French astronomer and hydrographer who, with Jean Delambre (Delambre, Jean-Baptiste-Joseph), measured the meridian arc from Dunkirk, Fr., to Barcelona. The measurement was made between 1792 and 1798 to establish a basis for the unit of length in the metric system called for by the French national legislature. Mechain also discovered 11 comets (comet) and calculated the orbits of these and other known comets.

Born the son of a master ceiling plasterer, Mechain early in life showed mathematical prowess and worked as a hydrographer for the Naval Map Archives at Versailles during the 1770s. He turned to astronomy, and in 1782 his work with comets won him admission to the Académie Royale des Sciences. In addition, Mechain discovered numerous nebulae that were later incorporated by Charles Messier into his famous catalog of clusters and nebulae.