Fig. 1. (Left) Original drawing by Wurzelbau (Eimmart Archive Coll. 998, v. 5, f. 47); (Right) reproduction of a horizontally mirrored image with imposed heliographic grids (Vokhmyanin and Zolotova 2023).
Observations by Johann Wurzelbau
11–12 June 1684
Johann Philipp von Wurzelbauer (or Wurzelbaur, Wurzelbau, Wurtzelbaur, Wurtzelbau, 1651–1725) is a German astronomer. One drawing by Wurzelbau is stored in the archives of the Manuscript department of the National Library of Russia (Eimmart Archive Collection, fond 998, volume 5, folio 47).
Text description says: "Schema Macularum in Sole visarum ope telescopii sinistra dextris permutantis" i.e. a figure of the sunspots viewed by means of a telescope swapping left and right. This suggests that projection was used; therefore, on the horizontally mirrored image, Meridies (the Noon direction) is located on the eastern limb, as expected for an afternoon observation (left panel of Fig. 1). Further, the description says that sunspots a and b were observed at 4 p.m. of 1 June, while α and β were observed at 2 1/2 p.m. on 2 June 1684 according to the Julian calendar. These dates are 11 and 12 June of the Gregorian calendar.
In order to reconstruct the heliographic grid, we assume that Wurzelbau observed in Nürnberg in his house at Spitzenberg 4 (Gaab 2002). Since just two observing days are available, we use only Method I, yielding the time of observation in UT format. The only image orientation giving an acceptable motion of the sunspots on nearly constant latitudes is shown in Figure 1. The average latitude of the sunspot group is −9o, and the longitudes of two sunspots vary from 260o to 280o.
Wurzelbau heterogeneously painted sunspots with ink. In order to calculate the sunspot area, we painted over the center of each sunspot (Fig. 2).
Fig. 2. (Left) Original drawing by Wurzelbau and a part of it; (Right) intensity of the image. The centers of the sunspots are painted over. Red denotes the intensity I<150.
