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Tom Siegfried



The first Cassini to explore Saturn was a person

Space probe preparing to crash into ringed planet was named for an astronomical pioneer


TWO CASSINIS  The Cassini spacecraft has become famous for its stunning views of Saturn, including this image of the unlit side of the rings taken in 2012. But what do we know about the man Cassini was named for?

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As the Cassini spacecraft plunges toward its death on Saturn, the world’s knowledge of the famous ringed planet continues to accumulate. Thanks to years of observations by the versatile probe, astronomers now know Saturn as intimately as macaroni knows cheese. But still hardly anyone outside the world of astronomy knows anything about Cassini — and I don’t mean the spacecraft, but the guy it was named for.

Gian Domenico Cassini was an Italian astronomer, born in Perinaldo in 1625, around the time that Galileo was battling the church over Copernicus’ revelation that the Earth orbits the sun. Cassini was attracted to poetry but was also good at math. He got his start in science via astrology, which back then was not considered quite as completely idiotic as it is today. In fact, astronomy itself was often supported by wealthy people in order to get better astrological forecasts. One such wealthy Italian, an amateur astronomer, was impressed with a pamphlet on astrology that Cassini had written; it earned him an invitation to work at the amateur’s observatory, near Bologna.

From the leading scientists at Bologna, Cassini learned the importance of using high-quality instruments to make the most precise measurements possible. His talents were soon recognized; by 1650 Cassini’s accomplishments and reputation earned him the chair in astronomy at the university in Bologna. He continued his research during the 1650s, taking a particular interest in comets.

Cassini was an old-school conservative kind of scientist, not even inclined to take Galileo’s side on the Earth-orbiting-the-sun issue. Cassini preferred Tycho Brahe’s position that the other planets orbited the sun, but the sun then orbited the Earth. (Later Cassini accepted the Copernican sun-centered solar system, but only half-heartedly.) Cassini also was no fan of Newton’s law of gravity.

Cassini’s work as an eminent Italian scientist was not limited to astronomy. Called on to referee a political dispute over the course of a river, he mastered hydraulics. Later he spent some time studying insects and experimenting on blood transfusions. None of that was as fun as astronomy, though, so he returned to the stars often. In addition to comets, he specialized in the sun and especially solar eclipses. Some of his best work resulted from the use of a meridian — a large sundial-like device for recording solar movements — that he designed and had installed on the top of a church steeple.

Cassini was also clever enough to know the value of cultivating useful friendships. In particular, he was on good terms with some skillful Roman lens makers who provided him with especially powerful telescopes. With such instruments Cassini was able to calculate the rotation rates of Jupiter and Mars accurately. (He gave Venus a shot, but it was harder.)

Even more impressively, Cassini accomplished a goal that had eluded Galileo by accurately describing the motions of the moons of Jupiter. It was Cassini’s work on Jupiter’s moons that allowed the Danish astronomer Olaus Rømer to measure the speed of light, establishing that light did not transit space instantaneously, as many (including Cassini) had believed.

Even before Rømer’s results, Cassini’s accomplishments made him famous enough to get offered a job in Paris. That job was actually a prime position with the new French Academy of Sciences, whose founders recruited savants from all over Europe to enhance the new academy’s prestige. So in 1669 Cassini moved to Paris. He didn’t fit in all that well; his French was sketchy and his authoritarian approach to things in general ticked some people off. But he eventually managed to develop a world-class astronomical observation program. He became a French citizen in 1673 and married the daughter of a French official wealthy enough to offer as part of the dowry a nice castle for a summer home. Cassini then forgot about his original plan to return to Italy and spent the rest of his life in France.

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Had his astronomical achievements ended with his departure from Italy, Cassini would have been an odd choice for the name of a spacecraft sent to study Saturn. But besides his administrative role at the French academy’s observatory, Cassini continued his own studies. And, having mastered Jupiter, he naturally moved on to Saturn. In 1671 he discovered Saturn’s moon Iapetus and found another, Rhea, the next year. Much later he detected the moons Tethys and Dione. He also thoroughly examined the disk around Saturn and discerned that it was not just one ring. He identified two prominent rings separated by a small gap — the thin black band between the two is now known as Cassini’s division. Cassini even guessed, more or less correctly, that the rings are made of small particles all orbiting Saturn in concert.

And so Cassini knew Saturn as intimately as anyone from his era, establishing sufficient Saturnian credentials to earn the honor of having a space probe named for him. Cassini the probe has now seen Saturn much more clearly than Cassini the man. But soon Cassini the probe will die and go blind; Cassini the man first went blind, two years before he died, in 1712.

Follow me on Twitter: @tom_siegfried

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