The Problem of the Earth's Figure: Measurement, Theory, and Evidence in Physical Geodesy

This thesis tells the story of a surprisingly difficult problem in gravitational physics: deriving and measuring the shape of our planet. Chapter 1 reconstructs the emergence of gravitational theories of planetary equilibrium figures and quantitative measurements of the Earth’s shape and surface gravity (c. 1660-1730). I show that early geodesists made substantial empirical progress despite deep theoretical disagreements about the nature of gravitation. Chapter 2 reconstructs how geodesy came to offer a decisive test of the interactive nature and compositionality of gravitation (c. 1740-1825). I retell how Pierre-Simon Laplace established that the Earth’s external attraction is the resultant force of the mutual attractions between its constituent particles; a momentous but overlooked achievement in the history of gravitational physics. Chapter 3 traces the numerical conflicts in the measurement of the central quantity characterising Earth’s figure: its ellipticity. These conflicts persisted throughout the nineteenth century and complicated the coordination between geodetic models and measurements. Chapter 4 reconstructs how geodesists finally achieved convergent measurements of Earth’s ellipticity between 1880 and 1924. Jointly, the four chapters illustrate key steps in obtaining strong evidence for quantitative claims about a complex and partially inaccessible system: using measurements to initiate theoretical inquiry (chapter 1), using measurements to conduct severe tests of theoretical laws (chapter 2), using discordant measurements to study second-order phenomena (chapter 3), and using multiple alternative measures to identify the sources of persistent errors (chapter 4).