Solar Physics

, 292:4 | Cite as

A Recount of Sunspot Groups on Staudach’s Drawings

  • Leif SvalgaardEmail author
Sunspot Number Recalibration


We have examined the more than 1100 drawings of the solar disk made by the German amateur astronomer Johann Caspar Staudach during 1749 – 1799 and counted the spots on each image. Using the modern perception of how to group spots into active regions, we regrouped the spots as a modern observer would. The resulting number of groups was found to be higher on average by 25 % than the first count of groups performed by Wolf in 1857, which was used by Hoyt and Schatten (Solar Phys. 181, 491, 1998) in their construction of the group sunspot number. Compared to other observers at the time, Staudach’s drawings have a very low average number, about two, of spots per group, possibly indicating an inferior telescope that probably suffered from spherical and chromatic aberration, as would be typical of amateur telescopes of the day. We have initiated an ongoing project aiming at observing sunspots with antique telescopes having similar defects in order to determine the factor necessary to bring the Staudach observations onto a modern scale.


Sunspots Group numbers Staudach Recount 



We thank Rainer Arlt for making available the photographic images of the Staudach drawings. L. Svalgaard thanks Stanford University for continuing support.

Disclosure of Potential Conflicts of Interest

The author declares that he has no conflicts of interest.

Supplementary material

11207_2016_1023_MOESM1_ESM.txt (341 kb)
(TXT 341 kB)


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Copyright information

© Springer Science+Business Media Dordrecht 2016

Authors and Affiliations

  1. 1.Stanford University, Cypress Hall C3, W.W. Hansen Experimental Physics LaboratoryStanford UniversityStanfordUSA

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