Giovanni Cassini and the Saturn Moon Rhea

Giovanni Domenico Cassini
(1625 – 1712)

On December 23, 1672, Italian astronomer Giovanni Domenico Cassini discovered Rhea, the 2nd largest of the 62 Saturn moons that are known by today.

Giovanni Domenico Cassini studied in Genoa as well as Bologna and was occupied with a professorship at the University of Bologna for studies in astronomy and mathematics. There he taught euclidean geometry and due to the church’s restrictions, ptolemy astronomy. For a long time he favored the geo-heliocentric system by Tycho Brahe before switching to Copernicus’ theory later on and also, just like Tycho, Cassini began observing the sky more accurate than many scientists before him. He published numerous charts on his observations and due to the fact that he used different kinds of telescopes, he was able to make some critical findings. He calculated Jupiter’s and Venus’ rotation and observed these planet’s surfaces in detail.

Cassini’s reputation grew fast, wherefore he was soon director of an observatory aiming to research on Earth’s form as well as to measure the solar system. It was during these years, when Cassini discovered Saturn’s Moons Iapetus in 1671 and one year later Rhea, named by Cassini to honor King Louis XIV. It was also him to discover the gap in Saturn’s Rings, now called the ‘Cassini Division‘. Another important part of Cassini’s research was the calculation of the Earth-Sun distance, today known as an Astronomical Unit. Through this unit, it was then possible to indicate all distances in the solar system standardized but this new kind measurement was not initially accepted by all astronomers, for instance Edmund Halley was ne of its opponents.

In general, Cassini was known to represent traditional theories instead of new discoveries of his young colleagues. When Jean Richer and Philippe de La Hire published their findings on the oblateness of the Earth at its poles, he rejected it as well as Ole Rømer’s theories about the finiteness of the speed of light. But despite his attitudes to certain theories, Giovanni Domenico Cassini counts as one of the most influential astronomers in the 17th century. He passed away on September 14, 1712 in Paris.

At yovisto you may enjoy the video lecture by Caroline Crawford on ‘Saturn, its Rings and Moons‘ at Gresham College.

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