Gnomon structure at Saint Sulpice
The gnomon of Saint-Sulpice is composed of different parts that span the breadth of the transept of the church. The Church itself is a huge building, the second largest church in Paris after Notre-Dame de Paris.
The system is first built around a meridian, a line which is strictly oriented along the north-south axis, represented by a brass line set in a strip of white marble on the floor of the church. This is not the Paris Meridian, established by Louis XIV in 1667, which is located a few hundred metres to the east and goes through the Observatory of Paris.
Gnomon hole in the stained glass window at Saint Sulpice
The sunlight passes through a small round opening in the southern stained-glass window of the transept, at a height of 25 metres, forming a small light disk on the floor; this disk will cross the meridian each time the sun reaches its zenith at true noon. The sun will cross different parts of the meridian depending the time of year, as the sun will be more or less high in the sky at noon. A point on the meridian is marked with a gold disk which shows the position of the sun at an equinox. It is located right in front of the altar.
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.
At the other end is an obelisk, which is lit near its top when the sun is at it lowest at midday (17°42' at the location of Saint-Sulpice). If the obelisk did not exist, the sun disk would hit an area about 20 metres beyond the wall of the church.