Gnomon of Saint-Sulpice

It is a gnomon, a device designed to cast a shadow on the ground in order to determine the position of the sun in the sky.

In early modern times, other gnomons were also built in several Italian and French churches in order to better calculate astronomical events.

[5] At one end of the meridian is a square marble plaque, which corresponds to the position of the sun at the highest at midday (64°35' at the location of Saint-Sulpice), during the summer solstice about 21 June.

[4] The time the sun disk crosses the Saint-Sulpice meridian gives the "true" local midday at that place.

[4] After this first attempt, Languet de Gergy resumed the project in 1742, this time with the objective of properly defining the Easter Equinox.

[2][5][6] The inscription at the base of the obelisk mentions Charles Claude Le Monnier, as well as the mission of the gnomon in Latin: "Ad Certam Paschalis Æquinoctii Explorationem" ("To determine precisely the Paschal Equinox").

Since the Council of Nicaea in 325, the Western Church had required that Easter be celebrated on the Sunday on or after the full moon following March 21, which at that time corresponded indeed to the vernal equinox.

[5] The endeavour is recorded on the plaque at the southern end of the meridian, in the South transept: "Pro nutatione axios terren.

[5] Perihelion occurs close to the winter solstice, during the period of the year when the sun's image at noon is on the obelisk, rather than on the floor of the church.

[7] A similar gnomon built to calculate the exact date of Easter also exists in the Santa Maria degli Angeli e dei Martiri in Rome.

The obelisk portion of the gnomon of Saint-Sulpice Church , with the meridian line in the middle
Gnomon structure at Saint-Sulpice
Gnomon hole in the stained glass window at Saint-Sulpice
The equation of time — above the axis the sundial will appear fast when compared with a clock, and below it the sundial will appear slow .
Latin and French inscriptions at the base of the obelisk. The mentions of the King and his Ministers were deleted. The name P.C.CL.LE MONNIER survives above right after the deletion.
Central equinoctial marker
The plaque at the southern end of the meridian, mentioning the work on the obliquity of the ecliptic
The obelisk (in the background), the meridian brass line on the floor, and the southern plaque