Alexis-Claude Clairaut is best remembered for his book Théorie de la figure de la terre, an outgrowth of studies he made during a 1736 journey to Lapland. In it, he discussed the curvature of the Earth, and thus proved Sir Isaac Newton's (1642-1727) theory that the shape of the Earth is that of an oblate ellipsoid. This in turn led to widespread acceptance of Newton's gravitational theory.
Born in Paris on May 7, 1713, Clairaut was the son of Jean-Baptiste, a mathematics teacher, and Catherine Petit Clairaut. The couple had some 20 children, of which few survived. Their son made an early display of his talent, studying calculus at age 10 and writing erudite mathematical treatises when he was barely a teenager. He went on to conduct a lively correspondence with some of the leading mathematical figures of the day, including Leonhard Euler (1707-1783) and Johann Bernoulli (1667-1748).
When he was 23 years old, Clairaut traveled to Lapland in the far north as part of an expedition directed by Pierre Louis Moreau de Maupertuis (1698-1759). The group's mission was, in part, to measure the curvature of the Earth inside the Arctic Circle, and later Clairaut published the results of their activities in Théorie de la figure de la terre (1743). In addition to ensuring the acceptance of Newton's ideas concerning gravitation and the shape of the Earth, the book presented a means for determining the shape of the Earth by timing the swings of a pendulum. Clairaut also analyzed the effects created by gravity and centrifugal force on a rotating body, putting forth a proposition later known as Clairaut's theorem.
The unmarried Clairaut maintained an active social life, and became heavily involved in societies to promote the advancement of young mathematicians. Most prominent was his involvement with the distinguished Académie Royale des Sciences, to which he was elected a member at the age of 18. Later studies on tides won him an award from the Académie in 1740, and three years later he was appointed associate director of the organization. He also became a Fellow of the Royal Society of London.
Among Clairaut's other ventures into applied mathematics was his measurement of Venus, the first accurate reckoning of that planet's size. He also conducted important studies on the gravitational relationship between the Earth and the Moon, as well as predictions regarding Halley's comet, and published a number of works. Clairaut died on May 17, 1765, after a brief illness, at the age of 52.