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'Jean Baptiste Joseph Delambre' (September 19, 1749 in Amiens – August 19, 1822 in Paris) was a French mathematician and astronomer.
Delambre was born in Amiens, France.
After a childhood fever, Delambre suffered from very sensitive eyes and the belief that he would soon go blind. For fear of losing his ability to read, he devoured any book available to him and practised his ability to memorise. He thus immersed himself in Greek and Latin literature, acquired the ability to recall verbatim entire pages of books he may have read weeks beforehand, became fluent in Italian, English and German and even published ''Règles et méthodes faciles pour apprendre la langue anglaise'' (Rules and methods to easily learn English).
In order to establish a universally accepted foundation for the definition of measures, in 1790 the National Constituant Assembly asked the French Academy of Sciences to introduce a new unit of measurement. The academics decided on the metre, defined as 1 / 10,000,000 of the distance from the North Pole to the equator, and prepared to organize an expedition to measure the length of the meridian between Dunkerque and Barcelona. This portion of the meridian, which also passes through Paris, was to serve as the basis for the length of the quarter meridian, connecting the North Pole with the Equator. In April 1791, the academy's Metric Commission confided this mission to Jean-Dominique de Cassini, Adrien-Marie Legendre and Pierre Méchain. Cassini was chosen to head the northern expedition but, as a royalist, he refused to serve under the revolutionary government after the arrest of King Louis XVI on his Flight to Varennes. On February 15, 1792, Delambre was elected unanimously a member of the French Academy of Sciences and in May 1792, after Cassini's final refusal, was placed in charge of the northern expedition, measuring the meridian between Paris and Dunkirk. Pierre Méchain headed the southern expedition, measuring between Paris and Barcelona. The task was completed only in 1799.
Named director of the Paris Observatory and professor at the Collège de France, Delambre was one of the first astronomers to derive astronomical equations from analytical formulas, was the author of Delambre's Analogies and, after the age of 70, also the author of works on the history of astronomy like ''Histoire de l'astronomie''.
Delambre died in 1822 and was interred in the Père Lachaise Cemetery in Paris.
Delambre crater on the Moon is named for him.

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★ A history article on Delambre