Fig. 1. Superposition of sunspot drawings and tabular computations of sunspot position (red circles) made by Manfredi (Eimmart Archive Collection 998, volume 4, folio 173 Vokhmyanin and Zolotova 2023).
Observations by Eustachio Manfredi
Eustachio Manfredi (1674–1739) is an Italian mathematician, astronomer and poet. We examine his sunspot observations (drawings and table data) stored in the Collection 998 of the Manuscript department of the National Library of Russia, St. Petersburg.
28 May – 29 June 1703
Eustachio Manfredi, an observer based in Bologna, also reported on solar activity during May – June 1703. He made two images showing a sunspot transit. The drawings are stored in volume 4, folio 173 and are accompanied by a date, an observing time (hours after sunset of the previous day), sunspot position in arcminutes and arcseconds, and other information. The x-coordinates of the sunspots are scaled to the solar diameter of 2′16.5′′, while the y-coordinates are scaled to 31'42''.
Figure 1 compares the original drawings by Manfredi and calculated sunspot positions according to micrometer measurements (red circles). Apart from the observation of 29 June 1703, drawings and tabular data are consistent. Similarly to Hoffmann, Manfredi has drawn sunspots not as they were seen in a telescope but already aligned. Therefore, we apply Method II to define sunspot latitudes. Also, following Hoffmann’s speculation, we consider this series of observations as an activity nest living for several rotations. Data contains the sunspot parameters reconstructed from both the drawings and micrometer measurements.
Figure 2 presents a comparison between the drawings by Hoffmann, Maria Eimmart, and Manfredi. It depicts the replicas of the drawings superimposed with the corrected orientation. Observing time is converted to UT. Hoffmann apparently depicted only sunspot umbre, while Maria Eimmart and Manfredi did not include small sunspots that accompanied the largest spot (see Hoffmann). This leads to an underestimation of the sunspot-group area.
Fig. 2. Superposition of sunspot drawing reproductions by Hoffmann, Maria Eimmart, and Manfredi from Eimmart Archive Collection 998, volume 3, folios 423–424; volume 35, folios 38–39; and volume 4, folio 173, correspondingly. Heliographic grids correspond to 26 May (a) and 18 June 1703 (b) seen at the Eimmart Observatory. Observing times reported by the observers are given in UT. Underlined notes suggest instances of probable time imprint (Vokhmyanin and Zolotova 2023).
On 30 May, Maria Eimmart drew the sunspot in a uniform gray color, yet the next day the sunspot umbra was accompanied by an asymmetric penumbra. The observations by Eimmart also allow estimating the latitudinal uncertainty to be slightly greater than 3o. During ~15 hours between these two observations, the longitudinal position of the spot decreased by 5.5o, implying that the associated longitudinal uncertainty is roughly twice that of the latitudinal uncertainty.
Manfredi apparently drew the penumbra and umbra schematically (Fig. 2, blue). On average, the sunspot is located at −1.7o in the southern hemisphere. Observations made by Manfredi from 28 May to 1 June yield the sunspot sidereal rotation period 14.3 ± 1o d-1, and 14.2 ±0.9o d-1 based on 20–28 June 1703 (spots near the limbs were excluded from the calculations). These results are compatible with the equatorial velocity of about 14.5o d-1 derived by Balthasar et al. (1986) from the Greenwich Photo-heliographic Results. We believe that there are no misprints in the time of observation noted by Manfredi. Therefore, the observations by Hoffmann (Fig. 2a, underlined red notes) of 29 and 30 May 1703 have been apparently made at 5 p.m. rather than at 5 a.m. The correction is further supported by the fact that the observation on the afternoon of 28 May is separated from the observation of 29 May by almost 13o in longitude, which corresponds to roughly 24 hours (red sunspots in Fig. 2a). The first observation of 31 May should have also been made in the evening, and the next observation of that day likely corresponds to 1 June at ~5 a.m. The note "1 Jun 5:36" is apparently the evening observation at 17:36, indicating a 12-hour shift in these records. In Figure 2b, the observation of 24 June was apparently made in the evening rather than in the morning hours. Once Hoffmann's time of observation is corrected, the velocity of the red sunspot transit from 26 May to 1 June 1703 equals 14.1 ± 2.1o d-1, and 14.5 ± 3.1o d-1 from 20 to 28 June 1703.
