Jacques Cassini (18 February 1677 – 16 April 1756) was a French

astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, moons, comets and galaxies – in either o ...

, son of the famous Italian astronomer Giovanni Domenico Cassini. Cassini was born at the

Paris Observatory The Paris Observatory (french: Observatoire de Paris ), a research institution of the Paris Sciences et Lettres University, is the foremost astronomical observatory of France, and one of the largest astronomical centers in the world. Its hist ...

. Admitted at the age of seventeen to membership of the

French Academy of Sciences The French Academy of Sciences (French: ''Académie des sciences'') is a learned society, founded in 1666 by Louis XIV at the suggestion of Jean-Baptiste Colbert, to encourage and protect the spirit of French scientific research. It was at th ...

, he was elected in 1696 a fellow of the

Royal Society The Royal Society, formally The Royal Society of London for Improving Natural Knowledge, is a learned society and the United Kingdom's national academy of sciences. The society fulfils a number of roles: promoting science and its benefits, r ...

of London, and became ''maître des comptes'' in 1706. Having succeeded to his father's position at the observatory in 1712, in 1713 he extended the Paris meridian, measuring the arc of the

meridian Meridian or a meridian line (from Latin ''meridies'' via Old French ''meridiane'', meaning “midday”) may refer to Science * Meridian (astronomy), imaginary circle in a plane perpendicular to the planes of the celestial equator and horizon * ...

from

Dunkirk Dunkirk (french: Dunkerque ; vls, label= French Flemish, Duunkerke; nl, Duinkerke(n) ; , ;) is a commune in the department of Nord in northern France.Perpignan Perpignan (, , ; ca, Perpinyà ; es, Perpiñán ; it, Perpignano ) is the prefecture of the Pyrénées-Orientales department in southern France, in the heart of the plain of Roussillon, at the foot of the Pyrenees a few kilometres from the ...

, and published the results in a volume entitled ''Traité de la grandeur et de la figure de la terre'' (1720). His two separate calculations for a degree of

meridian arc In geodesy and navigation, a meridian arc is the curve between two points on the Earth's surface having the same longitude. The term may refer either to a segment of the meridian, or to its length. The purpose of measuring meridian arcs is to ...

were 57,097 '' toises de Paris'' (111.282 km) and 57,061 toises (111.211 km), giving results for Earth's radius of 3,271,420 toises (6,375.998 km) and 3,269,297 toises (6,371.860 km), respectively.Traité de la grandeur et de la figure de la terre
Jacques Cassini, 1723. pp.182-3 & pp.302 He also wrote ''Eléments d'astronomie'' on

proper motion Proper motion is the astrometric measure of the observed changes in the apparent places of stars or other celestial objects in the sky, as seen from the center of mass of the Solar System, compared to the abstract background of the more dista ...

(1740), and published the first tables of the

satellite A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioiso ...

s of

Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; ...

in 1716. He died at Thury, near Clermont, France. The asteroid 24102 Jacquescassini is named after him. Jacques Cassini married Suzanne Françoise Charpentier de Charmois. Their second son was astronomer

César-François Cassini de Thury César-François Cassini de Thury (17 June 1714 – 4 September 1784), also called Cassini III or Cassini de Thury, was a French astronomer and cartographer. Biography Cassini de Thury was born in Thury-sous-Clermont, in the Oise departme ...

, who was also known as Cassini III.

Works

A number of his publications about astronomy
are preserved at the

library and available online on th
digital library
among them :
De la grandeur et de la figure de la terre
1720 (About the size and features of Earth)
Méthode de déterminer si la terre est sphérique ou non
1738 (Method to determine if Earth is a sphere or not)
Éléments d'astronomie
1740 (Anstronomy elements)
Traité de la Comète qui a paru en décembre 1743 & en janvier, février & mars 1744
(About the comet that appeared December 1743, January, February and March 1744)

References

External links

* *
Paris Observatory digital library
{{DEFAULTSORT:Cassini, Jacques 1677 births 1756 deaths Scientists from Paris 18th-century French astronomers French geodesists French people of Italian descent Members of the French Academy of Sciences Fellows of the Royal Society