Defining the form of the Earth with the help of pendulums in 1765–1815

Abstract The article treats the first 50 years’ history of the use of the seconds pendulum for defining the form of the earth. It starts at the late 18th century and continues to the early 19th century. Section 2 derives the relationship between gravity at a given latitude and the length of the simple pendulum at the same latitude. Then Sect. 3 deals with the relationship between the gravitational flattening and the geometrical flattening known as Clairaut’s formula. Sects. 4–6 report the historical data analyses made by Lambert, Laplace and Hällström. The Appendix explains the statistical methods applied to pendulum data and provides the list of data sources.