Giovanni Domenico Cassini

The rings and moons of Saturn

Giovanni Domenico Cassini (1625-1712, picture: Paris observatory)

Near Nice in the town Perinaldo, Giovanni Domenico Cassini was born at the 8th of June 1625. Until 1650 he studied in Genua and Bologna and became (in the year of the death of Christoph Scheiner) professor for astronomy and mathematics at the university of Bologna. He let built an observatory on the tower of the church St. Petronio and was mainly interested in the observation of comets.

Cassini's Valles Marineris
Mars according to G.D. Cassini
1663 he was ordered to fortify the citadel of the Italian town and archbishopric of Urbino. But while doing this he also continued to do astronomy, so he calculated the deformation of Jupiter and its rotation time. He watched the phases of Venus, discovered by Galileo Galilei (1564-1642), and also refined the visible surface marks of Mars. So besides of the Great Syrte discovered by Christiaan Huygens he discovered the polar caps of the red planet.

1668 the "sun king" Louis XIV. called Cassini to Paris as an observer to the academy of sciences. Here latest he met personally the university professor Christiaan Huygens. Cassini - now Jean-Dominique Cassini - became director of the Paris observatory, but this palace like building was not ready before 1672.

Before the completion of the observatory some of the astronomical instruments, made by Giuseppe Campani arrived 1671 in Paris. These were up to 40 meter long refractor telescopes. Just in the presence of Huygens, the discover of Titan in 1655, Cassini saw another moon of Saturn: Japetus. The moon consists mainly of ice and has the special feature at the surface that nearly one half is very dark. Because of this the moon has a magnitude of around 10 at western elongation, while at eastern elongation it is about 2 magnitudes dimmer. This fact Cassini discovered at continuing observations of "his" moon. The dark area on Japetus, possibly originating by dark dust, is called after his discoverer Cassini region.

1672 Cassini together with Jean Richter (1630-1696) could made parallel observations of the Mars in Paris (France) and Cayenne (French Guyane), using this observation for the indirect determination of the solar parallax as of 9.5 arc seconds as a first approximation. The solar parallax is the virtual shift of the Sun at the sky when seen from two maximum different positions near the Earth equator. The real mean value of the solar parallax is 8.8 arc seconds and can be used for the correct determination of the distance Earth-Sun.

Also in 1672 Cassini discovered Rhea. This is another Saturnian ice moon, which has a mean brightness better than Japetus. With Rhea 8 moon and 6 planets of the solar system were discovered - 14 celestial bodies circeling around the Sun. Cassini interpreted this as a glorification of is chief Louis XIV., so maybe this was the reason that it lasted twelve years before Cassini still as the director of the Paris observatory announced the discovery of two more moons of Saturn: Dione and Tethys. These are a half magnitude dimmer than Rhea, but they are also about a half magnitude brighter than Japetus in eastern elongation. In the meantime in 1675 Cassini was able to sign also for the discovery of the main 4450 kilometer gap within the Saturn rings, the Cassini division.

Collage of Saturn and its moons

Saturn and its moons (collage: NASA)

Another discovery of Cassini is the zodiacal light, an exaterristical sky illumination on the ecliptic plane caused by interplanetary dust and the reflected sunlight within the dust. He assumed this zodiacal light as the pointer from the Christmas star to the stable of Bethlehem as within the gospel of Matthew.

At the 14th of September 1712 Cassini died in Paris. His son Jacques Cassini (1677-1756) became director of the observatory of Paris, then.

Fritz Krafft, Adolf Meyer-Abich (Eds.): Große Naturwissenschaftler, Fischer 1970
Joachim Herrmann (Ed.): Das Große Lexikon der Astronomie, Orbis 1996
Carl Sagan: Blauer Punkt im All, 1994, Droemer-Knaur 1996

Page history:
created 1999-03-23
modified 1999-11-26 Mars map
modified 2000-02-04 History template

To the Sky over Berlin...To the astronomy links...-- jd --