenmark
  3. 3.Leibniz-Institut für Astrophysik PotsdamPotsdamGermany

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