The Birthday of Jean‑Baptiste Delambre (1749) – Arc Measurement

Delambre and Méchain measured the meridian arc from Dunkirk to Barcelona (1792–1799) to define the metre. They used triangulation: a chain of triangles between landmarks, measuring base lines with rods, then angles with a repeating circle (invented by Borda). This required solving spherical triangles (the Earth is not flat), using the law of cosines and haversine formula. The final metre was defined as one ten‑millionth of the quarter meridian. Their work was the largest geodetic survey of its era, accurate to about 5 cm over 1,000 km.