Fig. 1. Sunspot drawing from the letter of Cigoli (1611) to Galilei on 13 September 1611.
Observations by Lodovico Cardi known as Cigoli
Lodovico Cardi (1559–1613), also known as Cigoli, was an Italian painter and architect. We examine his sunspot drawings stored in the archives of the Biblioteca Nazionale Centrale of Florence (BNCF).
September 1611
Bredekamp (2019) mentioned two schematic sketches of the solar disk with sunspots from a letter of Cigoli to Galileo Galilei (Cigoli 1611) dated 16 September 1611, which we do not process as the sketches are too schematic to extract sunspot areas and positions. Cigoli reported about observations by Domenico Passignano on 13 September 1611 (Figure 1). The upper sketch shows the diurnal A – B – C path of the sunspot group consisting of four parallel spots. The lower sketch shows this sunspot group and one large individual sunspot.
18 Febr – 23 Mar 1612
The first set of 26 images drawn with pen and ink by Cigoli (1612a) are attached to the letter addressed to Galilei dated 23 March 1612. The average diameter of the disks is 3.7 cm. Cigoli observed through a thick green glass at the Basilica of Santa Maria Maggiore in Rome (Vaquero and Vázquez 2009).
The solar disks are accompanied by a date, a day of the week, an observing time (hours after sunset of the previous day), and number of spots. Two drawings on 23 February were made at 17 and 21 hours. Moreover, the first drawing contains two sunspots from the previous observation on 22 February at 18 hours. Note that sunspots of the previous day were imposed without additional turn of the solar disk due to its diurnal rotation in the sky. Similarly, on 24 February, two drawings were made at 18 (near 11:00 UT) and 12 hours (near sunset). The sunspots observed at 18 hours were included in the drawing made at 12 hours, again without taking into account the diurnal rotation of the disk. The size, shape, and position of the sunspots were illustrated roughly, and the drawings are schematic. Sunspot areas may differ by more than a factor of two, even on the same day drawings. This affects the reliability of the reconstructed sunspot parameters.
Two solar disks on 2 and 4 March 1612 are blank and accompanied by the short note "non vidi niente" (I did not see anything). In Data, these days are filled with NaN (Not-a-Number) instead of zeroes, as the schematic observations of the time may have ignored small sunspots.
Vaquero and Vázquez (2009) translated the description that Cigoli gave of his telescope in a letter to Galileo: "I do not believe I have told you that I have a telescope, and it is sufficiently good to enable me to see the clock of St. Peter's and its hand from St. Maria Maggiore, but not the numbers of the hours as distinctly as I saw it with yours." Analyzing the Moon on the paint by Cigoli on the dome of the Pauline Chapel, Panofsky (2013) suggested that Cigoli exploited a Galilean telescope. According to the sunspot positions defined here, sunspots move from left (east limb) to right (west limb) with a large longitude scatter up to 20o and with an average value of 7o. This also suggests to us that Cigoli used a Galilean telescope.
29 Apr – 6 May 1612
These five drawings by Cigoli (1612b) were sent in a letter to Galilei on 30 June 1612 from Rome. Diameter of the solar disks is about 5 cm. The drawings are accompanied by a date and a time (in hours after sunset). The spots were drawn in detail, but their areas are significantly enlarged and positions seem to be inaccurate. Sunspot parameters from these drawings should be used with extreme care, and especially on 29 and 30 April 1612 sunspot positions suffer from great uncertainties.
18 – 25 Aug 1612
On 31 August, Cigoli (1612c) sent seven sunspot observations to Galilei. These drawings of stipulated size (disk diameter occupies a full page width) were made to compare observations of the two researchers (Bredekamp 2019). Apparently, Cigoli exploited an improved telescope, or a new one. Drawings are accompanied by a day of the week, a date, and an observing time (again, in hours after sunset). For several sunspot groups, umbra and penumbra are distinguished by darker and lighter ink. In the original image, these drawings are shown reversed. Therefore, we flipped images from top to bottom.
This orientation of the images suggests that Cigoli observed by means of projection behind a Galilean telescope. Bredekamp (2019) cited the following text from the letter by Cigoli to Galilei on 3 May 1613, where Cigoli retroactively writes: "me le aveva fatte osservare di quella stressa sua grandezza, et dettomi il modo da farto" (he had pointed them [drawings] out to me of that great size, and told me how to do it).
Figure 2 compares daily numbers of sunspot groups from the database by Hoyt and Schatten (1998) with the results of this work. Further, the numbers of groups with the prefix G and the numbers of spots with the prefix S are indicated according to the numbering used in the supplementary information. We would like to mention several discrepancies. On 24 February 1612, Hoyt and Schatten assigned seven sunspot groups. On that day, Cigoli made two drawings. In the first one, we assigned seven sunspot groups, while on the second one, eight sunspot groups. Moreover, group G7 appears only on the first drawing, while groups G9 and G10 appear only on the second one. We think that Cigoli reported 9 sunspot groups on 24 February 1612. Additionally, we compared the observations by Cigoli with Harriot’s. Note that groups G7 and G9 are absent from Harriot's drawing, instead group G48 was missed by Cigoli. Eventually, we believe that on 24 February 1612 there were at least 10 sunspot groups on the Sun.
Another discrepancy (Figure 2) appears on 22 March 1612, when Hoyt and Schatten detected four groups, though we identified only one large sunspot and two paper defects, one of them might be facula. On this drawing, Cigoli himself also wrote that there was one object. On 23 March, Hoyt and Schatten detected two groups, while we identified one large group. We think that the tiny spot at the lower edge of the solar disk is a paper defect, because it differs from the other spots by the ink color (see Sorting sunspots into groups, Fig. 4).
Figure 3(Top) compares sunspot group latitudes restored by two methods. Due to early drawings are small and schematic, there is significant discrepancy in results (Fig. 3 Bottom).
Fig. 3. (Top) Time-latitude diagrams reconstructed by Methods I and II (Vokhmyanin, Arlt, Zolotova 2021); (Bottom) Latitude discrepancy of two methods.