Digitization of sunspot drawings by Sp\"orer made in 1861-1894

TL;DR

This paper details the digitization and processing of 19th-century sunspot drawings by Gustav Spörer, enabling modern analysis of historical solar activity through advanced image techniques and data aggregation.

Contribution

It introduces a comprehensive method for digitizing, correcting, and analyzing historical sunspot drawings, facilitating new scientific insights from archival solar data.

Findings

Created a digital archive of Sp"orer's sunspot drawings

Developed methods to measure sunspot sizes and tilt angles

Generated a sunspot area distribution consistent with modern data

Abstract

Most of our knowledge about the Sun's activity cycle arises from sunspot observations over the last centuries since telescopes have been used for astronomy. The German astronomer Gustav Sp\"orer observed almost daily the Sun from 1861 until the beginning of 1894 and assembled a 33-year collection of sunspot data covering a total of 445 solar rotation periods. These sunspot drawings were carefully placed on an equidistant grid of heliographic longitude and latitude for each rotation period, which were then copied to copper plates for a lithographic reproduction of the drawings in astronomical journals. In this article, we describe in detail the process of capturing these data as digital images, correcting for various effects of the aging print materials, and preparing the data for contemporary scientific analysis based on advanced image processing techniques. With the processed data we create a butterfly diagram aggregating sunspot areas, and we present methods to measure the size of sunspots (umbra and penumbra) and to determine tilt angles of active regions. A probability density function of the sunspot area is computed, which conforms to contemporary data after rescaling.