Fig. 1. Original sunspot drawings on 17 and 18 March 1612 with orientation of the solar grids corresponding to the time of observation indicated by the observer: Cigoli (1612a), Scheiner (1612), and Galilei (1612). These two drawings by Galilei were rotated by 40o and 35o correspondingly. Blue ovals define sunspot groups (Zolotova, Arlt, Vokhmyanin 2021).
Observations by Galileo Galilei
Galileo Galilei (1564–1642) was an Italian astronomer, physicist, mathematician, philosopher, who had a significant impact on the science of his time.
12 February – 3 May 1612
23 drawings with pen and ink were made by Galilei (1612). Circles vary in diameter between 3.6 and 5.9 cm (Bredekamp, 2019). The first and the last solar disks are almost perfect circles, while the interjacent disks are freehand circles. This affects the accuracy of sunspot parameters reconstruction. We suppose that Galilei observed at 15 km from Florence at the Villa Le Selve. Drawings are supplemented by a date and short notices on sunspots. In most cases, Galilei wrote that he observed at sunset (occasus) or sunrise (exortus). On 16 April, he observed "mane H. 1. ab ortu", on 19 April and 3 May, "mane ab ortu H. 3.". This means the observations were made in the morning in the first and third hours from sunrise. On 28 april, Galilei observed "H. 21" that means 21 hours after sunset of the previous day.
Bredekamp (2019) noted that after 23 February until 16 March 1612 Galilei interrupted his observations due to bad weather. In the manuscript, Galilei wrote "D. 1. Martij nec in ortu nec i occi aderat macula... D. 2. Martij ... nulla apparuit macula... Die 15. Martij nulla apparuit macula...". We translated as On 1 March, there were no sunspots at sunrise or sunset... On 2 March, there were no sunspots. On 15 March, there were no sunspots. Again, these days are filled with NaN in the Electronic Supplementary Materials (Method I and Method II.
Galilei's drawing of 16 April is flipped upside down. It is seen by comparison with the next day drawing and with Harriot's images containing the same sunspot group. Another uncertainty of the image orientation occurred on 17 and 18 March 1612. Comparing with observations by Scheiner (1612) and Cigoli (1612a), we rotated Galilei's drawing of 17 March by 40o and the drawing of 18 March by 35o clockwise (Figure 1). We do not know the origin of such orientations.
3 – 11 May 1612
These drawings from Galilei's letters to Cardinal Maffeo Barberini (Pope Urban VIII) and to his nephew Cardinal Francesco Barberini are under revision. For details Vokhmyanin and Zolotova (2018).
2 June – 8 July 1612
These drawings from the book Istoria e dimostrazioni intorno alle macchie solari e loro accidenti comprese in 3 lettere are under revision. For details Vokhmyanin and Zolotova (2018).
19 – 21 August 1612
These drawings by Galilei (1613) were published in Istoria e dimostrazioni intorno alle macchie solari. We compared these engravings with those by Cigoli (1612b) and found that Galilei's images in the book were rotated 90o clockwise.
This may have been done for the following reason. Sunspot drawings on 2 June – 8 July 1612 were made by Galilei after 11:30 UT, when the solar rotation axis inclined to the right from the vertical and hence sunspots move from the top left corner to the bottom right corner. However, on 3 August drawings sunspots have to move from the bottom left to the top right corner, because these drawings were made in the morning, when the solar rotation axis is inclined to the left from the vertical. Nevertheless, in Istoria August sunspots progress across the solar disk in the same way as on drawings of June and July. We hypothesize that Marcus Welser, the publisher, or the engraver, or someone else involved in publishing Galilei's letters to Welser, mistakenly rotated August drawings by 90o.
The August drawings are accompanied by the time notes "Hor. 14". Unaware that images were rotated, neither Casas, Vaquero, and Vazquez (2006) nor Vokhmyanin and Zolotova (2018) could find a relation between these notes and a suitable orientation of the solar grid. Hence, sunspot positions defined by Vokhmyanin and Zolotova (2018) for August 1612 are incorrect due to the incorrect orientation of the printed engravings. Here, to define sunspot positions we derotated drawings, and now "Hor. 14" (hours after sunset) clearly corresponds to the time at about 8:00 UT in the morning. This is a nice example of how every little piece of information matters in such researches.
Particular attention should be paid to the quality of the etchings in Galilei (1613) made by Mattheus Greuter. Thanks to the exceptional way of engraving, the fine structure of the sunspot umbra and penumbra was thoroughly reproduced (Noyes, 2016, for details).
