Science in Saint-Sulpice

I was totally fascinated by the Gnomon of Saint-Sulpice and went right down the rabbit hole.

Please skip this post rather than roll your eyes at me – OR – join me on this investigation into 18th-century technology.

For even more info see: Gnomon of Saint-Sulpice (Wikipedia)


The gnomon at Saint-Sulpice is a scientific instrument used for precise astronomical observations including:

  • determining the equinoxes,
  • measuring solar noon (useful for calibrating watches),
  • tracking the apparent movement of the Sun (it’s actually the Earth that is moving),
  • verification of the Gregorian calendar (adopted in France in 1582).

In reading about the gnomon it quickly became clear that I was hazy about some terms that cropped up repeatedly.

  • Solstice => the time or date (twice each year) at which the sun reaches its maximum or minimum declination, marked by the longest and shortest days (about 21 June and 22 December in the northern hemisphere).

  • Equinox => the time or date (twice each year) at which the sun crosses the celestial equator, when day and night are of approximately equal length (about 22 September and 20 March in the northern hemisphere).

  • Easter => celebrated between 21 March and 25 April, on the first Sunday after the first full moon following the northern spring equinox.


A Bit of Background

Jean-Baptiste Languet de Gergy, the parish priest at Saint-Sulpice from 1714 to 1748, had the gnomon built. Initially he wanted to establish the exact astronomical time in order to ring the bells at the most appropriate time of day.

Henry Sully, an English clockmaker and inventor, was commissioned to construct the gnomon. He started work in 1727 but died the following year without completing the project.

In 1742 Pierre Charles Le Monnier, a member of the French Academy of Sciences, resumed the project and completed it in 1743.

Cathy sitting near the meridian line running along the floor.


Dan Brown Made Stuff Up

Dan Brown immortalised the gnomon in The Da Vinci Code. But he used a lot of artistic licence. Many tourist visit the church because of the book, but there is a cautionary note on display:

Contrary to fanciful allegations in a recent best-selling novel, this [the line in the floor] is not a vestige of a pagan temple.

No such temple ever existed in this place.

It was never called a «Rose-Line».

It does not coincide with the meridian traced through the middle of the Paris Observatory which serves as a reference for maps where longitudes are measured in degrees East or West of Paris.

Please also note that the letters «P» and «S» in the small round windows at both ends of the transept refer to Peter and Sulpice, the patron saints of the church, not an imaginary «Priory of Sion».


So What is the Meridian Line?

The start of the meridian line below the south transept window.

The meridian line runs on a north-south axis.

A beam of sunlight passes through a hole in the southern stained-glass window. This sunbeam will cross the meridian line each time the sun reaches its zenith at true noon.

It crosses different parts of the meridian line at different times of the year.

There is a gold disc (in front of the altar) that shows where the sunbeam will cross the meridian line when the sun is at the an equinox.

Note: the sunbeam crosses the meridian line at true local midday at the location of Saint-Sulpice (which will be different to ‘official’ Paris time).


Gnomon structure at Saint Sulpice.

The square marble plaque (bottom right) is crossed when the sun is at its highest point during the summer solstice (around 21 June).

The top of the obelisk (left) is crossed when the sun is at its highest point during winter solstice (around 22 December).

The gold disc (centre) is crossed when the sun is at its highest point during spring equinox (around March 21).

The south transept window is shown top right.


The Official Explanation

Here is a translation of the information board provided by the church:

The Astronomical Gnomon of Saint-Sulpice

The “gnomon” of Saint-Sulpice is a sophisticated scientific device installed here in 1743 by astronomers from the Paris Observatory in collaboration with the parish priest.

A plaque fixed high on the wall to the right of the south transept window has a small hole in it. Directly beneath this opening, a “meridian line” is embedded in the church’s floor, marked by a brass ruler that extends onto a trompe l’oeil obelisk placed at the end of the north transept.

During its daily journey from east to west, the sun’s beam appears on the ground at the base of the obelisk, through the window’s oculus, as a luminous disk that moves in the opposite direction.

