Number of sunspot groups from the Galileo–Scheiner controversy revisited

Abstract

ABSTRACT We revise the sunspot observations made by Galileo Galilei and Christoph Scheiner in the context of their controversy regarding the nature of sunspots. Those of their sunspot records not included in the current sunspot group database, used as a basis to calculate the sunspot group number, are analysed. Within the documentary sources consulted in this work, we can highlight the sunspot observations by Scheiner included in the letters sent under the pseudonym Apelles to Marcus Welser and the first sunspot observations made by Galileo, which can be consulted in Le opere di Galileo Galilei. These sunspot observations would extend the temporal coverage for these two observers and fill some gaps in the current group database in the earliest period, where the data available are sparse. Moreover, we have detected changes in the quality of the sunspot drawings made by Galileo and Scheiner in their observation series, affecting the number of groups recorded by the two observers. We also compare these records with sunspot observations made by other astronomers of that time. According to this comparison and regarding the same observation days, Scheiner was generally the astronomer who reported more sunspot groups, while Harriot, Cigoli and Galileo recorded a similar number of groups. We conclude that these differences are mainly because of the observational methods used by the observers.