French astronomer who determined the positions of nearly 10,000 stars of the southern hemisphere 1750-54. He also performed a number of geodetic investigations; in particular, he made the first measurement of the arc of meridian in the southern hemisphere.
Lacaille was born in Rumigny, near Rheims, and studied theology in Paris. He began to make astronomical observations in 1737 and participated in two Academy of Sciences projects, in 1738 and 1739. At the age of 26 he became professor of mathematics at the Collège Mazarin (now the Institut de France), Paris. He was subsequently given an observatory.
Despite the lack of equipment at the Cape of Good Hope, South Africa, Lacaille catalogued a large number of seventh-magnitude stars, invisible to the naked eye. He charted and named 14 new constellations, abandoning the traditional mythological naming system and instead choosing names of contemporary scientific and astronomical instruments.
Simultaneous observations by Lacaille at the Cape and Joseph de Lalande in Berlin provided them with a baseline longer than the radius of the Earth and gave an accurate measurement of the lunar parallax.
Coelum australe stelliferum 1763 catalogued all Lacaille's data from the southern hemisphere.