Jacques Cassini (18 February 1677 – 16 April 1756) was a French astronomer, son of the famous Italian astronomer Giovanni Domenico Cassini. 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, he was elected in 1696 a fellow of the Royal Society of London, and became maître des comptes in 1706. While in England, he was acquainted with other famous astronomers such as Newton and Halley. His two separate calculations for a degree of meridian arc 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. 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.
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.