At some point in the early 1660’s Giovanni Domenico Cassini (1625—1712) told Pope Alexander VII that he would not to be ordained as a priest. Cassini was a gentle and holy man, but he did not feel called to the priesthood even when asked by the Pope. Cassini had worked for Pope Alexander on several projects over the years and when King Louis XIV of France asked Cassini to come work on a project in Paris, Cassini asked the Pope for permission to go. The Pope, presuming it would be a short absence, said yes. Oops.
In France Cassini met a lovely girl named Genevieve, married her and had children. He lived to be 87. His career as a scientist continued in France, and four generations of Cassini scientists followed.
Cassini did not originally believe in Copernicus, being rather a disciple of Tycho Brahe, but his observations were very accurate and his interests wide. He studied rivers, a perennial source of misery in Italy. He helped to build a gnomon in the Church of San Petronio. A gnomon is more or less a sundial, but constructed by creating a hole in a ceiling or wall that lets you do observations of the sun by following the light it casts across the floor. The floor will be marked out in such a way that the light gives information on days of the year and times of day. The big advantage of building such a thing in a church is that it is HUGE. You can make observations that would be impossible otherwise.
Cassini studied comets, Jupiter, fortifications, and Mars. He made observations that suggested that light had a speed but discounted this idea. Roemer used Cassini’s data years later to calculate the speed of light for the first time.
https://mathshistory.st-andrews.ac.uk/Biographies/Cassini/
The younger of two sons born to Giovanni, Jacques, also known as Cassini II (1677 -1756) did geodesic work with his father, now also known as Jacques, helping him to measure the size and shape of the earth. Jaques’s measurements were impeccable but his conclusions were not. He wrote on electricity and mirrors as well as astronomy and geodesy. He was an excellent observer. He studied the moons of Jupiter and Saturn and the proper motion of the stars. (Proper motion is the actual motion of the stars rather than parallax. This is incredibly hard to see because stars are very far away and it takes 100’s of years to confirm most proper motion.) Jacques also studied enough law to make contributions in that field as well.
https://mathshistory.st-andrews.ac.uk/Biographies/Cassini_Jacques/
Like his grandfather and father, Cesar Francois Cassini de Thury (1714-1784), worked on exacting measurements, trying to figure out the shape of the earth. Cesar, also known as Cassini III, worked on creating the most accurate map possible of France. This requires a lot of math and careful measurement. Cesar switched sides in the great debate of the times on whether the earth was elongated at the poles rather than flattened at the equator. Flattened is the correct answer. It’s worth noting that others involved in the measuring and discussion ended up seeing that the instruments they were using were simply incapable of measuring with enough precision to distinguish between the choices. It took new instruments to settle the question properly. Cesar was made the hereditary Director of the Paris Observatory, after his father and grandfather, but though he made observations his reputation does not rest on them.
https://mathshistory.st-andrews.ac.uk/Biographies/Cassini_de_Thury/
Dominique Cassini, aka Cassini IV (1748-1845) long lived like his great grandfather, studied at an oratory that was founded to train candidates for the priesthood. Like his great-grandfather, Dominique decided he was not called to be a priest and instead studied physics, astronomy and mathematics. The Paris Observatory was run by his father and the job of director was made hereditary in the Cassini family so Dominique always knew he would end up there.
He also knew that his father had spent his life trying to produce a wonderful map of France and it was Dominique’s ultimate job to complete this map after his father died.
Unfortunately, when he was in his forties the French Revolution broke out and eventually consumed his position. The Academy of Sciences was disbanded. His three assistants all turned upon him. One was an elderly monk who ended up marrying a serving-girl, one was a promising astronomer who was inflamed by the Revolution, got drunk one night, and tried to get Cassini, an aristocrat and Catholic, killed. The third assistant was just as useless. Cassini, whose wife had died, gave up and stopped doing any science. He moved out of Paris and the position of Director of the Observatory was given to the assistant who tried to get him killed.
https://mathshistory.st-andrews.ac.uk/Biographies/Cassini_Dominique/
Dominique Cassini had five children, one of whom, Alexandre, had a career as a lawyer. However, Alexandre had inherited the family talent for acute observation and spent his leisure studying sunflowers. His contributions were sufficient to get a genus named for him.
One of the most amazing sources of information about all kinds of scientists and mathematicians is the website run by Saint Andrews University, in I think Scotland. When I want a lot information about a Catholic or Protestant or atheist scientists, they have it. Except for Alexandre…
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