Time waits for gnomon

During the turbulent years that followed the French Revolution, the Church of Saint-Sulpice in Paris may have benefited from the wisdom of one its previous pastors who, half a century earlier, had placed an each-way bet on science and religion.

That wasn’t quite the intention of Jean-Baptiste Languet de Gergy. His main concern was the calculation of the spring equinox, and from that Easter. But in this cause, he commissioned scientists to construct an elaborate gnomon – a sun-dial, essentially – whose applications went well beyond his basic requirement.

The key to the gnomon was a north-sound meridian traced across the church floor, marked by a thin inlaid line of brass, and cutting through the sanctuary in front of the altar. On one side, the brass line climbed a 16-metre marble obelisk. On the other, it was overlooked, from 25 metres up, by a small hole in a window, fitted with a lens.

Through this lens, the noon sun cast a disc of light on the floor that crossed the line at a different point each day, ranging between the winter solstice, when the sun at its lowest hit the top of obelisk, to June 21st, when at its highest, it pointed to a marble plaque in the floor of the south transept.

In between, the spring and autumn equinoxes were delineated by a copper plaque in front of the altar, where the sunbeam fell at noon on those days. So accurate was the instrument, however, that it also shed light – literally and metaphorically – on more difficult astronomical problems. Thus the Cassinis, a father and son who were successive directors of the Paris Observatory, used the gnomon to calculate the decreasing “obliquity of the ecliptic”, the minute but measurable shifting of the Earth’s axis over time.

They put it at 45 seconds a century, creditably close to the figure arrived at by today’s much more sophisticated methods.

The church’s accidental respectability among atheists, thanks to the gnomon, gave it an argument against worse treatment during the Revolution, when religion fell from favour.

Even so, it still suffered much damage. The main altar was destroyed, most objects of value stolen, and the crypt vandalised.

Of potentially mobile treasures, only the pipe organ – one of the world’s greatest – survived, thanks to a quick-witted church employee who put fake seals on the loft door.

(From the same door, last Sunday, I saw the St Sulpice deputy organist Sophie-Véronique Cauchefer-Choplin emerge after a recital to be greeted by a group of admirers the like of which you might see at the backstage entrance of a rock concert. Talking to her briefly, I learned she was just returned from playing at the Irish Organ Festival in various venues, north and south. She spoke very highly of a pipe organ in Dundalk.)

In 1791, after a period of uneasy cohabitation with the Revolution, the church ceased to be a place of religion and was used instead as a hall for public meetings. It must have been quite a hall – among Paris churches, only Notre Dame is bigger. There then followed a brief interlude during which it again became a place of worship, but under the new “Cult of the Supreme Being”. This was essentially a religion invented by Robespierre and, like Robespierre, it would not last long.

In fact, the grim progress of the revolution can be summed up in the details of a wedding Robespierre attended there in 1790. The bridegroom was Camille Desmoulins, journalist and revolutionary, whose public rejection of the Catholic Church made the priest at first reluctant to perform the ceremony.

Then Desmoulins had his way, and the stellar line-up of guests included his two witnesses – the aforesaid Robespierre and yet another revolutionary, the leader of the moderate Girondist faction, Jacques Pierre Brissot.

Within three years, Desmoulins sent Brissot to the guillotine. But to paraphrase an old Irish saying, the execution of one wedding witness is sometimes the making of another. A few months later, Robespierre had Desmoulins beheaded. Then Robespierre in turn was given an appointment with Dr Guillotine, and his short-lived religion was decapitated with him.

For a time in more recent years, the church attracted worshippers from an even more esoteric cult, the Church of Dan Brown. This was the result of a highly spurious sub-plot in the Da Vinci Code. But that too is now waning.

The gnomon remains fully intact, meanwhile. So does the magnificent organ. And of the church’s most turbulent years, the only hint these days is a barely legible inscription over the main entrance. A summary of the new religion of 1794, it reads simply: “The People of France recognise the Supreme Being and the immortality of the soul.”

@FrankmcnallyIT

fmcnally@irishtimes.com