Jacques Cassini

Jacques Cassini ( born February 18, 1677 Paris, † April 16, 1756 in Thury at Clermont (Oise ), also called Cassini II) was a French astronomer and surveyor, which dealt among other things with the earth's shape.

Life and work

Cassini was born on February 18, 1677 in Paris, the son of Giovanni Domenico Cassini, director of the Paris Observatory. He studied at the College Mazarin in Paris, which he left at the age of 14 or 15 years with a work on optics.

1694 he was admitted to the Académie des sciences, elected on March 23, 1698 during a stay in England in the Royal Society of London. Meridian or degree measurements that he along with his father performed in France in 1700, led him to believe that the polar radius of the earth is greater than the equatorial radius, so that the earth is egg-shaped. In contrast, English astronomer represented under the leadership of Isaac Newton's view that the Earth was flattened at the poles because the Earth's rotation would cause a centrifugal force. To that time but being able to refute one of the two hypotheses empirically, the degree of measurement were still a little too inaccurate ( see the later introduced triangulation on the basis of the work Snells ).

Throughout his life denied both the Cassini Earth flattening at the poles as well as the law of gravity of Newton. From 1709, when the sight of his father wore off, he became more and more his duties at the Paris Observatory.

Further surveys in France he undertook in 1718 and 1733, where he was assisted in the latter by his son César -François. The measurement results also supported the egg shape of the earth, but were challenged after measurements by Pierre Bouguer and Charles Marie de La Condamine 1735 in Peru and supported by Pierre de Maupertuis in Lapland in 1736, the thesis of the flattening at the poles.

The Astronomy Cassini gave valuable impulses through its exact tables of the sun, moon, planets and the moons of Jupiter and Saturn as well as by measuring the proper motion of the fixed stars.

Cassini died on 16 April 1756 the consequences of an accident, had at the turn over his carriage the day before. His successor as director of the observatory was his son César François Cassini de Thury.

The lunar crater Cassini is named after him and Giovanni Domenico Cassini.

Works

• Cassini: observation of the eclipse of Mars by the moon, on the Parisian Observatorio gemachet, on the 18th January, 1726 in The Royal. . Academy of Sciences in Paris physical treatises. Volume 7, 1751st grain (eds. ), pp. 660-661, (online at uni-goettingen.de )