Cassini, Giovanni Domenico (1625-1712).

titled "Correction des moyens mouvemens des satellites de [Jupiter]", written in brown ink, in occasionally idiosyncratic French, unsigned, giving a comprehensive account of his own measurements of the orbit of the closest moon [Io] to Jupiter, from 1652 until 1680, and those made by Galileo in 1610, admitting to some errors he made in a paper in 1680, giving details of more recent measurements of all four Galilean moons, the manuscript including several autograph revisions, deletions and insertions, especially towards the end, relating to his investigations in 1688 and 1689, emending a number of dates and calculations, dated within the text ("...cette année 1689..."), 11 pages, 4to, in a single gathering, 1688-1689--together with a contemporary working manuscript of Cassini's essay, presumably by his assistant or pupil, written on similar paper, with Cassini's autograph additions and alterations, including four lines of text added by him at the end ("Cette annee 1688..."), 8 pages, 4to, 1688;  19 pages in all, 4to (22.5 x 17cms), all with watermark "P.Buisson" and a coat of arms, some creasing at lower margins, a few perforations through oxidization of the ink

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Giovanni Domenico Cassini is one of the great names in the history of astronomy, arguably the greatest seventeenth-century astronomer after Galileo. He observed the orbits of Jupiter's satellites over a period of forty years, detailed in the present manuscript, and first identified the Great Red Spot.  Cassini discovered four new moons of Saturn (Galileo only observed Titan) and the division in Saturn's rings, which is named after him.

This is a highly unusual pair of manuscripts, including Cassini's first main draft (1688) of his essay on the moons of Jupiter, a contemporary copy of the draft which the author revised and emended, with further additions and emendations added by by him to his original draft in 1689.  The essay contains Cassini's corrections to his "Tables" of the moons of Jupiter given in Nouvelles corrections aux tables astronomiques du mouvement des satellites de Jupiter [1680] (Houseau & Lancaster, vol II, section 80 (col.1389)), to which the author refers in the text ("...j'espere que la posterite ne trouvera guerre a corriger dans le moyen mouvement de ce satellite que je establi l'an 1680...").  The present essay is untraced and appears to be unpublished.

"...Ce qui m'oblige de supposer plustost que les distances entre ces satellites et Jupiter paruvent beaucoup plus petites a Galilee qu'elles ne l'estoient, comme il parut de sia de ce qu'il jugeoit que ce satellite [Io] touchoit presque Jupiter quand il estoit sorti de l'ombre a une distance d'environ un tiers de son diametre; d'ou l'on peut inferer que la lunete dont il se servoit, tout exellente quil la jugeoit, ne terminoit pas bien Jupiter, mais qu'elle y laissoit des rayons qui s'entendoient jusqu'au satellite eloigne et diminuoient sa distance ce que je confesse m'estre aussi arrive dans les premieres observations que je fis de ces satellites par de lunetes a occulaire concave qui m'augmentoient Jupiter et me firent determiner les degressions des satellites [deleted: 'mesurees par son diametre'] un peu trop petites..."