Giovanni Domenico Cassini
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Giovanni Domenico Cassini (June 8, 1625–September 14, 1712) was an Italian-French astronomer, engineer, and astrologer, was an accomplished astronomer who was the first to observe four of Saturn's moons. He compiled tables of the moons of Jupiter that were used to compute longitude and more accurately measure the circumference of the earth. He also made attempts to more accurately measure the size of the solar system by estimating the distance of mars from the earth.
Biography
Cassini was born in Perinaldo, near Sanremo, at that time in the Republic of Genoa. His father was James Cassini, who first assigned a tutor to Cassini's education, and later sent the young man to study theology and law under the Jesuits in Genoa. In 1646, the later published a book of poetry within which was contained some contributions by Cassini.
Cassini was early introduced to astronomy through a text devoted to astrology, and soon he was very knowledgeable about it; strangely enough, it was this knowledge that led to his first appointment.
Astronomer of Bologna
In 1644 the Marquis Cornelio Malvasia, who was a senator of Bologna with a great interest in astrology, invited Cassini to Bologna and offered him a position in the Panzano Observatory which he was constructing at that time. Most of their time was spent calculating newer, better, and more accurate ephemerides for astrological purposes, using the rapidly advancing astronomical methods and tools of the day. At the age of 25, Cassini was appointed by the senate to the post of astronomer at the newly-completed observatory, where he remained from 1648 to 1669. During this time he also served as professor of mathematics at the University of Bologna.
When Cassini arrived in Bologna, one of his first tasks was to improve the measurement of the meridian line passing through the Cathedral of Saint Petronius. He not only accompished this, but also constructed additional astronomical instrumentation, completed in 1655. The primary purpose of this undertaking was to fix the religious feasts of the Roman church at the right time of year. Cassini also redrew the meridian line passing through the cathedral according to his own observations.
Cassini published solar tables, and calculated the parallax of the sun, the arc subtended by the disk of the earth were it observed from the Sun. This, in combination with the same measurement of the sun from the earth, could yield comparative measurements of the two bodies. Cassini's measurements were not, however, particularly accurate, and his conclusion that the sun was about a million times larger than the earth was found to be on the high side.
Cometary observations
During his time in Bologna, Cassini established an international reputation by his many accomplishments. In 1652, Cassini penned an account of the appearance of a comet. At the time, the motions of comets were believed not to be governed by any particular law, but contrary to this contention, Cassini attempted to establish the orbit of the comet from a few observations. He believed that the same laws then recently discovered by Johannes Kepler should apply to comets as well. He was able to establish the orbit of comets with just a few observations, a task that had frustrated Kepler.
Public works and engineering
In 1657, Cassini was asked to accompany the ambassador to the Roman pontiff from Bologna, in order to settle a dispute between the city states of Bologna and Ferrara regarding the two major river systems that they shared. He showed marked skills as a diplomat, and was asked to remain in Rome to undertake a number of public works, including the repair of Fort Urban and other fortifications. Pope Clement IX was so impressed with Cassini that he encouraged the astronomer to become an ecclesiastic, but Cassini resisted this invitation. He did, however, become the supterintendant of the waters in the papal estates.
In 1664, Cassini observed the red spot on the surface of Jupiter, and by recording its movement, was able to estimate the jovian day to be 9 hours and 56 minutes. He also estimated the period of rotation of venus, mars and the sun, stating the last of these as 27 days, a figure not far from current estimates (25 days).
Cassini was the first to compile tables of the eclipses of Jupiter's moons for the purpose of measuring longitude by the method suggested by Galileo, using eclipses of the satellites as a clock. He published these tables in 1668.
Observatory director in France
In 1669, King Louis XIV of France, anxious to obtain Cassini's employ at a new observatory in Paris, offered the astronomer a handsome pension that was competitive with his income at Bologna. Cassini hesitated to accept the post until he could gain the approval of the pope and the Bolognese government, which was forthcoming on the condition that he would return in two years. These conditions were eventually dropped, and Cassini thoroughly adopted his new country, to the extent that he became interchangeably known as Jean-Dominique Cassini—although that is also the name of his great-grandson. He was appointed director of the Paris observatory in 1671, and remained so until his death.
In 1673 he became a French citizen. While in France Cassini also served as the court astronomer/astrologer of the Sun King for forty-one years, serving the expected dual role yet focusing the overwhelming majority of his time on astronomy rather than the astrology he had studied so much of in his youth.
Two important tasks that involved Cassini and his crew of astronomers and measurement-takers were map-making and determining longitude. Cassini instituted a project to map the world, and was asssisted in this venture by many of the world's leading observatories. These reported their observations to the Paris observatory under Cassini's direction, where they were tablulated and translated into a map projection that was viewed favorably by King Louis.
Expeditions
During the 1670s and 1680s, Cassini sent out several expeditions to different parts of the world to undertake astronomical observations with varying success. One problem these teams confronted was the change in the periods of pendulums when brought to different latitudes. This problem was not solved with great success, and the teams themselves, having no communication with their home base in Paris for years at a time, had to improvise their own solutions.
In 1672 he sent his colleague Jean Richer to Cayenne, French Guiana, while he himself stayed in Paris. The two made simultaneous observations of Mars and thus found its parallax to determine its distance, thus measuring for the first time the true dimensions of the Solar System.
Around this time, Cassini began work on a project to create a topographic map of France, using Reiner Gemma Frisius's technique of triangulation. The project was continued by his son Jacques Cassini and eventually finished by his grandson Cassini de Thury and published as the Carte de Cassini in 1789[1] or 1793.[2] It was the first topographic map of an entire country.
Cassini discovered the zodiacal light in 1683, a mild illumination seeming to emanate from the direction of the sun toward the ecliptic just before sunrise and after sunset. In 1684, he was the first to observe four of Saturn's moons, which he called Sidera Lodoicea; The first had been discovered by Christian Huyggens. Around 1690, Cassini was the first to observe differential rotation within Jupiter's atmosphere.
Cassini's method of determining longitude was used to measure accurately the size of France for the first time. The country turned out to be considerably smaller than expected, and the king quipped that Cassini had taken more of his kingdom from him than he had won in all his wars.
Cassini died in Paris in _____________.
Named after Cassini
- Cassini-Huygens Mission to Saturn
- The Cassini Division in Saturn's rings
- Cassini Regio, dark area on Iapetus
- Cassini crater on Mars
- Cassini crater on the Moon
- Cassini's Laws
- 24101 Cassini, an asteroid
- Cassini's identity for Fibonacci numbers
- Cassini Web Server
See also
Notes
ReferencesISBN links support NWE through referral fees
<<We need at least 3 reliable references here, properly formatted.>>
- Corney, Bolton, John Nichols, and Henry John Rose. 1839. A new general biographical dictionary. London: Shoberl, [Privately printed]. 101-102.
- Brown, Lloyd Arnold. 1979. The story of maps. New York: Dover Publications. 215-224.
External links
- Catholic Encyclopedia article
- John J. O'Connor and Edmund F. Robertson. Giovanni Domenico Cassini at the MacTutor archive
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