Gian Cassini

Jack Fitzgerald

Percival

Astronomy P1

10 October 2014

Gian Cassini

Gian Cassini is a well-accomplished Italian astronomer. Born in 1625, Cassini made major contributions to the field of astronomy for nearly 90 years before his passing. Some of his major contributions in astronomy include locating the moons of Saturn, finding the distance of Mars, and determining the obliquity of the ecliptic. His historical influence on astronomy as a whole is impeccable, evidenced by the naming of the Cassini space probe, which was the first to orbit Saturn.

Cassini was born in Perinaldo, Italy, on June 8, 1625 to Jacopo Cassini and Julia Crovesi, the former of which was a Tuscan. He studied at Vallebone and then the Jesuit college of Genoa and the abbey of San Fructuoso. His astronomical career began when Marquis Cornelio Malvasia, a rich amateur astronomer and senator of Bologna, invited him to come to work in his observatory at Panzano, near Bologna in 1648. Cassini left that post when he departed for France in February 1669. However, in that time, he completed his education under two prestigious professors, and made use of several instruments that allowed him to begin his first researches. He was a conservative theorist- he initially rejected Copernicus’ theories on the heliocentric model.

Cassini won the favor of the Bolognan community with his works to such an extent that the senate of Bologna actually designated him to be the principal chair of astronomy at the university. Cassini had a specific interest in planetary astronomy, and in 1653, he wrote to Pierre Gassendi to request precise measurements of the superior planets. Upon receiving these observations, Cassini concluded that the Earth was the center of the galaxy. Among the many of his great early feats that demonstrated his practical ability included the building of a large and accurate meridian in San Petronio after its current meridian was rendered useless after architectural changes to the church it was originally situated on blocked sun rays from reaching the meridian.

Using his meridian research, he made principal observations on the obliquity of the ecliptic, exact positions of solstices and equinoxes, the speed of the sun’s apparent motion and the variation of its diameter. He published these findings in Specimum observationum Bononiensium in 1656, and he continued to use the San Petronio meridian in later publications. In 1659, he presented a model of the planetary system that agreed with the hypothesis of Tycho Brahe. In 1661, he developed a method, inspired by Kepler’s work, of mapping successive phases of solar eclipses, and in 1662 he published new tables of the sun, based on his observations at San Petronio. He held a special interest at this time, however, in comets. In 1664, in front of Queen Christina of Sweden, he studied one and formulated a theory about the orbit of the comet being a great circle.

He maintained healthy friendships with Roman lens-makers, and in 1664, he obtained powerful celestial telescopes. With these telescopes, through studying the shadow of satellites on Jupiter, he was able to find the rotation period of the satellites and the planet itself, and he came very close to the modern commonly accepted time of 9 hours and 56 minutes. In the beginning of 1666, he did the same with mars, and came up with a rotation time, being 24 hours and 40 minutes, only 3 minutes less than the modern commonly accepted time. In the case of Jupiter, however, where Galileo failed, Cassini made a full, precise table of the determination of longitudes of the planet, and published this in 1668 as Ephemerides Bononienses mediceorem siderum.

Because of this publication, the Académic Royale des Sciences, recently founded in Paris, offered Cassini the honor of being a regular correspondent with the academy. He accepted in 1668, and in 1669, he left Italy for good to reside in Paris. Upon his entrance, he was cordially accepted by the king. Because of the financial contribution of the academy, he was amply able to replace his instruments whenever necessary. He took a special planetary interest in Saturn; in September 1671, he discovered a second satellite of Saturn, Iapetus. A year later, he discovered a third, being Rhea, and in March 1684, he discovered 2 more, being Tethys and Dione. In 1680, he made an observation on a comet that confirmed his method of observing comet trajectory.

The tables of the eclipses of Jupiter’s satellites were utilized internationally, being used by many French Astronomers. In 1672, he travelled to Cayenne to observe the parallax of Mars during its opposition, and they not only were able to determine the parallax of Mars at 25’, but also changed the accepted parallax of the Sun from 8.8’ to 9.5’.

Unfortunately, in the 1700’s, his activities dropped sharply, and in the last two years of his life, he went completely blind. He died on September 4, 1712, in Paris.