Cassini's craft

Contributions of a seventeenth-century astronomer.

Spectacular images of Jupiter's 'red spot' — assembled from data sent back by the latest space probe to skirt the planet — made the pages of many newspapers in the first days of 2001. Such probes have regularly been named after famous explorers of the heavens and Earth. Galileo is still orbiting Jupiter after five years, and Magellan let us look afresh at Venus in the early 1990s. By contrast, the name assigned to the new joint NASA–European probe, which is due to rendezvous with Saturn in 2004, rings a less obvious bell in the popular mind. How many people, outside the world of astronomy, could say what Cassini has done to merit such an accolade?

In fact, the choice of name is unusually apposite. Gian Domenico Cassini was renowned for his exceptional mastery of the astronomer's subtle art of educated seeing. He exploited the new generation of giant refracting telescopes to enlarge our knowledge of both Jupiter and Saturn. Cassini provided tables of the motions of Jupiter's satellites, formulated rotation periods for Jupiter and Saturn, established the oblate shape of Jupiter, and observed Saturn's surface belts and four of its satellites. In 1675, six years after his move to the splendid new observatory in Paris, he discovered the “Cassini Division” in the ring of Saturn. He also played a valuable role in the confirmation that the planet's 'ears' corresponded to a ring.

Astronomy at the highest level was then, as now, a matter of international prestige and big money. Scale was crucial if precise observations were to be made — one of the lenses sent to Cassini's Parisian observatory boasted a focal length of 136 feet. One factor, however, that distinguished the patronage of astronomy in Cassini's time from that of our era was the role of the Church. Notwithstanding the unhappy episode of Galileo's forced retraction of his Copernicanism, the Church generally found astronomers' revelations of the divine geometry of the heavens much to its liking.

The Church did more than provide an intellectual and theological base for astronomers during the sixteenth and seventeenth centuries. For example, the great cathedrals and basilicas, which enclosed by far the largest interior spaces, could be set up as massive scientific instruments, most notably as locations for extended meridian lines. These numerous church meridians attracted considerable public interest as well as serving their functions of time-keeping, astronomical recording and calendar reform.

The Bologna Meridian from G. D. Cassini's Meridiana (1695).

Notable among the church meridians is that found in the city of Bologna, where the devout Cassini first came to prominence. The city acquired its first meridian in 1576, when Egnatio Danti opened a small aperture in a chapel wall of the massive late Gothic basilica of San Petronio, and inlaid a line across the floor in order to track, as closely as was practicable, the direction of the midday sun as it slanted across the tiled pavement. In 1655, Danti's meridian was replaced by Cassini's meticulously engineered device, its inset metal strip aligned due north and passing diagonally with breathtaking precision between the massive piers of San Petronio. Bernard de Fontenelle hailed the feature as “a new oracle of Apollo where the sun can consult with confidence about all the difficulties of astronomy”.

For his part, Cassini promised the “illustrious nobles” in charge that “the kingdom of astronomy is now yours”. Restored on a number of occasions, not least by the astronomer himself in 1695, its enduring beauty provides testimony to his importance, which is now appropriately recognized through the 'new' Cassini.