Giovanni/Gian (Jean-Dominique) Domenico Cassini

Born on June 8, 1625- Cassini was an Italian-French Astronomer of enormous importance; one who established himself as an astrologer, an engineer, and an astronomer. Cassini was born in Perinaldo, which is known today as the Republic of Genoa. Early in life, Cassini was already showing interest in the cosmos; however it was not as an astronomer. Cassini started his career as an astrologer, (one who interprets information about celestial bodies in the hopes that the gleaned information would be practical in terrestrial affairs) which some theorize gave him a start in his eventual shift to astronomy. It should be noted that as Cassini aged, he moved so dramatically away from astrology that he denounced it completely, preferring to work solely on astronomy. Astrology however, is what landed Cassini his first job as an astronomer. In 1644, Cassini was invited to Bologna by its senator- Marquis Cornelio Malvasia to work in the Panzano Observatory which he was having constructed. He spent 21 years there- (1648-1669) until the King Louis XIV of France sent Cassini a grant to come head the then under construction Observatory of Paris, which would open in 1671. While in France, Cassini became a French citizen (and interestingly embraced France so warmly that he became known as Jean-Dominique Cassini) and worked with the opulent Louis XIV as his court astrologer/astronomer. While he fulfilled all aspects of his obligations to the king, he spent the vast majority of his time on astronomy. Some major highlights of his career include the discovery of the Cassini Divide, (a gap between rings A and B of Saturn) the Discovery of the Great Red Spot on Jupiter. He was also responsible for the discovery of four of Saturn’s moons- which he named collectively: Sidera Lodoicea, or Louisian Stars. Today however, the moons are named individually; Iapetus (1671), Rhea (1672), Tethys (1684), and Dione (1684). Amazingly, Cassini is also responsible for giving us the first “accurate” measure of scale in our galaxy. By sending an assistant to French Guiana to make simultaneous observations of Mars—he was able to use parallax to determine the planet’s distance. In approximately 1690, Cassini noted differential rotation in Jupiter’s atmosphere. Giovanni Cassini had a huge impact on astronomy in his time, indeed his impact was profound enough to carry over to modern times.