The moons of Jupiter
“We have moreover an excellent and splendid argument for taking away the scruples of those who, while tolerating with equanimity the revolution of planets around the Sun in the Copernican system, are so disturbed by the attendance of one Moon around the Earth while the two together complete the annual orb around the Sun that they conclude that this constitution of the universe is impossible. For ... our vision offers us four stars wandering around Jupiter like the Moon around the Earth.”
On the title page of the Sidereus nuncius, attention is directed 'especially' to 'four planets flying around the star of Jupiter at unequal intervals and periods with wonderful swiftness', which had remained 'unknown by anyone until this day'. These 'Medicean stars' also provide the central conceit of the work's famous dedication. Within the body of the work, nearly as much space is devoted to illustrating a series of 64 observations of Jupiter's moons, undertaken successively between 7 January and 2 March 1610, as to all the other telescopic discoveries put together.
The significance of this series of images can be more clearly perceived when these schematic representations of the relative position of the planet and its moons are stacked one on top of the other, without the interveneing text (in the manner of Image 2, which is derived from the translation of E.S. Carlos, published in 1880). In the central column in particular, the orbit of Jupiter's outermost moon is clearly evident, swinging from east (25. Jan) to west (3 Feb) and back again (11 Feb). Closer inspection likewise reveals the orbit of inner moons, for instance moving from east (17-18 Feb) to west (26 Feb.) and back again.
These sequences are somewhat distorted by several factors: by the imprecision especially of Galileo's earliest measurements; by the fact that one or more of the moons is sometimes hidden not only by the body of Jupiter but also by falling within its shadow; and not least by the fact that Galileo sometimes included more than one observation for a given night and sometimes made no observations at all due to cloudy weather. Image 3 is a montage which compensates for the latter source of distortion by selecting from the original imprint only one image for each night and adding blank spaces for the missing nights.
Credits
Images 1 and 3: Galileo Galilei, Sidereus Nuncius (1610), fols. E3v-E4r and composed from fols. F1r-F4v; all from archive-org. Image 2: composed from Galileo, The Sidereal Messenger, trans. Edward Stafford Carlos (Oxford and Cambridge, 1880), figures linked from p. 72; available on Project Gutenberg.
Translation: Albert Van Helden (Chicago, 1989).
Text: Howard Hotson (December 2018)