Fig. 1. (a) Superposition of the colored modifications of the original drawings by Siverus from Fogelius (1671a) is shown in red and from Fogelius (1671b) in blue with imposed heliographic grids. Drawing by Hook (1671) is shown in black. (b) Sunspot-group longitudes. Filled symbols indicate the data used to calculate the rotation rate (Zolotova and Vokhmyanin 2025).
Observations by Heinrich Siverus
Heinrich Siverus or Sievers (1625–1712) was German mathematician and geographer.
18 August – 15 September 1671
This series of sunspot observations continues the observations by Jean-Dominique Cassini from 11 to 20 August 1671. These observations were analyzed by Hayakawa et al. (2021), who proposed that Cassini and Siverus documented the same long-lived active region.
The observations by Siverus accompanies a letter by Martin Fogelius (1671a) to Henry Oldenburg, which is preserved in the Royal Society Library. Portion of these observations was also reprinted in Fogelius (1671b). Figure 1(a) illustrates the superposition of the Siverus's drawings, represented in red and blue. The heliographic grid overlaid on the image corresponds to the equatorial mounting of the telescope used. Compared to Cassini's engravings, Siverus's depictions are more schematic, with sunspots appearing enlarged.
The engraving discrepancies are approximately 2o in both latitude and longitude. Similar to the observations by Cassini (Figure 1, therein), the latitude of the sunspot group increases as the group approaches the western limb, likely resembling increasing observational uncertainty. The latitude range of the sunspot group is estimated to be 2 − 10o, with an average latitude of 7.3 ± 3.4o.
Figure 1(b) displays the longitude variations of the sunspot group, which exhibit considerable fluctuation. The rotation rate is estimated to be 14.2 ± 0.8o d-1 and 14.5 ± 0.8o d-1 for two distinct intervals, as indicated by the filled symbols. The actual value is likely in the range of 14.2–14.3o d-1, aligning well with Cassini's measurements.
Another sunspot observation by Robert Hook (commonly spelled Hooke) is represented as a black circle in Figure 1. Similar to Cassini, Hook (1671) referred to the sunspot as comprising the umbra alone, describing the penumbra as a "dusky cloud". He also provided a sketch of the sunspot group (enclosed in a black frame in Figure 1a) and noted that it was "of this form exactly". In addition to the sketch, Hook recorded the time, size, and position of this group. The black circle in Figure 1 depicts the position and size of the whole phenomenon based on Hook's measurements, as it would appear from Hamburg (where Siverus observed; Hook himself was in London). The size of the whole phenomenon (1/72 of the solar diameter) is evidently too small when compared to Siverus's drawings. However, the longitude of the sunspot group reported by Hook aligns closely with the first part of Siverus’s observations, as illustrated in Figure 1b.
By examining the first longitudes reported by Cassini (≈ 145o on 11–14 August), those by Siverus (≈ 148o on 5–8 September), and the one by Hook (148o on 11 September 1671), we infer that the group was likely recurrent, with a rotational rate of approximately 14.3o d-1 . However, it is noteworthy that Siverus generally mapped the sunspot a few degrees closer to the Equator compared to Cassini (Figure 1, therein). This positinal difference raises the possibility that the spots observed in August and September may have belonged to two distinct groups within the same activity nest.
