written by: Brian Jones•edited by: RC Davison•updated: 7/18/2009
Giovanni Cassini was a 17th century Italian/French astronomer who discovered the gaps is Saturn's rings, four of its moons, and the rotational period of Jupiter.
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Born: June 8, 1625
Birthplace: Perinaldo, Genoa, Italy
Credentials: Graduated from the Jesuit College at Genoa
1648 - Astronomer at Panzano Observatory
Professor of Astronomy – University of Bologna
1671- Director of the Paris Observatory under King Louis XIV
Died: September 14, 1712 (age 87)
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Giovanni Cassini made several important discoveries early on in our solar system. Cassini was the first to discover that there were spaces between the rings of Saturn in 1675. The gap he discovered between the A ring and B ring is still known today as the Cassini Division. He also studied extensively the moons of Saturn, discovering four of them himself: Iapetus (1671), Rhea (1672), Tethys (1684), and Dione (1684).
Cassini was the first to accurately measure the rotational period of Jupiter at 9 hours, 56 minutes.In 1664, Robert Hooke discovered the Great Red Spot of Jupiter and Cassini used his own observations to determine the measurement in 1665. During his Jupiter observations, he also cataloged the movements of all of Jupiter’s moons and noted the differential rotation of Jupiter’s atmosphere. Cassini later was able to measure the rotational period of Mars.
In 1662, Giovanni Cassini published accurate measurements of the sun’s motion through the sky and later he came within 7% of measuring the distance from the Sun to the Earth. Through triangulation, he also determined the distance to Mars. Cassini is credited with giving the most accurate dimensions of the solar system at the time.
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Giovanni Cassini had originally intended to study astrology but as he became more aware of the heavenly bodies he turned to science and later disavowed the practice.
After being given a grant by King Louis XIV of France to start the Paris Observatory, opening in 1671, Giovanni Cassini became a citizen of France in 1673, changing his name to Jean-Dominique Cassini.
Cassini was also an engineer and was hired by Pope Clement IX to work on management of the Po River and its flooding.
Giovanni led four successive generations of astronomers, Jacques Cassini (son), Cesar Francois Cassini (grandson), and Jean Dominique Cassini (great-grandson), all of which, in turn, became Director of the Paris Observatory.
Cassini was the first to investigate the phenomenon know as the Zodiacal Light in 1683. (Zodiacal Light is a faint triangular glow seen in the late evening or early morning moonless sky and is the reflection of sunlight off fine particles of dust space.)