Sunspot group tilt angle measurements from historical observations

TL;DR

This study analyzes historical sunspot drawings to measure bipolar sunspot group tilt angles over 270 years, revealing that tilt angles were normal before the Maunder minimum and only decreased afterward.

Contribution

It provides a comprehensive analysis of historical sunspot tilt angles, extending the dataset to 270 years and comparing pre- and post-Maunder minimum values.

Findings

Average tilt angles before the Maunder minimum were similar to modern values.

Tilt angles during cycles 0 and 1 after the Maunder minimum were significantly lower.

Normal tilt angles before the Maunder minimum suggest other factors caused the grand minimum.

Abstract

Sunspot positions from various historical sets of solar drawings are analysed with respect to the tilt angles of bipolar sunspot groups. Data by Scheiner, Hevelius, Staudacher, Zucconi, Schwabe, and Spoerer deliver a series of average tilt angles spanning a period of 270 years, additional to previously found values for 20th-century data obtained by other authors. We find that the average tilt angles before the Maunder minimum were not significantly different from the modern values. However, the average tilt angles of a period 50 years after the Maunder minimum, namely for cycles 0 and 1, were much lower and near zero. The normal tilt angles before the Maunder minimum suggest that it was not abnormally low tilt angles which drove the solar cycle into a grand minimum.