It crosses the line at noon, local solar time, which varies with the seasons depending on the sun’s position, higher or lower above the horizon. The sun’s movements correspond to the apparent movements of the earth in the sky, affected by the earth’s rotation on its axis and its annual orbit.

The gnomon indirectly allows for precise measurement of various orbital parameters and variations in the “equation of time” (the difference between “mean time” and “true time”) resulting from these measurements.

This device draws on work carried out in Italy in the 16th century, initiated by Pope Gregory XIII, to establish the current “Gregorian” calendar, specifying the date of the spring equinox, which also determines the date of Easter.

The gnomon also helped to demonstrate the change in the angle of the earth’s axis relative to the plane of its orbit around the sun, causing seasonal changes. The tilt of this axis decreases by one-hundredth of a degree per century. Though extremely slight, this variation results in a measurable change, made possible by the installation of the instrument in a large building such as Saint-Sulpice.

Contrary to some claims, the meridian line traced on the floor of Saint-Sulpice is not aligned with the Paris meridian, which runs slightly further east. It has never served as a reference for longitude calculations.

The first true meridian was established on the Greenwich meridian in England, adopted in 1884 at an international conference. It also forms the basis for “Greenwich Mean Time” (GMT). The solar noon at Saint-Sulpice is about 12 minutes and 50.39 seconds in winter and 13 minutes and 50.39 seconds in summer, according to French legal time.

The Wikipedia entry for Gnomon of Saint-Sulpice is a bit easier to understand than the translation above. I have relied heavily on it in writing this post.

👉 What I find so fascinating is that the church is part of the scientific instrument. It needed a large building to be able to measure the extremely slight variations in the tilt of the earth’s axis.


The Parts of the Gnomon

South transept window.

I don’t know why there are two holes.

The square marble plaque and the meridian line passing through the front of the altar area. The obelisk is in the background.

The meridian line passing through the gold disc in the altar area.

I guess it was part of the design that this should be right in front of the altar.

The obelisk at the north transcept.

If the obelisk did not exist, the sunbeam would hit an area about 20 metres beyond the wall of the church.


The Inscriptions Below the Obelisk

Inscription at bottom left of obelisk.

AI translation:

Gnomon Astronomicus

For the Certain Exploration of the Paschal [Easter] Equinox

What St. Martyr and Bishop Hippolytus Adorns. What the Nicene Council Demanded from Patriarch Alexander. What the Fathers of Constantinople and Lateran were Concerned with. What Among the Roman Pontiffs Gregory XIII and Clement XI, with Incredible Effort and with the Industry of Learned Astronomers, Tried to Accomplish. This Gnomon Emulates with a Subducted Meridian Line and an Equinoctial Point, Certain Solar Period Indices.

Latin:
What is there for me in Heaven? And what do I desire upon earth, besides you? God of my heart and my portion, God forever. (Psalm 73:25-26)

French:
What should I seek in Heaven, and what should I desire on earth, if not You, O Lord? You are the God of my heart and the inheritance I hope for Eternity. (Psalm 73:25-26)

Inscription at bottom right of obelisk.

AI translation:

Work Dedicated to God, the Best and Greatest

Executed by the Scientific Academy in the Name and with the Advice of P. C. C. Le Monnier, of the Same Academy and of London, Fellow. From the Autumnal Equinox to the Winter Solstice Completed Year of Salvation 1743.

Latin:
Behold, thou hast made my days measurable, and my substance is as nothing before thee. (Psalm 39:5)

French:
Thus, Lord, you have set limits to our days, and all our life is nothing in your eyes.

There are Biblical quotes to cover just about any situation.


But What If It’s Cloudy?

I did not find any references to what happens if it’s cloudy or raining heavily.

I guess they would make daily readings when it was sunny and keep good records of their observations. By averaging these observations over time (years ?) they would still have pretty accurate results for any given day.

My AI buddy says:

Gnomons and similar instruments are not intended to provide a single, instantaneous measurement but rather to collect data over extended periods. Consistent observations over many days, months, and years help to build an accurate understanding despite occasional poor weather.


What a testament to human ingenuity to conceive and build a gnomon in this church.

Another product of the Age of Enlightenment.


Stay tuned for more adventures on our European Odyssey!