Fig. 1. (a) Sunspot position derived from the tabular measurements in Le Monnier (1741). A few horizontally and vertically mirrored sunspot reproduced from the original engravings are highlighted in yellow frames. (b) Longitudes and rotation rate determined in this study (Zolotova and Vokhmyanin 2025).
Observations by Jean Picard
Jean Picard (1620–1682) was a French astronomer, priest, and cartographer.
26 June – 4 July 1676
A portion of the sunspot position measurements, presumably made by Picard near Noon using a quadrant with a radius of 32 inches from 26 June to 2 July 1676, was published by Le Monnier (1741). Originally the last measurement was listed as 1 July, but upon matching the sunspot longitudes, we suspect this is a typographical error. A detailed engraving of this sunspot group spans from 26 June to 4 July. Most of the drawings were made in the morning or evening, when the Sun’s altitude is low. A few reproductions of these drawings are shown in yellow frames in Figure 1(a).
Hypothesizing that Picard exploited the same equipment as Cassini (1671a, 1671b), we flipped the engraving both top to bottom and right to left, resulting in an anti-Joy orientation of the sunspot group. Additionally, the trailing spots may represent a different group, located approximately 5–8o closer to the Equator than the large leading spot. Since Picard did not measure the size of these sunspots, we cannot transfer the engraving directly to the solar disk. In the Electronic Supplementary Materials, we provide the count of umbras and pores based on the engraving.
The solar diameter must be known to determine sunspot coordinates from angular distance measurements. We interpolated the routine measurements by Picard from the period 1666 – 1670 and determined the angular size of the Sun’s disc to be 31' 37.5''. From this, we obtain a sunspot latitude of −12.4 ± 0.2o. Measurements published in Le Monnier (1741) later reprinted by De La Lande (1778), and Spoerer (1889), reported a latitude of −13o.
Figure 1(b) shows the derived longitudes and rotation rate calculated between consecutive observations. Due to the limited number of observations and the scatter of longitudes, the rotation rate is difficult to judge. The longitude increased by 3.8o over six days, which we suspect is an artifact. This apparent growth in longitude could be due to several factors: (i) variations in sunspot position caused by changes in sunspot shape (referred to as La tache in French, likely the largest umbra), and (ii) imperfections in the pendulum time measurements. An error of one second in time measurements results in an inaccuracy of 1–1.5o in the sunspot's position at the center of the solar disk, and about 3o for a spot located 60o from the center. De La Lande (1778) also questioned the accuracy of the sunspot position and solar altitude measurements at the Paris observatory, citing the lack of a micrometer and vernier and the irregularity of the sunspot shape. In contrast, Ribes and Nesme-Ribes (1993) argued that the Paris observatory was equipped with a micrometer.
9 – 14 Aug 1676
Analyzing the sunspot's rotation, Cassini (1676a) concluded that the sunspot (Macula) observed in August and the one from late June (Fig. 1) were distinct objects. He noted that the late-June sunspot was located farther from the Equator than the August sunspot. Hook (1677) also observed this active region and wrote that, on 8 August 1676, he counted "about six greater and smaller [spots] in one knot with their proper Nubecules".
Figure 2(a) illustrates the position of the sunspot (la tache) as retrieved from the measurements by Picard and/or Cassini at the Paris observatory using quadrants with radii of 3 foot and 32 inches, recorded from 9 to 14 August 1676 and later published by Le Monnier (1741). Additionally, we embed sunspot drawings from Cassini (1676a, 1676b) in green. The sizes of these spots are chosen arbitrarily, and the observation times were not noted. The latitudinal position of the sunspots (3' from the disk center, as stated by Cassini) align well with the measurements of Flamsteed and Halley (Fig. 2 therein).
To derive the heliographic coordinates of the sunspot, we assume the angular size of the solar disc to be 31' 46'' based on Picard's measurements. The resulting latitudes range from −4.6 to −9.6o, with an average of −7.6 ± 1.8o. Figure 2(b) shows the corresponding longitudes with a derived rotation rate of 14.2 ± 0.3o d-1, which aligns with Flamsteed's and Halley's measurements. This result is considered more reliable due to small scatter in longitudes (less than 1o).
30 October – 27 December 1676
Astronomers at the Paris Observatory, presumably Picard, believed they observed the same long-lived recurrent sunspot. On 19 November 1676, Picard wrote about the return of the sunspot, which had appeared on 30 October and 1 November. On 15 December, he noted that at Noon the sunspot returned for the second time; it almost reached the eastern edge of the Sun and could only be observed with a 20-foot telescope.
Figure 3(a) depicts the sunspot's position from 30 October to 27 December 1676, reconstructed from the tabular measurements in Le Monnier (1741). These measurements were typically taken around Noon and are accompanied by sunspot engravings. We include horizontally and vertically mirrored reproductions of these engravings. We suggest that the detailed drawings were made during the morning or evening hours (similar to Figure 1) and that the sunspot may have had small trailing spots that were not considered as sunspots (Cassini, 1671c, 1671d).
To restore sunspot coordinates, we interpolated the solar diameter based on Picard's measurements: 32' 25'' in October, 32' 37'' in November, and 32' 42'' in December 1676. The time of the solar disc’s passage through the Meridian also varied and was taken into account. Our estimate for the average latitude, −5 ± 0.7o, aligns with Spoerer (1889) table, who reported −5.2o for the latitude in October, −4.6o in November, and −4.9o in December 1676, based on the Parisian measurements reprinted by De La Lande (1778). The longitudes of the sunspot are shown in Figure 3(b) as yellow diamonds.
The exact observation time on the first day is unknown; we assumed it was Noon to derive heliographic coordinates. Picard wrote: "A spot appeared on the Sun, which due to bad weather could not be observed earlier". This implies the observation may have occurred a few hours later, which would reduce the calculated longitude of the sunspot.
In November, excluding the near-limb observations, we derive a rotation rate of 14±0.6o d-1. December observations are too sparse to determine the rotation rate. For comparison, in Figure 3(b) also includes the longitudes derived from Flamsteedius (1725), indicated by pink squares. Both the French and English reports yield that in November, the rotation rate was slower than the sidereal Carrington rotation of 14.18o d-1. Consequently, after a half-turn on the Sun's far side, the longitude of the sunspot would have decreased by 2.3o at 14o d-1 or by 3.6o at 13.9o d-1.
However, at each reappearance of the sunspot at the eastern limb, its longitude slightly increases (Figure 3). In conclusion, we we found no evidence to support the i dea that the active regions observed from October to December 1676 were the same long-lived recurrent sunspot group. It is important to note that the derived longitudes and rotation rates are compromised by numerous uncertainties. This sunspot group was also reported by Flamsteed (Fig. 3 therein) and Cassini (Fig. 4 therein).