Analyses on Anselm Desing’s Sunspot Drawings: Additional Data to the Sunspot Datasets in 1742

Solar magnetic activity has been quantified using sunspot data on a centennial timescale based on historical records. However, It is challenging to calibrate sunspot records, particularly from the 1740s backward, due to data scarcity. In this study, we present and analyse Anselm Desing’s sunspot records of 1742. His records had been previously overlooked in databases and discussions in the scientific community. His observational site and telescope were located in Salzburg (N47°48′, E013°03′) using an 11-ft telescope without a projection board. From Desing’s original manuscript, we identified 18 days of whole-disk sunspot drawings and six days of partial sunspot drawings between 14 May and 29 June 1742, counting sunspot groups (up to four) and individual sunspots (up to 15). Contextualising Desing’s data to the existing sunspot records of 1742 from previous studies (17 days known), we doubled the data coverage to 34 days, suggested a slight upward modification of the greatest group number for this year in raw group count, and influence the brightest star estimate. Our measurements confirmed sunspot positions in Desing’s sunspot drawings almost symmetrically within < 21° from the solar equator in both solar hemispheres, showing equatorial migrations of reported sunspot positions in the late phase of Solar Cycle −1. This case study underscores the significance and importance of further investigations of historical sunspot records for developing a data series of sunspot number, sunspot group number, and sunspot positions.