Digitization of Sp\"orer's sunspot drawings

TL;DR

This paper digitizes and processes Gustav Spörer’s historical sunspot drawings to create precise, accessible data on sunspot positions, areas, and active region complexities, extending the solar activity record into the late 19th century.

Contribution

It introduces a novel digitization and correction method for historical sunspot drawings, enabling detailed analysis of sunspot morphology and dynamics from past observations.

Findings

High-precision sunspot position data for nearly 450 solar rotations

Accurate measurements of sunspot areas and tilt angles

Assessment of active region complexity using morphological techniques

Abstract

Much of our knowledge about the solar dynamo is based on sunspot observations. It is thus desirable to extend the set of positional and morphological data of sunspots into the past. Gustav Sp\"orer observed in Germany from Anklam (1861-1873) and Potsdam (1874-1894). He left detailed prints of sunspot groups, which we digitized and processed to mitigate artifacts left in the print by the passage of time. After careful geometrical correction, the sunspot data are now available as synoptic charts for almost 450 solar rotation periods. Individual sunspot positions can thus be precisely determined and spot areas can be accurately measured using morphological image processing techniques. These methods also allow us to determine tilt angles of active regions (Joy's law) and to assess the complexity of an active region.