Giovanni Cassini

Giovanni Domenico Cassini (also known as Cassini I) (Perinaldo, Republic of Genoa, June 8, 1625-Paris, France; September 14, 1712) was an Italian naturalized French astronomer, geographer, and engineer. From 1669 he lived in France and in 1673 he became a French citizen, for which he is also known by the French version of his name, Jean-Dominique Cassini . Louis XIV of France appointed him in 1671 director of the Paris Observatory and member of the Academy of Sciences. Cassini would remain as director for the rest of his life. After forty years of observing the sky, he became completely blind and died in 1712. Cassini was a contemporary of Isaac Newton who made numerous observational contributions to the astronomy of the solar system, which would become instrumental in underpinning the theory of gravitation.

Astronomical works

After being educated with the Jesuits, his first job consisted of calculating astronomical tables for a nobleman interested in astrology, a discipline he would soon renounce, influenced by Giovanni Pico della Mirandola's book, Disputationes aversus astrologiam divinatricem.

At the age of 25 he was appointed professor of astronomy at the University of Bologna, succeeding Galileo's disciple, Bonaventura Cavalieri. Between 1652 and 1668, after years of patient work with his observations of Jupiter's satellites, he published some tables (ephemerides) of its eclipses, giving on one page the appearance of the satellites with respect to Jupiter and on the opposite the time of the eclipse both of immersion and emergence in hours, minutes and seconds.

He measured the periods of revolution of Mars and Jupiter and discovered four satellites of Saturn. In 1675 he discovered the division of Saturn's rings that bears his name. With the help of his colleague Jean Richer (1630-1696) he measured the distance to Mars by triangulation. With this, he measured the size of the solar system, obtaining for the Astronomical Unit a value that was only 7% less than the real value (previous values underestimated it by factors of 3 or more).

He observed the motion of comets and the apparent motion of the Sun. He used the most advanced telescopes of his time to observe Jupiter's satellites and made accurate tables of their motions, allowing navigators to determine their longitude using the satellites as a "celestial clock". He discovered the seasonal changes of Mars and measured its rotation period, as well as that of Saturn. After working for Pope Clement IX, in 1669 he went to Paris to participate in the creation of the new Paris Observatory, of which he became director two years later. There he discovered Iapetus (1671), Rhea (1672), Dione (1684) and Tethys (1684), satellites of Saturn, and observed a gap in the planet's ring system (Cassini division). In 1683 he observed the zodiacal light and in 1693 he discovered the laws that regulate the libration movements of the Moon.

Although he is considered one of the greatest observational astronomers of all time, in the theoretical aspect he made errors such as not fully accepting the heliocentric theory (his position on the matter was somewhat ambiguous, on some occasions adopting Tycho's model and in others an agnostic position) or the elliptical movement of the planets discovered by Kepler. However, it should be clarified that these theories were still discussed in his time, since it was at the beginning of the XVIII century when, thanks to differential calculus and observations of improved precision, it was possible to verify the unquestionable superiority of Newton's theory of gravitation. In fact, Cassini himself ended up contributing to the verification of the second when he verified with the sundial of the Basilica of San Petronio in Bologna that the variation of the distance between the Sun and the Earth was better adjusted to an oval orbit than to an oval one. circular with the central object located at a certain distance from its center (Ptolemy's, Copernicus' and Tycho's systems used circular orbits, not Kepler's, which was the only one to use elliptical orbits and dispense with epicycles). Cassini also used the sundial of San Petronio (which he himself remodeled) to measure the length of the year and the inclination of the ecliptic with unequaled precision until then, thanks to the precise correction he applied for atmospheric refraction. His observations of Jupiter's rotation, his discovery of eclipses produced by the Galilean moons, and his verification that the Jovian and Saturn satellite systems followed Kepler's third law also eventually contributed to the acceptance of the Keplerian heliocentric system.

Disputes

Recorded by the Paris Observatory during the time of Cassini. The tower on the right is the Marly Tower, a dismantled part of the Marly machine, brought here by Cassini to mount long-distance and air telescopes.

Cassini did not accept Kepler's elliptical orbits of the planets, instead proposing a series of quadratic curves (Cassini oval) for these orbits.

In 1676 Ole Rømer, working for Cassini at the Paris Observatory, was able to more accurately predict an eclipse of Jupiter's moon Io by taking into account that the speed of light is finite. Although Cassini initially seemed to support this important discovery, it finally did not accept Rømer's argument, clinging to the idea that light was instantaneous. Despite the fact that at the same time Huygens and Newton gave credit to Rømer, it took many years until the idea that the speed of light is finite was universally accepted.

Cassini and his son Jacques, based on geodetic measurements made by themselves, claimed that the Earth was elongated at the poles contrary to what Newton's mechanics predicted, that is, that it is flattened at the poles. For this reason Cassini did not accept Newton's theory of universal gravitation. The controversy over the shape of the Earth lasted for many years, and was finally settled in 1736 in the expedition to Lapland led by Pierre Louis Moreau de Maupertuis that clearly confirmed Newton's theories.

Geographical works

During his time as a French citizen, he contributed to a more accurate map of France, thanks to his method of determining Earth longitudes by more precisely measuring the time differences of eclipses in different meridians. From the results of the measurements carried out, France turned out to be smaller than previously believed. He also drew a complete world map, where the various parts of the world were described by meridians and parallels. His work was continued by Guillermo Delisle.

Her legacy

He is the father of a generation of astronomers and geographers: his son Jacques Cassini (1677-1756) was a geographer. His grandson César Cassini (1714-1784) drew a map of France on a scale of 1/86400 and his great-grandson Jacques Dominique Cassini (1748-1845) was director of the Paris Observatory and finished the map that César's father had begun. he. The last name is the Martian crater Cassini, the lunar crater Cassini (in this case, an honor shared with his son Jacques) and the asteroids (24101) Cassini and (24102) Jacquescassini (the latter dedicated to his son).. He also named after him the NASA-built Saturn system exploration ship, the Cassini spacecraft in conjunction with the Huygens probe, built by the European Space Agency. Together they formed the Cassini-Huygens unmanned space mission, which was in operation until Friday, September 15, 2017, the date on which it was destroyed upon entering Saturn's atmosphere.