Jacques Cassini
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Jacques Cassini (18 February 1677 – 16 April 1756) was a French
astronomer An astronomer is a scientist in the field of astronomy who focuses on a specific question or field outside the scope of Earth. Astronomers observe astronomical objects, such as stars, planets, natural satellite, moons, comets and galaxy, galax ...
, son of the famous Italian astronomer
Giovanni Domenico Cassini Giovanni Domenico Cassini (8 June 1625 – 14 September 1712) was an Italian-French mathematician, astronomer, astrologer and engineer. Cassini was born in Perinaldo, near Imperia, at that time in the County of Nice, part of the Savoyard sta ...
. He was known as Cassini II.


Biography

Cassini was born at the Paris Observatory. He was first admitted to Collège Mazarin after brief studies at his home observatory under his father. Later, he was admitted at the age of seventeen to membership of the
French Academy of Sciences The French Academy of 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 method, scientific research. It was at the forefron ...
, 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, re ...
of London, and became ''maître des comptes'' in 1706. While in England, he was acquainted with other famous astronomers such as Newton and Halley. 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 from
Dunkirk Dunkirk ( ; ; ; Picard language, Picard: ''Dunkèke''; ; or ) is a major port city in the Departments of France, department of Nord (French department), Nord in northern France. It lies from the Belgium, Belgian border. It has the third-larg ...
to
Perpignan Perpignan (, , ; ; ) is the prefectures in France, prefecture of the Pyrénées-Orientales departments of France, department in Southern France, in the heart of the plain of Roussillon, at the foot of the Pyrenees a few kilometres from the Me ...
, 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 (geometry), curve between two points near the Earth's surface having the same longitude. The term may refer either to a arc (geometry), segment of the meridian (geography), meridian, or to its ...
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
This work represented Cassini's delve into the Earth figure debate. There were two main schools of thought, the Cartesians believed in a prolate spheroid, and the Newtonians in an oblate spheroid. Cassini sided with the Cartesians over the Newtonians, advocating for a prolate spheroid shaped earth. The Cartesians had support, but the Newtonians made separate observations seeming to disagree with the Cartesian idea. Cassini II likely realized that the Newtonian observations were more accurate, and stepped out of the scientific field. He only returned publicly after an attack from
Anders Celsius Anders Celsius (; 27 November 170125 April 1744) was a Swedes, Swedish astronomer, physicist and mathematician. He was professor of astronomy at Uppsala University from 1730 to 1744, but traveled from 1732 to 1735 visiting notable observatories ...
. For his last few years he took up cartography, working with his son, Cassini de Thury or Cassini III, to create a new French map. This map was known as the Carte de Cassini, and was to be a very accurate map of France. Jacques Cassini's work on the ballistic pendulum has been a topic of controversy among historians of science. While some credit him with developing original work on the topic, others argue that he built upon the ideas of earlier scientists. It is difficult to determine the extent of Cassini's original contributions, as the available historical records are vague and incomplete. However, subsequent work on ballistic pendulums by Benjamin Robins (1707–1751), a British mathematician and engineer, suggests that he independently repeated many of the same results. Regardless of the extent of Cassini's original contributions, his work on the ballistic pendulum was significant in its time and helped to advance the field of ballistics. The device allowed for the measurement of the velocity of projectiles, which was important for the development of more accurate firearms and artillery. He also wrote ''Eléments d'astronomie'' on
proper motion Proper motion is the astrometric measure of changes in the apparent places of stars or other celestial objects as they move relative to the center of mass of the Solar System. It is measured relative to the distant stars or a stable referenc ...
(1740), and published the first tables of the
satellite A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation ( GPS), broadcasting, scient ...
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 9 times that of Earth. It has an eighth the average density of Earth, but is over 95 tim ...
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, who was also known as Cassini III.


Works

A number of his publications about astronomy
are preserved at the Paris Observatory 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