Fig. 1. (a) Superposition of two original engravings by Cassini (1672) and Bion (1751). The heliographic grid corresponds to 21 October 1672 with bold letters marking the cardinal points. (b) Largest sunspot positions based on tabular measurements: observations from 18 – 26 October 1672 are shown in red, and those from 12 – 22 November 1672 in blue. The heliographic grid corresponds to 21 October, and the Equator position on 13 November marked in blue. Small circles highlight discrepancies in sunspot mapping. (c) Sunspot positions and sizes as deduced by us: observations from 18 – 26 October are shown in red and those from 12 – 14 November 1672 in blue. (d) Sunspot position derived from tabular measurements in Le Monnier (1741). (e) Sunspot longitudes derived from the engraving are represented in black and those from the tabular measurements are in color with the corresponding rotation rates. The gray shaded area illustrates the expected range of the longitudes in November (Zolotova and Vokhmyanin 2025).
Observations by Ole Christensen Roemer and Jean Picard
Ole Christensen Rømer or Roemer (1644–1710) was a Danish astronomer, and Jean Picard (1620–1682) was a French astronomer, priest, and cartographer.
18 October – 22 November 1672
The series of observations initiated by Roemer and continued in collaboration with Picard at the Paris observatory from 18 October to 22 November 1672 was published by Cassini (1672) and Bion (1751, reprinted from 1699). These publications include engravings, text, and conclusions, though they differ in some aspects.
Tabular measurements of the largest sunspot's position are available only in Cassini (1672). Figure 1(a) presents a superposition of two original engravings from Cassini (1672) and Bion (1751). The engraving discrepancy in latitude and longitude ranges from 1 − 2o. Detailed sunspot parameters reconstructed from both engravings are provided in Data.
Further, the text description is derived from Cassini (1672). The author referred to the umbra as the sunspot and described the penumbra as a "cloud". Observations were interrupted by bad weather on 26 October; however, Cassini, observing from Provence, recorded seeing the sunspot the following day at Noon as it touched the western limb. He noted that the sunspot appeared narrower toward the limb due to the projection effect. This apparent reduction led to the conclusion that the spot might reappear on the eastern limb.
By 9 November, the spot had not yet reappeared, and cloudy conditions prevailed the following day. Observations resumed on 12 November. Poor weather hindered observations until 18 November, when the sunspot was visible again as a small black dot. At least six-foot telescope was required to observe it. These measurements were employed to determine the rotation rate of the Sun, the tilt of the solar rotation axis, and the regularity of sunspot motion (see Cassini, October – December 1676). Cassini concluded that the object observed over two months was the same recurrent sunspot, continuously located 15o south of the Equator. He attributed the divergence in sunspot tracks in the October and November engravings to the annual variation in the solar axis tilt relative to the observer. Based on Cassini's assumption of a long-lived active region, we evaluate its average latitude to be −13.4 ± 0.2o in Bion (1751) and −14.1 ± 0.1o in Cassini (1672).
Figure 1(b) illustrates the positions of the largest sunspot based on tabular measurements: October observations are marked in red, and November observations in blue. Measurement on 22 October is absent. The heliographic grid corresponds to 21 October (in red), and the Equator position on 13 November 1672 is marked in blue. It is suggested that Roemer and Picard used the equatorial setup of a telescope for these observations. The distance from the western limb was given in time units, while that from the South was measured in angular units. The ratio of the solar diameter measured in these units decreases by 5% over the observation period, which was accounted for in transferring the tabular measurements onto the solar disk.
On 18 October 1672, the recorded distance of the sunspot from the South is given as 9' 5'' (small red circle in Figure 1b). We suspect this to be a misprint and have assigned a corrected value of 7' 5'' (regular red circle). The corrected data for this day is provided in Data.
On 18 November at 7 a.m. local time, the measured distance from the western limb was recorded as 1' 5'' (regular blue circle). This measurement significantly differs from the sunspot position shown in the engravings indicated by two small blue circles). It seems that either author or publisher of the original observations proposed that the measured distance was incorrect, leading to a corrected sunspot mapping in the engraving. In contrast, we intentionally hypothesize that the measured distance was accurate, but a time error occurred. We suggest the correct time might have been 17 November 15:51 UT. The corrected data based on this assumption is included in Data. Overall, the measurements yield an average sunspot latitude of −14.6 ± 1.5o.
Roemer and Picard also produced ten detailed drawings of the observed sunspots and provided measurements of the penumbra's linear size. These detailed drawings allow us to transfer the sunspot positions onto the solar disk with accuracy limited by the orientation of the active region (Figure 1c): October observations are in red and November observations in blue.
Figure 1(d) presents the sunspot position measured by Picard, as published by Le Monnier (1741), later reprinted by De La Lande (1778), and incorporated by Spoerer (1889). Spoerer evaluated the sunspot latitude as −13o, while our analysis, based on the solar altitude measurements near Noon, determined an average latitude of −13.7 ± 1.8o. However, two November observations present in Cassini (1672) are missing in Le Monnier (1741).
Figure 1(e) shows the sunspot longitudes of the largest sunspot derived from engraving (black) and measurements (red and blue) as reported by Cassini (1672). Based on the most reliable October observations, the engraving indicates a sidereal rotation rate of 14.2 ± 0.9o d-1 (13.3 ± 0.9o d-1 in synodic units), while the measurements suggest 14.1 ± 0.7o d-1 (13.1 ± 0.7o d-1 in synodic units). The gray shaded area indicates the potential range of longitudes for the active region in November, assuming a motion velocity of 14.1 − 14.2o d-1. The November data (black, blue, and yellow) shows greater scatter, but are shifted toward 14.1o d-1. Thus, matching longitudes confirms that the observers likely saw an active region spanning two solar rotations.
Casas, Vaquero, and Vazquez (2006) analyzed the engraving from the second edition of Bion's book and obtained significantly different results for observation dates, sunspot latitudes, and synodic rotation rate. Their findings highlight that relying solely on the engraving, which contains incomplete information, may lead to the conclusion that the solar equatorial rotation during the Maunder minimum was slower.
A rough translation of the report accompanying the measurements reads as follows: on 12 November 1672, Monsieur Picard and Monsieur Romer, while at the Royal Observatory, discovered a sunspot resembling an ant. On 13 November, the spot split into two parts, and on the 14th, a cloud (penumbra) appeared around it, on the edge of which appeared a third spot. On 18 November, they saw the spot as a small black dot. On 22 November, observing the spot required at least a six-foot telescope. It was surrounded by small intermittent clouds and faculae.
It seems that on 26 October, the sunspot retained its penumbra, whereas on 12 and 13 November, it appeared only as a pore (without penumbra) measuring 8 msh, as shown in both engravings and our reconstruction (Figure 1d). The penumbra reported on 14 November implies the emergence of a new sunspot, reinforcing the hypothesis that the observers were witnessing an activity nest, rather than a single, long-lived sunspot group.
