RAS public lecture, 14th January 2014

by Dr Simon Mitton, Department of the History and Philosophy of Science, University of Cambridge

For more than three millennia watchers of the skies have used geometry as a means of understanding the mechanism of the heavens. In Egypt and Mesopotamia classical surveying techniques were essential for the stellar alignments of the largest structures in the ancient world. Geometry allowed the ancients to measure the size of the Earth, and to attempt to measure the relative distances of the Sun and Moon. In his great book the *Almagest*, Claudius Ptolemy (second century AD) describes a geometrical model of the solar system that served as the standard work for 1300 years. In the thirteenth century Robert Grosseteste of Lincoln wrote *De Luce*, the first attempt at describing the origin of the universe in geometrical language. Copernicus, Galileo and Newton all employed geometry as the essential tool for doing cosmology. In 1915 Einstein published the general theory of relativity, a classical theory of gravity using four dimensional geometry, and this continues to be the preferred mathematical tool for modelling the universe. This is a non-technical lecture with an emphasis on the remarkable astronomers who used geometry to advance our understanding of the universe