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Composite image showing photograph of Saturn’s rings from the Cassini Spacecraft 2017 and a diagram by Jean Dominique Cassini published in Philosophical Transactions in1676.
Composite image showing photograph of Saturn’s rings from the Cassini Spacecraft 2017 and a diagram by Jean Dominique Cassini published in Philosophical Transactions in1676. Photograph: Nasa/PA
Composite image showing photograph of Saturn’s rings from the Cassini Spacecraft 2017 and a diagram by Jean Dominique Cassini published in Philosophical Transactions in1676. Photograph: Nasa/PA

Cassini: the 17th-century astronomer who shrank France and inspired a spacecraft

This article is more than 6 years old

The Cassini spacecraft and its dramatic dive towards Saturn have been in the news this week, but the human Cassini is no less memorable

As a historian of science, when I scroll through my Twitter timeline and see mentions of Cassini, my thoughts tend to go not to the spacecraft that is, at the time of writing, somewhere between Saturn’s rings and the planet itself. Rather, they turn to Cassini I, II, III and IV, the 17th and 18th-century dynasty of Paris Observatory directors. With the word Saturn appearing alongside, I fix on Cassini I, Giovanni Domenico (or, after he moved to France, Jean-Dominique) Cassini.

Giovanni Domenico Cassini was the first director of the observatory founded by Louis XIV and, among much else, he discovered two of Saturn’s moons, the planet’s equatorial belt and a division in its rings. This last has been named the Cassini Division in his honour. Thus the Cassini Spacecraft has imaged the Cassini Division that was first depicted by Cassini I. It can just be seen in the image at the top, which was published in 1676 in the Philosophical Transactions of the Royal Society.

Like Christiaan Huygens and Johannes Kepler, who also had Nasa spacecraft named after them (the probe that landed on Saturn’s moon, Titan, and the space observatory respectively), Cassini is certainly worth remembering. His portrait perhaps suggests that he felt the same way.

Giovanni Dominique Cassini gestures to the Paris Observatory and long telescope of the kind used to observe Saturn’s rings. Photograph: Print Collector/Getty Images

Cassini was employed as an astronomer from his early 20s until his death in 1712, aged 87. He began as an assistant at Panzano Observatory before becoming professor of astronomy at the nearby University of Bologna. He reached sufficient renown to have been poached by Jean-Baptiste Colbert, Louis XIV’s influential Minister of Finance. He was to help plan and build the grand new observatory, a key part of the new Academy of Sciences and a strong signal of France’s power and ambition.

While the observatory and its equipment helped Cassini to find new objects in the heavens, and new levels of detail on the surface and surroundings of the planets, its main purpose related to the more mundane business of mapping and navigation. Like the Royal Observatory at Greenwich (founded shortly after but completed before its French equivalent), the initial prompt for the Paris Observatory’s foundation was to solve the problem of finding longitude at sea. For France, however, mapping land territory proved to be more significant.

Longitude had long been a tricky matter here too, but Cassini made good use of a method using observations of Jupiter’s moons, first suggested by Galileo, their discoverer. Now known as the Galilean Moons, he had called them the Medicean Moons - making a gift of both spectacular novelty and potential utility to his patron Cosimo de Medici. Galileo’s hope that timing the eclipses of the moons would help find longitude at sea was never to be realised, despite at least two centuries of trying.

Cassini showed, however, how well the method could be used when telescopes could be mounted on solid ground. He and his team helped redraw the east-west boundaries of France, in the process revealing France to be rather less large than it had been thought. Louis XIV is said to have quipped that his astronomers had lost him more territory than his generals had won.

Cassini began an ambitious plan to map the whole of France using triangulations. This project was based on the meridian, or north-south line, on which the chief instrument of the Paris Observatory was aligned. The work was continued by Cassini’s son and grandson, but the Carte de France (or, of course, Cassini) was completed in 1744. It shows the meridian running down the centre of France, the observatory as the beating heart of the whole. This line can now be seen laid out on the floor of the observatory in the grand Meridian (or, of course, Cassini) Room.

None of the human Cassinis are much remembered as we report on the exploits of the Cassini Spacecraft. This is despite the fact that the spacecraft is itself anthropomorphised. We lament the tragedy of its doomed flight into the planet, although it has not reached the emotive heartstring pulling evoked by plucky little Philae’s Twitter feed.

Rebekah Higgitt will be on @beckyfh continuing to get confused by historical names. On 9 June she will appear on Inside Versailles discussing Louis XIV as a patron of science with Greg Jenner and Kate Williams.

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