Gnomon of Saint-Sulpice, Astronomical instrument in Saint-Sulpice Church, France

The Gnomon of Saint-Sulpice is an astronomical device spanning the transept of the church. A brass line set in white marble runs north-south across the floor; sunlight enters through a round opening in the south stained-glass window 25 metres up and forms a disc that crosses the line at true noon. At the line's north end stands a pale stone obelisk on a marble base, lit near its top at the winter solstice.

Jean-Baptiste Languet de Gergy, parish priest from 1714 to 1748, commissioned the gnomon to set the exact astronomical time for ringing the bells. The English clockmaker Henry Sully laid the meridian line in the floor but died in 1728 before completing the work; the Paris Observatory finished it a year later. In 1742 Languet de Gergy resumed the project with Pierre Charles Le Monnier to fix the date of Easter, and from 1744 Le Monnier used it to study the obliquity of the ecliptic.

The Latin inscription on the obelisk base, "Ad Certam Paschalis Æquinoctii Explorationem", states the device's purpose: to determine the exact date of the Paschal equinox and thus Easter. The meridian line is not the Paris Meridian of 1667, which runs a few hundred metres east through the Paris Observatory.

The meridian runs across the floor of the transept, with the marble plaque at its southern end and the obelisk at the northern end; the round window opening sits 25 metres up in the south stained-glass window. Look for the small light disc crossing the brass line at true noon, clearest on sunny days. The line is not the Paris Meridian, which lies a few hundred metres east.

The obelisk at the north end exists only because the winter-solstice light would otherwise strike the floor about 20 metres beyond the church wall. Its top is lit at the lowest noon sun, 17°42′, while the square marble plaque at the south end marks the summer-solstice height of 64°35′.