Daniel Mögling’s Sunspot Observations in 1626–1629: A Manuscript Reference for the Solar Activity before the Maunder Minimum

Abstract

The sunspots have been observed since 1610, and their group numbers have been used for evaluating the amplitude of solar activity. Daniel Mögling recorded his sunspot observations for more than 100 days in 1626–1629 and formed a significant data set of sunspot records before the Maunder Minimum. Here we have analyzed his original manuscripts in the Universitäts- und Landesbibliothek Darmstadt to review Mögling’s personal profile and observational instruments and derived the number and positions of the recorded sunspot groups. In his manuscript, we have identified 134 days with an exact sunspot group number and 3 days of additional descriptions. Our analyses have completely revised the observational dates and group numbers, added 19 days of hitherto overlooked observations, and removed 8 days of misinterpreted observations. We have also revisited the sunspot observations of Schickard and Hortensius and revised their data. These results have been compared with the contemporary observations. Moreover, we have derived the sunspot positions from his sunspot drawings and located them at 2°–23° in heliographic latitude in both solar hemispheres. Contextualized with contemporary observations, these results indicate their temporal migration to lower heliographic latitudes and emphasize their location in the declining phase of solar cycle −12 in the 1620s. Mögling’s observations were probably conducted using a pinhole and camera obscura, which likely made him underestimate the sunspot group number by ≥33%–52%. This underestimation should be noted upon comparison with modern data sets.