Sunspot Positions and Areas from Observations by Galileo Galilei
Sunspot records in the seventeenth century provide important information on the solar activity before the Maunder minimum, yielding reliable sunspot indices and the solar butterfly diagram. Galilei’s letters to Cardinal Francesco Barberini and Marcus Welser contain daily solar observations on 3 – 11 May, 2 June – 8 July, and 19 – 21 August 1612. These historical archives do not provide the time of observation, which results in uncertainty in the sunspot coordinates. To obtain them, we present a method that minimizes the discrepancy between the sunspot latitudes. We provide areas and heliographic coordinates of 82 sunspot groups. In contrast to Sheiner’s butterfly diagram, we found only one sunspot group near the Equator. This provides a higher reliability of Galilei’s drawings. Large sunspot groups are found to emerge at the same longitude in the northern hemisphere from 3 May to 21 August, which indicates an active longitude.
KeywordsSunspots Sun: activity Solar cycle, observations
We thank the referee for the substantial revision and helpful comments. The reported study was partially funded by RFBR according to the research projects 15-02-06959-a and 16-02-00300-a.
Disclosure of Potential Conflicts of Interest
The authors declare that they have no conflicts of interest.