The idea that the French state, like other colonial powers, enlisted scientists in support of its colonial enterprises is basically true. This happened both in the First and Second colonial Empire and I'll address this in conclusion. However, in this particular case, the names of Huygens and Cassini seem to be there mostly for clickbait. The Cassini–Huygens space mission is a popular one, so claiming that these two men were in fact supporters of slavery is meant to demonstrate that modern science, even an ethereal one like astronomy, is tainted by the legacy of colonialism and slavery. But what exactly happened?

Part 1: The Académie des Sciences and the problem of the longitude

In 1666, Jean-Baptiste Colbert, the influential minister of Louis XIV, created the Académie des Sciences, as a part of a general project to put artistic and intellectual creations at the service of the King. Like the other Royal academies (Letters, Dance, Painting), it was financed by the State. The cultural Academies employed the best artists to increase the prestige of France and of its King: today we would call this "cultural influence" and "soft power".

The objective of the new Académie des Sciences was more materialistic: it aimed at increasing France's economic and military power. To achieve that, Colbert enlisted the best scientists of the time, even foreign ones. He tried to hire Newton and Leibnitz without success, but he was able to convince Dutch scientist Christiaan Huygens to come to Paris. Huygens was only 37 at the time and already one of the most respected savants in Europe (Sturdy, 1995). Italian astronomer Giovanni Domenico Cassini (aka Cassini I) accepted Colbert's invitation in 1669. He became French and was the first of the Cassini dynasty of scientists.

The Académie des Sciences had a primarily utilitarian goal, and its researches on mathematics, chemistry, physics, mechanics, and astronomy had practical applications, at least at medium and long term. The Academicians spent time examining concepts and machines proposed by independent inventors. The Académie was also involved in less applied science: considerable work was done for instance on the description of the anatomy of local or exotic plants and animals.

One major objective of the Académie was to solve the long-standing question of determination of the longitude at sea. Measuring latitude is straightforward, but measuring longitude is not, due to the rotation of the Earth. When seafarers were in unknown waters, they calculated their ship's position by dead reckoning, ie by assessing the current position from a previous one using estimates of speed, direction, and time. This imprecise method was often good enough, but resulted in navigational errors that led ships astray or to their loss. Like squaring the circle or perpetual motion, the longitude problem was one of those eternal intellectual challenges and was sometimes considered as a fool's errand, except that solving it had practical implications (Dunn, 2014).

By the 1600s, Spain, England, France and the Netherlands had established maritime colonial empires that required vessels for carrying goods (including slaves) and bullion to and from the colonies, and warships to protect them. As traffic grew, it became even more crucial to make transoceanic navigation as fast, efficient, safe, and profitable as possible. This question was of utmost importance for all naval powers, but it was felt more strongly in France, whose navy was smaller and outnumbered (Ferreiro, 2011). Spain (in the 16th century) and the Netherlands (since 1600) had established incentive schemes with life-changing rewards for those who found the solution to the longitude problem, but little had come out of it.

In France, the new Académie des Sciences made it one of its ongoing topic of investigation. In 1669, its Academicians reviewed (and rejected) three poorly-thought solutions about the longitude question, including one by a country priest who had dedicated 35 years of his life to invent a system based mostly on measuring wind temperature. This was "a strange edifice of assumptions and ideas" wrote Fontenelle in his Histoire de l'Académie Royale des Sciences: the problem, like others of its kind, tended to attract cranks. Fortunately, the Académie had some of the smartest brains in Europe working on the issue. Huygens and Cassini, among others, became directly involved in solving the longitude problem.

Huygens had invented the pendulum clock in 1656, an invention that generated considerable interest since the accurate measurement of time at sea was one of the potential solutions for establishing longitude. Two experiments were conducted in the Mediterranean in 1669 during the Cretan War. Huygens considered the second one to be promising: he noted with satisfaction that the cannonades and the explosion of a warship near the one where the clock was installed had not affected the instrument! (Huygens, Oeuvres, VI, 501).

Another theoretical solution to the longitude problem was the use of eclipses. Lunar and solar ones were easy to observe but too infrequent. The eclipses of the satellites of Jupiter (discovered by Galileo Galilei in 1610), on the other hand, were frequent but they required a telescope. In Italy, Cassini established reliable ephemerides of Jupiter's satellites that were usable on land to measure longitude, and he was invited by Colbert to become a member of the Académie and to run the Observatory.

-> Part 2: The Guiana expedition of 1672-1673