Number of Sunspot Groups from the Galileo-Scheiner controversy revisited

TL;DR

This study revisits historical sunspot observations by Galileo and Scheiner, analyzing previously unused records to improve the sunspot group database and understand observational differences of the early 17th century.

Contribution

It uncovers new sunspot records from Galileo and Scheiner, extends historical data coverage, and analyzes how observational methods affected sunspot group counts.

Findings

Scheiner generally reported more sunspot groups than Galileo.

Differences in sunspot counts are mainly due to observational methods.

New records fill gaps in early sunspot data.

Abstract

We revise the sunspot observations made by Galileo Galilei and Christoph Scheiner in the context of their controversy on the nature of sunspots. Their sunspot records not included in the current sunspot group database, used as a basis to calculate the sunspot group number, are analyzed. 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 filling some gaps in the current group database in the earliest period where the data available is sparse. Moreover, we have detected changes in the quality of the sunspot drawings made by Galileo and Scheiner in their observation series affecting to 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 these differences are mainly because of the observational method used by the observers.