A chronology of some lunar drawings by Galileo Galilei

2023 October 10

Giancarlo Favero

By taking into consideration the morphological features along the terminator in some of Galileo’s watercolour portraits of the Moon, found among his personal belongings, new datings are suggested for the observations reported in Galileo’s Sidereus Nuncius, earlier than those proposed by Ewen Whitaker in 1978. The new dates solve some inconsistencies found in Whitaker’s paper, and in the literature generally, concerning the first telescopic observations made by Galilei during the last months of 1609.

Introduction

Galileo Galilei, Professor of Mathematics at Padua University from 1592 to 1610, published a 41-page booklet titled Sidereus Nuncius (‘Starry Messenger’; Nuncius from now on) in the early days of March in 1610. The printer, Thomas Baglioni in Venice, reported Mar 8 as the date of issue.

Some tables in the end matter (Figure 1) depict five images of the Moon (two of them, nos. 3 and 4, are identical to each other, probably indicating hasty editing) which are not Galilei’s work, as may be immediate realised when comparing his watercolours (Figure 2, e.g., nos. 5 and 4) with the corresponding tables of the Nuncius (nos. 2 and 4 in Figure 1). The tables were engraved on copper for printing, likely by a worker of Baglioni, and this can explain the otherwise puzzling fact that in some of the tables a large crater appears (nos. 4 and 5 in Figure 1), which is not clearly delineated in Galilei’s watercolours (Figure 2, nos. 4 and 3).

The book can be considered the first example and the template of a modern scientific paper. It begins with a short summary of the content (the Abstract), followed by the description of the instrument used in the observations (the Materials and Methods section). The Main Results part follows, first reporting the observations of the Moon, of some fixed and wandering stars, and of the Milky Way, and afterwards accurately recording the appearance of Jupiter and its ‘Medicean Moons’ from Jan 7 to Mar 2, in 1610 (the latest observations had clearly been added to the text while it was being printed).

Galilei wrote that the instrument he used to observe the Moon and the other celestial bodies described in the booklet was a refractor (made of a positive objective lens and a negative eyepiece) yielding a ×30 magnification. As for the light-gathering power, Galilei wrote: ‘Stellula enim quintae aut sextae magnitudinis, per Perspicillum visa, tamquam magnitudinis primae repraesentatur’ (‘a little star of fifth or sixth magnitude, seen with the instrument, looks like a first-magnitude star’). According to the same Nuncius, the instrument had been built well after 1609 Aug 25, when Galilei showed to the Venetian government (whose authority Padua University was subject to) an early ×8 version: this means that the observations reported without a date had probably been performed between September and December of 1609.

The purpose of this paper is to propose new dating for some of Galilei’s lunar drafts.

The observations

After some attempts by different authors to identify the dating of Galilei’s drawings, in 1978 a well-documented paper appeared by Ewen A. Whitaker,¹ an English selenographer working in Kuiper’s group in the United States after 1960. Whitaker had been an astronomer at the Royal Greenwich Observatory after World War II, and he served as the Director of the BAA Lunar Section between 1956 and 1958.

After estimating the selenographic colongitude in each of Galilei’s drawings (both the tables in the end matter and the watercolours found among his personal papers; see Figures 1 & 2) Whitaker used the lunar ephemerides to deduce their dating, thus obtaining Table 1.

Discussion

Although impressive, Whitaker’s analysis is flawed by two significant weaknesses. The first is that Galilei’s watercolours were not meant to give a cartographic representation of the Moon, but rather aimed at sketching an idea of the appearance of its rough surface along the terminator. This suggests that evaluating the dating of the drawings should be achieved by taking into consideration, rather than an estimate of the colongitude, the (few) morphological features that can be clearly identified, such as those in the paintings nos. 3, 4, and 5 of Figure 2. A map of the Moon is reported here as Figure 4,² which identifies some lunar features with bold letters for the convenience of the reader.

In watercolour no. 3, Mare Serenitatis (Se in Figure 4) is shown partially flooded with light, with Montes Caucasus (d) as its north-western edge and Montes Haemus (mh) as its south-western edge. In watercolour no. 4, Mare Imbrium (Im) has Montes Alpes (a) as the north-eastern edge and Montes Apenninus (b) as the south-eastern edge. In no. 5, the western part of Mare Serenitatis (Se) is bordered to the north by Montes Caucasus (d) and to the south by Montes Haemus (mh). The massifs near the terminator probably looked exaggeratedly prominent to Galilei, because of the light diffusion by the hand-polished lenses of his instrument.

A simulation of these features generated with Virtual Moon Atlas software (Figure 5b) shows that the configuration of watercolour no. 3 (Figure 2) can only have occurred on Nov 3, 18h. On Nov 2 or 4, 18h, the terminator is significantly different to that recorded by Galilei (Figures 5a & c, respectively). Given the duration of 29.5 days of the lunar phase cycle, in early October and December of 1609 the terminator would be compatible only if the observations had been done about 12 hours earlier or later than 18h, and thus with the Moon in full daylight or under the horizon.

Figure 6 shows a terminator like that in watercolour no. 4 on Nov 18, 00h. Figure 7 shows a terminator like that in watercolour no. 5 on Nov 16, 23h. Times in Nuncius were expressed by Galilei in Italic hours, measured between two successive sunsets, the interval being divided into 24 equal parts. To do the simulations, they are expressed as UT+1h.

Another weak point in Whitaker’s analysis is the identification of the star reappearing from the occultation in watercolour no. 8* (Figure 3), which he deemed to be theta Librae. This is puzzling for two reasons. First, it would be surprising for Galilei to have accomplished a Moon observation at the time of such an occultation, which occurred in the early hours of 1610 Jan 19: according to the accurate diary reported in the Nuncius, Galilei was observing Jupiter and its moons both in the early hours of Jan 18 and between 19 and 22h on Jan 19, and no lunar observation is reported on those nights. Secondly, the star in watercolour no. 8* is depicted as a simple six-pointed asterisk, but this is inconsistent with the stellar maps of the Nuncius: since theta Librae is a magnitude 4.1 star, it is reasonable to expect that Galilei would draw it as a polygonal six-pointed star, just like he did elsewhere for stars with similar magnitudes (see, for instance, Taygete, magnitude 4.3, in his map of the Pleiades, Figure 8).

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