Around 4 centuries ago, Giovanni Domenico Cassini, an Italian astronomer and engineer, was born. 8 June 1625 would mark the beginning of a life full of discoveries, and pioneering work, notably the discovery of several of Saturn’s satellites as well as the composition and repartition of its rings: enough groundbreaking work to instill a notable legacy, of which the Cassini-Huygens space probe which was named after him is a perfect example. So let us uncover the mysteries of Cassini’s work and genius.

Giovanni Cassini

Cassini’s early life in Italy

Growing up in an Italian household near Imperia, Cassini studied at the Jesuit College at Genoa and then moved to San Fructuoso. Over the years, Cassini developed keen interests in mathematics and astronomy, which brought him towards his first position as an astronomer. In an observatory near Bologna, he worked under the supervision of a mentor: Cornelio Malvasia, and became increasingly involved in ongoing scientific developments.

Cassini continued his work and studies in Bologna until, in 1669, an astronomer known under the name of Colbert invited him to Paris to help set up the Paris Observatory, which would open two years later, in 1671. Cassini would eventually become more and more assimilated into French Culture, receiving French citizenship and changing his name from Giovanni Domenico to Jean-Dominique. He would remain in France and uphold his position as the director of the Paris Observatory, all while pursuing his research until his death.

The discovery of the Cassini division

The Inspiring Life and Works Cassini, The Man Who Unraveled The Secrets of Saturn. 2
Image: Astronomy Now

One of Cassini’s most notable breakthroughs was discovering the dark gap between the rings A and B of Saturn using a refracting telescope at the Paris Observatory. Now known as the Cassini Division, this gap is the 4800 km region that separates two of Saturn’s rings. Particularities of the Cassini division include strong orbital resonance. In effect, objects located in the division will orbit Saturn and its moon Mimas with an exact 1:2 ratio. Hence, the ring particles at this location orbit twice for every orbit of the moon Mimas. However, Cassini was not the only astronomer working on the particular division, and scientist Christiaan Huygens gave name to the inner edge of the Cassini Division, now known as the Huygens gap.

Uncovering Saturn’s Moons

Saturn's Moons

Before fully focusing on Saturn and its mysteries, Cassini was turned towards the Sun and Jupiter. Some of his earlier works involved, for instance, the experimental determination of Jupiter’s orbital period. Indeed, he used the following of Jupiter’s spots to map its rotation and, in doing so, in 1665, alongside Robert Hooke, discovered the Great Red Spot. He compiled a table of the positions of Jupiter’s satellites that were used in 1675 by the Danish astronomer Ole Rømer to calculate the speed of light (Read this article to learn how it was done).

However, once the construction of the Paris Observatory was complete, his attention shifted towards Saturn, a planet for which Cassini would remain well-known to date. Indeed, he discovered some of the moons which orbited Saturn, such as Iapetus in 1671, Rhea in 1672, Tethys later on in 1984, and finally Dione the same year.

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The Cassini-Huygens space probe, a legacy

The Inspiring Life and Works Cassini, The Man Who Unraveled The Secrets of Saturn. 3

Launched in 1997, entering in orbit of Saturn in 2004, and completing its mission in 2017, the Cassini-Huygens Space Probe was named in honor of Giovanni Cassini and Christiaan Huygens. It was the first Space Probe to enter into Saturn’s orbit. As we previously uncovered, Cassini’s most notable contributions in astronomy touched upon the planet Saturn. Thus, the Cassini-Huygens Space Probe symbolizes Cassini’s dense contribution to our current knowledge of Saturn, its rings, and its moons.

To add to the complex legacy of Giovanni Cassini, the astronomer was not only at the roots of intricate astronomical discoveries but also of a long line of Cassini astronomers who would add to the list of notable breakthroughs in the field. Therefore Giovanni Cassini is sometimes also referred to as Cassini I, the first of an era.

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