Pierre Bouguer

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Pierre Bouguer. Jean-Baptiste Perronneau , 1753.

Pierre Bouguer (born February 16, 1698 in Le Croisic , † August 15, 1758 in Paris ) was a French astronomer , geodesist and physicist . He became internationally known as a participant in the Peru Expedition, which the Paris Academy carried out to precisely determine the figure of the earth .

 The geophysically important Bouguer gravity anomalies were named after Bouguer in the 19th century because of his investigations into the earth's gravitational field . He was also a professor of marine studies at Le Havre .

Life and scientific work

Bouguer studied at the Jesuit College in Vannes , the capital of the Breton department of Morbihan , and later at the University of Paris . His astronomical research dealt with the photometry of the sun and moon ; He developed the heliometer for precise angle measurements in astrometry .

In geodesy and geophysics he is primarily known for the gravity anomalies named after him , for the first investigations of the vertical deviation as well as the vertical gradient and the large degree measurement of long meridian arcs in South America. The latter took place from 1735 to 1741 in what was then the Viceroyalty of New Granada at the instigation of the Paris Academy ( Académie des sciences ). With the expedition to Swedish Lapland carried out by Pierre-Louis Moreau de Maupertuis , Alexis-Claude Clairaut , Anders Celsius and others in 1736, she had the goal of clarifying the question of whether the polar radius of the earth is larger than the equatorial radius, as some older ones and contemporary measurements u. a. by Jean-Dominique Cassini , or vice versa, as would be expected from Newton's theory; these measurements should also be significant for the later definition of the meter . Bouguer's colleagues were Louis Godin (leader of the expedition) and Charles Marie de La Condamine . However, there was considerable tension between the three scientists, which ultimately led to the split-up of the expedition team. The degree measurement along a profile extending from slightly north of Quito to slightly south of Cuenca resulted in the length of one degree of longitude of the earth at the equator 56753 Toisen (110.612 km) and an equatorial radius of the earth of 3281013 Toisen (6394.694 km ). With the arctic measurements of Maupertuis the flattening of the earth was found to be 1: 179 (modern value 1: 298.25), but was improved to 1: 305 a few years later by measurements by Cassini in France.

Bouguer wrote several books about his research; his best-known work is La figure de la terre: déterminée par les observations de messieurs Bouguer, & de la Condamine (Paris 1749). In the last part of the book, in which he deals with gravity measurements, he also applies the two reductions for the first time when processing data from the summit of Pichincha , Quito and the coast, which are known today as the open-air and the Bouguer reduction are. For a long time, La Condamine and Bouguer had a bitter dispute about the results of the expedition, which only ended with the death of Bouguer.

With his book Traité du navire, de sa construction et de ses mouvemens (1746), Bouguer laid the foundations for the hydrostatic constitution, which Archimedes had already established, but which was lost again, alongside Euler . Bouguer also dealt with the navigation and steering of ships. In 1731, for example, he wrote a textbook Manière d'observer en mer la déclinaison de la boussole , which deals with navigation by compass.

Through his investigations into light intensity , Bouguer became the founder of photometry . He wrote an account of these investigations in the Essai d'optique, sur la gradation de la lumière (Paris 1729) and even more extensively in the Traité d'optique sur la gradation de la lumière , which was only published by Lacaille after his death in 1760 . Bouguer also invented the heliometer in 1748 .

In the field of geodesy he wrote the work Traité de navigation (Paris 1753), which was substantially supplemented by Lacaille in the second edition ( 1769) and by Lalande in the third edition (1792). He made the first observations in the vicinity of Chimborazo about the deviation of the lead in the earth's gravitational field due to the attraction of the mountains and about the height of the snow line .

In 1750 he was elected a member ( Fellow ) of the Royal Society . He was a member of the Académie des siences since 1731. The lunar crater Bouguer , the Martian crater Bouguer and the asteroid (8190) Bouguer are named after him.

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personal data
SURNAME Bouguer, Pierre
BRIEF DESCRIPTION French geodesist and astronomer
DATE OF BIRTH February 16, 1698
PLACE OF BIRTH Le Croisic , Brittany
DATE OF DEATH August 15, 1758
Place of death Paris