Fig. 1. (a) Original drawings by Saxonius (Eimmart Archive Coll. 998, v. 11, f. 99; courtesy of MD NLR); (b) clustering of sunspots into groups and reconstructed heliographic grid assuming projection behind a Galilean telescope; (c) the number of groups defined by Hoyt and Schatten (1998), for which we shifted dates to the Gregorian calendar, and in Vokhmyanin and Zolotova (2023); (d) spatio-temporal distribution of sunspot groups.
Observations by Petrus Saxonius
Petrus Saxonius (Petri Saxonis in Latin transcription, 1591–1625) was a German mathematician and astronomer.
5–27 March 1616
We examine reproduction of twelve sunspot drawings stored in the archives of the National Library of Russia (Manuscript department: the Collection 998, volume 11, folio 99). The diameter of the solar disks is about 5 cm. The original dates refer to the Julian calendar. As can be seen from the examples of drawings, sunspots are marked with letters (Fig. 1a).
Neuhauser and Neuhauser (2016, Figure 7 therein) analyzed the full copy of this reproduction available in the German National Museum in Nuremberg. They concluded that the drawings by Saxonius are oriented upside down after comparing them with the observations by Tarde (1620). After an appropriate rotation, the spots move from the left to the right. This may imply that Saxonius exploited a projection apparatus behind a Galilean telescope.
Figure 1b shows sunspot groups assigned by us. In complex activity nests (Castenmiller et al. 1986), however, various researchers may assign different numbers of groups. Figure 1c compares the groups defined by Hoyt and Schatten (1998) and by us. The former used the original dates, but we shifted them to the Gregorian calendar since Nürnberg and Altdorf were protestant at the time. The blue square of 14 March 1616 (Gregorian) is apparently a misprint in Hoyt and Schatten (1998); it should be the next day. Significant discrepancies occur on 15 – 19 March 1616. We suppose that Hoyt and Schatten assigned groups in accordance with Saxonius’s lettering while we clustered several spots into probable groups. The historical archive of sunspot observations (HASO) of Vaquero et al. (2016) provides the same group numbers (version 1.21) as Hoyt and Schatten, but corrected the dates to the Gregorian calendar, as we also did.
Saxonius observed at the University of Altdorf near Nürnberg and did not provide any notes on the time of drawings, except on the first one on 5 March (see below). Thus, we can only suppose that the images are oriented as they were seen on the projection screen. Applying Method II, we reconstruct the orientation of the solar axis, which provides us with the time of observation and the sunspot positions. We obtained the transit of 12 sunspot groups, of which only one was located in the southern hemisphere (Fig. 1d).
In the drawing of 5 March (Gregorian), Saxonius wrote “24 Februar. in. m.”, which could mean the drawings are made in the morning “in mane” or at Noon “in meridie”. The time calculated by Method II is about 11:00 UT, i.e. around local Noon. The figurative Sun in the middle of the reproduction repeats the drawing of 5 March. The sunspot positions therein deviate by a maximum of 3o, while sunspot areas on the figurative Sun are increased by a factor of two to three, giving us an idea of the accuracy of the drawings.
Saxonius drew the contours of sunspots. In order to calculate the sunspot area, we painted over the center of each sunspot (Fig. 2).
Fig. 2. (Left) Intensity of an image and a part of it. Image reversed vertically. Red denotes the intensity I<150; (Right) the same, but the centers of the sunspots are painted over.
