Pierre Bouguer (February 16, 1698 – August 15, 1758) was a French mathematician.

His father, John Bouguer, one of the best hydrographers of his time, was regius professor of hydrography at Croisic in lower Brittany, and author of a treatise on navigation. In 1713 he was appointed to succeed his father as professor of hydrography. In 1727 he gained the prize given by the French Academy of Sciences for his paper On the best manner of forming and distributing the masts of ships ; and two other prizes, one for his dissertation On the best method of observing the altitude of stars at sea, the other for his paper On the best method of observing the variation of the compass at sea. These were published in the Prix de l’Academie des Sciences.

In 1729 he published Essai d'optique sur la gradation de la lumière, the object of which is to define the quantity of light lost by passing through a given extent of the atmosphere. He found the light of the sun to be 300 times more intense than that of the moon, and thus made some of the earliest measurements in photometry. In 1730 he was made professor of hydrography at Havre, and succeeded Pierre Louis Maupertuis as associate geometer of the Academy of Sciences. He also invented a heliometer, afterwards perfected by Joseph von Fraunhofer. He was afterwards promoted in the Academy to the place of Maupertuis, and went to reside in Paris.

In 1735 Bouguer sailed with Charles Marie de La Condamine for Peru, in order to measure a degree of the meridian near the equator. Ten years were spent in this operation, a full account of which was published by Bouguer in 1749, Figure de la terre determine. His later writings were nearly all upon the theory of navigation.

A crater on Mars was named in his honor.

His name is also recalled as the meterological term Bouguer's halo (also known as Ulloa's halo, after Antonio de Ulloa, a Spanish member of his Peru expedition) which an observer may see infrequently in fog when sun breaks through (for example, on a mountain) and looks down-sun -- effectively a "fog-bow" (as opposed to a "rain-bow"). An infrequently observed meteorological phenomenon; a faint white, circular arc or complete ring of light that has a radius of 39 degrees and is centered on the antisolar point. When observed, it is usually in the form of a separate outer ring around an anticorona. Reference: Tricker, R. A. R., 1970: An Introduction to Meteorological Optics, pages 192–193.

This article incorporates text from the public domain 1911 Encyclopædia Britannica.