Sunspot positions, areas, and group tilt angles for 1611-1631 from observations by Christoph Scheiner

TL;DR

This study digitized and analyzed Christoph Scheiner's 1611-1631 sunspot observations to produce detailed sunspot positions, sizes, and tilt angles, revealing historical solar activity patterns and tilt angles comparable to modern values.

Contribution

The paper provides the first detailed digitization and analysis of Scheiner's early 17th-century sunspot data, including positions, sizes, and tilt angles, with improved accuracy and historical context.

Findings

Sunspot butterfly diagram constructed for 1611-1631.

Average tilt angle of sunspot groups ~4 degrees, similar to modern values.

Data quality varies, with most accurate from 1618-1631.

Abstract

Digital images of the observations printed in the books "Rosa Ursina sive solis" and "Prodromus pro sole mobili" by Christoph Scheiner as well as the drawings from Scheiner's letters to Marcus Welser are analysed in order to obtain information on positions and sizes of sunspots that appeared before the Maunder minimum. In most cases, the given orientation of the ecliptic is used to set up the heliographic coordinate system for the drawings. Positions and sizes are measured manually on the screen. Very early drawings have no indication of their orientation. A rotational matching using common spots of adjacent days is used in some cases, while in other cases, the assumption of images being aligned with a zenith-horizon coordinate system appeared to be the most probable. In total, 8167 sunspots were measured. A distribution of sunspot latitudes versus time (butterfly diagram) is obtained for Scheiner's observations. The observations of 1611 are very inaccurate, the drawings of 1612 have at least an indication of their orientation, while the remaining part of the spot positions from 1618-1631 have good to very good accuracy. We also computed 697 tilt angles of apparently bipolar sunspot groups observed in the period 1618-1631. We find that the average tilt angle of nearly 4 degrees is not significantly different from 20th-century values.