Sunspot Positions and Areas from Observations by Galileo Galilei

Abstract

Sunspot records in the seventeenth century provide important information on the solar activity before the Maunder minimum, yielding reliable sunspot indices and the solar butterfly diagram. Galilei’s letters to Cardinal Francesco Barberini and Marcus Welser contain daily solar observations on 3 – 11 May, 2 June – 8 July, and 19 – 21 August 1612. These historical archives do not provide the time of observation, which results in uncertainty in the sunspot coordinates. To obtain them, we present a method that minimizes the discrepancy between the sunspot latitudes. We provide areas and heliographic coordinates of 82 sunspot groups. In contrast to Sheiner’s butterfly diagram, we found only one sunspot group near the Equator. This provides a higher reliability of Galilei’s drawings. Large sunspot groups are found to emerge at the same longitude in the northern hemisphere from 3 May to 21 August, which indicates an active longitude.

Appendix

Figures 15, 16, 17, 18, 19, and 20 show sunspots sorted into groups. Black rectangles with numbers denote individual sunspots or subgroups of sunspots. Green ovals mark sunspot groups. Note that “not a sunspot” shows paper defects. For convenience, blue and pink lines represent the $x$-and $y$-axes in the rectangular coordinate system, and the solar limb is shown as a red circle. Note that on 14 June, one tiny spot is missing in the image provided by the Galileo Project; this spot was added by us for completeness.

Figure 15

Schematic sketches of sunspot drawings of 3 – 10 May 1612. Black rectangles with numbers denote individual sunspots or subgroups of sunspots. Green ovals mark sunspot groups. The solar limb is shown as a red circle.

Figure 16

Schematic sketches of sunspot drawings of 11 May and solar images of 2, 3, 5 – 9 June 1612. Note that “not a sunspot” shows paper defects. Black rectangles with numbers denote individual sunspots or subgroups of sunspots. Green ovals mark sunspot groups. The solar limb is shown as a red circle.

Figure 17

Solar images of 10 – 17 June 1612. Black rectangles with numbers denote individual sunspots or subgroups of sunspots. Green ovals mark sunspot groups. The solar limb is shown as a red circle.

Figure 18

Solar images of 18 – 25 June 1612. Black rectangles with numbers denote individual sunspots or subgroups of sunspots. Green ovals mark sunspot groups. The solar limb is shown as a red circle.

Figure 19

Solar images of 26 – 29 June and 1 – 4 July 1612. Black rectangles with numbers denote individual sunspots or subgroups of sunspots. Green ovals mark sunspot groups. The solar limb is shown as a red circle.

Figure 20

Solar images of 5 – 8 July and 19 – 21 August 1612. Black rectangles with numbers denote individual sunspots or subgroups of sunspots. Green ovals mark sunspot groups. The solar limb is shown as a red circle.