June 8: Giovanni Domenico Cassini

Giovanni Domenico Cassini

June 8, 1625 – September 14, 1712

Giovanni Domenico Cassini was an Italian/French mathematician, astronomer, engineer, and astrologer.

Cassini was an astronomer at the Panzano Observatory from 1648 to 1669. He was a professor of astronomy at the University of Bologna and in 1671 became director of the Paris Observatory. He 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.

Along with Robert Hooke, Cassini is given credit for the discovery of the Great Red Spot on Jupiter (ca. 1665). Cassini was the first to observe four of Saturn's moons, which he called Sidera Lodoicea; he also discovered the Cassini Division (1675). Around 1690, Cassini was the first to observe differential rotation within Jupiter's atmosphere.

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.

Cassini was the first to make successful measurements of longitude by the method suggested by Galileo, using eclipses of the satellites of Jupiter as a clock.

The Lunar crater Cassini is named in his honor.