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Great Lives from History: The 17th Century

Great Lives from History: The 17th Century
Gian Domenico Cassini

Cassini accurately determined the rotation periods of Jupiter and Mars, observed a gap in the rings of Saturn, and observed the phases of Venus. He also discovered that Jupiter and Saturn were not spherical but were slightly flattened at their poles. His astronomical measurements, dismissed as inaccurate by Cassini himself, were nevertheless used by Danish astronomer Olaus Rømer in 1676 to calculate the speed of light.

Born: June 8, 1625; Perinaldo, Imperia, Republic of Genoa (now in Italy)
Died: September 14, 1712; Paris, France
Also Known As: Jean Dominique Cassini
Areas of Achievement: Astronomy, mathematics

Early Life
Gian Domenico Cassini (jahn doh-MAY-nee-koh kah-see-nee) was born in Perinaldo on the Mediterranean coast, near Nice, France. His father, Jacopo Cassini, was from Tuscany, and his mother was Julia Crovesi. Cassini was raised by an uncle, his mother's brother.

Cassini studied poetry, mathematics, and astronomy at the Jesuit College in Genoa. Initially, Cassini was interested in astrology, but he soon became convinced that astrological predictions were not accurate. Nonetheless, Cassini's knowledge of astrology led to his first professional employment. In 1644, a senator from Bologna, the marquis Cornelio Malvasia, who had a deep interest in astrology and was familiar with Cassini, offered him a position at the new Panzano Observatory in Bologna. Beginning in 1648, Cassini would observe there, using instruments purchased with money obtained by Malvasia. Moving to Bologna was important for Cassini, since he had the opportunity to learn from two outstanding Jesuit scientists: Giovanni Battista Riccioli, an astronomer who made highly detailed telescopic observations of the Moon, and Francesco Maria Grimaldi, a physicist who discovered the diffraction of light.

Life's Work
In 1650, at the age of twenty-five, Cassini became a professor of mathematics and astronomy at the university in Bologna, Italy. While at Bologna, from 1650 to 1671, Cassini made many planetary observations using telescopes built by two contemporary Italians, Eustachio Divini and Giuseppe Campini.

His first serious astronomical observations were of the Sun, and he measured its position with an instrument called a gnomon, a device that used a small hole to allow the Sun's rays to form an image on the floor and, thus, allow Cassini to chart its position accurately. He had this gnomon built to replace an older device that was in the church of San Petronio in Bologna. Cassini was able to determine the precise times of the solstices, that is, the time the Sun reaches its farthest north and farthest south positions in the sky, marking the beginning of winter and summer respectively. In 1652-1653, Cassini observed a comet.

Cassini had initially believed in an Earth-centered solar system. In 1659, however, he presented his own version of an Earth-centered system, in which the Moon and the Sun orbited the Earth while the other planets orbited the Sun. He would later come to accept the Copernican model, in which all the planets, including Earth, orbit the Sun.

In July of 1664, using a telescope built by Campini, Cassini observed that Jupiter was not a perfect sphere, as first believed, but instead was flattened at its poles; he also saw that there were bands and spots on the planet. He measured Jupiter's period of rotation by determining how long it took for the large red spot in Jupiter's atmosphere to circle the planet. His value for the rotation period of 9 hours and 56 minutes, published in 1665, is within a few minutes of the best value obtainable with modern instruments.

Cassini observed the moons of Jupiter between 1666 and 1668, and discovered discrepancies in his own measurements that, at first, he attributed to light having a finite speed. However, he appears to have rejected his own idea. In 1676, Danish astronomer Olaus Rømer used Cassini's measurements to calculate the speed of light.

In 1666, he observed surface features on Mars, including Syrtis Major, a feature discovered by Christiaan Huygens in 1659. As with Jupiter, Cassini observed these features as they rotated around the planet and measured the red planet's rotation period. His value of 24 hours and 40 minutes is within 3 minutes of the number accepted in the early twenty-first century.

Cassini also attempted to determine the rotation period of Venus: 23 hours, 20 minutes. However, it is not clear what Cassini observed or what he interpreted as surface features. Modern telescopes show no features on Venus, which is entirely covered by bright clouds, and its rotation period, which can be determined only by mapping surface features with radar that penetrates the clouds, is about 243 days.

Cassini's measurements made him famous in scientific circles throughout Europe, and his work came to the attention of Jean-Bapiste Colbert, the French minister of finance during the reign of King Louis XIV. Colbert had been attempting to strengthen science in France, so, at Colbert's suggestion, the king invited Cassini to Paris to head the new Paris observatory. Cassini arrived in Paris on April 4, 1669, and immediately joined the new Académie Royale des Sciences (Royal Academy of Sciences), which Colbert had founded. When Cassini moved to France, he adopted the French version of his name, Jean Dominique Cassini.

Saturn was a major focus of Cassini's efforts at the Paris observatory. Huygens had discovered Titan, the largest moon of Saturn in 1655. Cassini discovered two other moons, Iapetus in 1671 and Rhea in 1672. In 1675, he recognized that Saturn's ring, discovered by Huygens in 1656, was divided into two parts, separated by a dark gap that is now known as the Cassini Division. In 1677, Cassini demonstrated that Saturn was not a perfect sphere but was flattened at its poles, and he discovered two more moons, Dione and Thetys, in 1684. In 1705, he suggested that Saturn's ring might not be a solid disk, but rather a swarm of objects moving around the planet, objects so small that they could not be seen individually.

