Inside the book

Cysat sets out his treatise in seven chapters that move methodically from raw observation to physical cause, a structure he announces in the preface. The first chapter assembles the observations themselves, running from 1 December 1618 to 22 January 1619, each position of the comet fixed by its distance from two fixed stars; a table on p. 20 gathers the results in parallel columns of latitude, longitude, declination, right ascension, digression from the sun, and diurnal motion. From this foundation, the later chapters build outward. Caput IV traces the coma and tail across the sky in a sequence of full-page woodcuts, carrying the comet through Boötes, Virgo, and Draco, while a large plate sets its path against the orb of the sun. Caput VI explains and depicts the head resolving over the course of December from a solid disc into a swarm of starry points; Cysat likens three bright starlets within it to the luminous companions of Jupiter, "aeque clarae atque lucidissimi Iovis Comites" ("as bright as Jupiter's brightest satellites"). The same chapter contains his much-discussed remark on a nebulous appearance, on the strength of which he is sometimes, probably wrongly, credited with the discovery of the Orion Nebula. Cysat's work formed part of a wider contemporary discussion of the comet: Kepler, who corresponded with Cysat and visited Ingolstadt, and Orazio Grassi both published treatments of the same apparition in 1619. The provenance is particularly apt: this copy comes from the library of Owen Gingerich (1930-2023), the Harvard historian of astronomy and author of the celebrated census of surviving copies of Copernicus's De revolutionibus.