Sunspot characteristics at the onset of the Maunder Minimum based on the observations of Hevelius

TL;DR

This study re-evaluates Hevelius's 1642-1645 sunspot observations, revealing detailed characteristics and early signs of the Maunder Minimum, and confirms the reliability of these historical records for understanding solar activity.

Contribution

It provides a meticulous re-analysis of Hevelius's sunspot data, offering new insights into pre-Maunder Minimum solar activity and validating the authenticity of historical observations.

Findings

Hevelius's sunspot group counts are 7% higher than previous databases.

Active day fraction before the Maunder Minimum was around 70%.

No hemispheric asymmetry was observed during 1642-1645.

Abstract

An analysis of the sunspot observations made by Hevelius during 1642-1645 is presented. These records are the only systematic sunspot observations just before the Maunder Minimum. We have studied different phenomena meticulously recorded by Hevelius after translating the original Latin texts. We re-evaluate the observations of sunspot groups by Hevelius during this period and obtain an average value 7% greater than that calculated from his observations given in the current group database. Furthermore, the average of the active day fraction obtained in this work from Hevelius' records previous to the Maunder Minimum is significantly greater than the solar activity level obtained from Hevelius' sunspot observations made during the Maunder Minimum (70% vs. 30%). We also present the butterfly diagram obtained from the sunspot positions recorded by Hevelius for the period 1642-1645. It can be seen that no hemispheric asymmetry exists during this interval, in contrast with the Maunder Minimum. Hevelius noted a ~3-month period that appeared to lack sunspots in early 1645 that gave the first hint of the impending Maunder Minimum. Recent studies claim that the Maunder Minimum was not a grand minimum period speculating that astronomers of that time, due to the Aristotelian ideas, did not record all sunspots that they observed, producing thus an underestimation of the solar activity level. However, we show the good quality of the sunspot records made by Hevelius indicates that his reports of sunspots were true to the observations.