Mars: earliest detailed sketches

Originally posted at, the now-discontinued blog of the OU History of Science Collections. Neither this post nor any of its content should be taken as an official communication of the University of Oklahoma.

MarsThe Collections have recently acquired a rare 1666 first edition of three separately issued broadsides (foglie volante) in which Giovanni Domenico Cassini (1625-1712, also known as Jean-Dominique Cassini) reported his observations of Mars, including the first detailed sketches of its surface, preceded only by highly schematic renderings by Fontana in 1636/1638 and Huygens in 1659.

Cassini, De aliis (1666)

Cassini was professor of astronomy at the University of Bologna until becoming the founding director of the Paris Observatory in 1671. In addition to his discoveries about Mars, Cassini is well-known for his observations of Saturn, including the discovery of four of Saturn’s moons and of the largest division between its rings, which now bears his name. In 1672 he devised a way to measure the distance between Mars and Earth, thereby allowing a better estimate of the solar system’s dimensions.

These broadsides are of great interest not only for their content but more generally for what they represent about the practice of astronomy in Italy in the generation after Galileo: all three are brief reports separately issued in quick succession in order to stake out what would nowadays be called an intellectual property claim. 

Among other details, they expressly identify the telescopes used and their maker; most notably, the 17-foot long telescope crafted by Giuseppe Campani. Also scrupulously credited are the correspondents who provided information and even a list of ten witnesses who could in principle verify the observations, including Geminiano Montanari.

Altogether, these documents provide highly interesting and little known documentation for how scientific results were disseminated. It is no accident that the present titles appeared just a year after the founding of the first two scientific journals in Europe: the Journal des Savans and the Philosophical Transactions of the Royal Society of London, in January and March of 1665 respectively. The present pamphlets share a version of experimental methodology like that championed by these journals: precise recording of observational data, especially by means of illustration and tables; description of the means and conditions by and under which the data was obtained; crediting other researches with corroborative data when appropriate; and naming a list of witnesses who could attest to the author’s observations if challenged.

Cassini, De aliis Romanis, 1666

In the first document, Martis circa axem proprium revolubilis observationes Bononiae (below), Cassini provided a page-long description of his observations of the dark areas of Mars, taken over a series of days in February, March, and April of 1666. The planet’s different phases, illustrated on the facing page (above), allowed Cassini to deduce that Mars rotated on its own axis, as well as to detect the planet’s large, Earth-like inclination to the ecliptic.

Martis, 1 Martis, 2 Martis, 3

In the second document, De aliis Romanis observationibus macularum Martis (below), Cassini discussed other interpretations of Mars’ markings. Referring back to both the text and diagrams of the previous document, he responded to the contentions of Francesco and Salvatore Serra that Mars’ rotational period was approximately 13 hours. By Cassini’s calculation, on the other hand, it was 24 hours 40 minutes — just 3 minutes off the modern value for Mars’ sidereal rotation.

De aliis, 1 De aliis, 2 De aliis, 3

Mars’ rotational period is the focus of the third document, De Periodo quotidianæ revolutionis Martis (below), in which Cassini offered a more detailed explanation of his data.

De periodo, 1 De periodo, 2

All three pamphlets were published by the same Bologna publisher in the same year. While issued separately, they form a consecutive series. The present set conforms to the Instituto Centrale per il Catalogo Unico delle Biblioteche Italiane collation of 6 leaves, of which the verso of the 4th leaf is blank and the 6th leaf is an integral blank.

CASSINI, Giovanni/Gian Domenico. Martis Circa Axem Proprium Revolubilis Observationes Bononiae. Bologna, HH de Ducius, 1666. Large 4to. [26.5 x 19.5 cm], (2) ff., including a full-page engraving.
__________. De aliis Romanis observationibus macularum Martis. Bologna, HH de Ducius, 1666. Small folio [30.5 x 21 cm], (2) ff., including 1 full-page engraving.
__________. De Periodo quotidianæ revolutionis Martis. Bologna, HH de Ducius, 1666. Small folio [30.5 x 21 cm], (2) ff., the second blank and integral. Disbound. Some staining but plates clean.
View catalog record.

Of the greatest rarity, we have located only 10 other copies: 2 in Florence, 4 in Paris, and 1 each in Bologna, Munich, Zurich and London (none in America).

View the entire work in the Online Galleries.

With our thanks to Seth Fagan and Nick Dew (McGill University) for their assistance with this description.

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