Cassini also observed several comets between 1672 and 1707. From December 5 and December 23, 1690, Cassini observed a feature that appeared in the atmosphere of Jupiter. That feature is similar to the features observed in 1994, when more than twenty observable fragments of Comet Shoemaker-Levy 9 impacted Jupiter. Amateur astronomer Isshi Tabe and professional astronomer Junichi Watanabe, both Japanese, believe that Cassini's drawings show that he observed the effects of a similar comet impact in 1690. Watanabe and colleagues published the findings in a 1997 journal article.

During his years at the Paris observatory, Cassini served as the organizer of a renowned group of astronomers called the Paris School, and it included Jean Picard, Huygens, Rømer, Giacomo Felippo Maraldi, and Philippe de La Hire. Cassini was also trained in engineering, and published several manuscripts on flood control. He served as inspector of water and waterways, and as superintendent of the fortifications of Fort Urban.

In 1673, Cassini became a naturalized French citizen and married Geneviève Delaître, the daughter of an adviser to the king. They had two sons. The younger son, Jacques Cassini (1677-1756), followed in his father's footsteps, taking over his duties as head of the Paris observatory in 1712. Cassini's grandson César-François Cassini de Thury (1714-1784) and his great-grandson, Jacques-Dominique de Cassini (1748-1845) also became noted astronomers. By 1711, Cassini was blind, and he died in Paris on September 14, 1712.

Significance
Gian Domenico Cassini's improvement of the gnomon in the church of St. Petronius allowed him to determine precisely the times of the solstices. His measurements were influential in the adoption of the Gregorian calendar in the Protestant countries in 1700, by which time the Julian calendar was eleven days behind the actual beginning of each season. The Gregorian calendar, which corrected errors in the earlier Julian calendar, had been introduced in the Catholic countries of Europe in 1582.

Civil and Jesuit missionaries came to the Paris observatory to be trained in Cassini's methods of observing the eclipses of the moons of Jupiter. In 1668, Cassini had published tables listing the times of future eclipses of Jupiter's moons. The missionaries would go on to make their own observations while in Africa, the Americas, and China, and they used Cassini's tables to determine the longitudes of their remote sites. Also, other astronomers of his and later times used Cassini's methods to determine the longitudes of astronomical observation sites around the world.

Although Cassini never followed up on his own speculation that the apparent errors in the timing of the eclipses of the moons of Jupiter were a result of the finite speed of light, his measurements were used by Rømer, who announced in 1676 that he had determined the speed of light.

Cassini's achievements were so notable that the National Aeronautics and Space Administration (NASA) named its Saturn orbiter after him; the orbiter was launched in October, 1997. In addition, named in his honor are a crater on the Moon, a crater on Mars, and the Cassini Regio region on Saturn's moon Iapetus.

George J. Flynn

Further Reading
Alexander, A. F. O'Donel. The Planet Saturn: A History of Observation, Theory, and Discovery. New York: Dover, 1980. This work describes Cassini's many contributions to the understanding of Saturn, its rings, and his discovery of the moons Iapetus, Rhea, Dione, and Thetys.

Beatty, J. K. "A Comet Crash in 1690?" Sky and Telescope 93 (April, 1997): 111. Beatty summarizes a new Japanese interpretation of Cassini's 1690 sketches of Jupiter, suggesting Cassini saw the results of a comet impact.

Débarbat, S., and C. Wilson. "The Galilean Satellites of Jupiter from Galileo to Cassini, Rømer, and Bradley." In Planetary Astronomy from the Renaissance to the Rise of Astrophysics, edited by René Taton and Curtis Wilson. New York: Cambridge University Press, 1989. Part A of this edited volume provides an excellent account of Cassini's contributions to the measurement of the positions and eclipses of the moons of Jupiter.

Schorn, Ronald A. Planetary Astronomy: From Ancient Times to the Third Millennium. College Station: Texas A&M University Press, 1999. This work describes Cassini's numerous contributions to the observations of the planets, particularly Jupiter and Saturn.

See Also
Giovanni Alfonso Borelli; Jean-Baptiste Colbert; Galileo; Francesco Maria Grimaldi; Robert Hooke; Christiaan Huygens; Johannes Kepler; Hans Lippershey; Marin Mersenne; Sir Isaac Newton; Nicolas-Claude Fabri de Peiresc.

Related Articles
In Great Events from History: The 17th Century, 1601-1700: September, 1608: Invention of the Telescope; 1609-1619: Kepler Issues His Laws of Planetary Motion; 1610: Galileo Confirms Heliocentric Model of the Solar System; 1632: Galileo Publishes Dialogue Concerning the Two Chief World Systems; 1656: Huygens Identifies Saturn's Rings; 1665: Cassini Discovers the Great Red Spot; December, 1671: Newton Builds First Reflecting Telescope; December 7, 1676: Römer Calculates the Speed of Light; Summer, 1687: Newton Formulates Theory of Universal Gravitation.