Gnomon of Saint-Sulpice

A thin brass line runs across the floor of the Saint-Sulpice church, cutting through the stone pavement toward a white marble obelisk. It is a solar meridian, a gnomon built in seventeen twenty-seven. A tiny hole in a window pane high above lets a sliver of sunlight strike the brass line at noon. The astronomer Pierre Charles Le Monnier used this instrument to calculate the exact timing of the equinoxes, aiming to settle long-standing disputes over the date of Easter.