Gnomon astronomique

Hidden within the grandeur of the Saint-Sulpice Church lies a scientific marvel, the Gnomon astronomique. This isn’t just an astronomical instrument. It’s a testament to a time when science and religion intertwined. The Gnomon astronomique, commissioned by Jean-Baptiste Languet de Gergy, the parish priest, was originally intended to establish the precise time for ringing the church bells. Imagine him, standing in this vast space, listening for the perfect moment to resonate the call to prayer. The Gnomon astronomique stretches across the transept of the church. A brass line embedded in white marble marks the meridian, a north-south axis. A small opening in the southern stained-glass window, 25 meters high, allows a beam of sunlight to create a disc of light on the floor. As the sun reaches its zenith at true noon, this disc crosses the meridian. A gold disc marks the sun’s position during the equinox, directly in front of the altar. At one end of the meridian, a square marble plaque marks the sun’s highest position at midday during the summer solstice. At the other end stands an obelisk, catching the sunlight near its top when the sun is at its lowest during midday. The Gnomon astronomique could also have been used to calibrate clocks by determining mean time, the average of true time shown by sundials. The Gnomon astronomique at Saint-Sulpice has a fascinating history intertwined with the determination of Easter. Languet de Gergy wanted to verify the vernal equinox’s exact date to accurately calculate Easter. Pierre Charles Le Monnier used the Gnomon astronomique to study the variations of the ecliptic, the Earth’s axial tilt. The inscription at the base of the obelisk reflects this purpose. He also used the gnomon to determine the Earth’s perihelion, its closest point to the sun, by observing the size of the sun’s image on the obelisk. The Gnomon astronomique at Saint-Sulpice, more than just a timekeeping device, embodies the intersection of science and faith in the heart of Paris.