Born in Rumigny, near Reims, studied in Nantes and, from 1729, in Paris. After his ordination as abbot, became interested in mathematics and astronomy. In 1737, joined the Paris Observatory. In 1738, began a series of geodesic assignments, including the measurement of the meridian arc between Perpignan and Dunkerque in 1739. Continued working on the subject until he demonstrated that the meridian arcs are longer at the equator than at the poles. His finding that the Earth is an oblate spheroid (i.e., slightly flattened at the poles) won him election to the Académie Royale des Sciences in 1741. In 1746, Lacaille was given the use of the observatory of the Collège de Lisieux, where he surveyed many types of astronomical phenomena. Promoted a scientific expedition to study the southern constellations: in late 1750, left for the Cape of Good Hope, which he reached the following year. From a small observatory built for the occasion, Lacaille measured the positions of almost 10,000 stars of the southern sky, creating 14 new constellations. In 1753, before returning to Europe, determined the position of today's Mauritius and Réunion islands, then French colonies. On his return to France in 1754, prepared the data gathered during his voyage and published several works, including Astronomiae fundamenta (Paris, 1757). His premature death was due to the rigor with which, in 1760, he undertook a new observation project to obtain accurate measurements of the positions of the zodiacal stars.