**Alexis Claude Clairaut** He was born on May 7, 1713 in Paris (France) and died on May 17, 1765, also in Paris. The son of a French mathematician from whom he received his training, he became one of the earliest and most celebrated mathematicians in history. He studied 10-year-old calculus, published his first 13-year mathematical work, and drafted an 18-year mathematical treatise. This treatise was considered a landmark in the study of analytic geometry in space. Even without reaching the legal age, the work made him elected member of the Academy of Sciences (*Academie des Sciences*) in 1731 by a special license.

Clairaut helped Maupertuis determine the length of the meridian. In 1743, he wrote a book that studied the shape of a rotating body, like the Earth, and suffered the effects of gravity and centrifugal force. It also showed how to determine the radius of the earth by measuring the force of gravity at various points on the globe, marked by the time of oscillation of a pendulum.

Clairaut is among those who have exhaustively studied lunar movements (lunar theory). He calculated the effects of the gravitational pull of Venus on the earth and compared it to the pull of the moon. By comparing these data with Lacaille's observations, he obtained in 1757 the first reasonable value for the mass of Venus (2/3 of the earth's mass). ) and a new value for the lunar mass (1/67 of the earth's mass).

It is known today that the first value is somewhat below the actual value, and the second just above, but they represented the best values hitherto obtained. For a period of time, his studies provoked a true revolution, as they seemed to contest Newton's theories. On Buffon's advice, he broadened his remarks and thought Newton was right.

As the year of Halley's comet approached, Clairaut began to study, with Lalande's aid, the effects of Jupiter and Saturn's gravity on the comet. He thought that the two giant planets would stop him in such a way that he would not reach the closest point to the Sun in its orbit (perihelion) before April 13, 1759. It appeared on Christmas Day 1758 and reached its peretheum in. of the deadline.

* Photo taken from *MacTutor History of Mathematics archive** (//www-history.mcs.st-andrews.ac.uk)*.