Wednesday, August 10, 2011

The Gnomon of Saint-Sulpice

One of the wonderful things about our long stay in Paris and hosting visitors at different times is that I have the opportunity to revisit places I've been, and re-photograph them in different ways and at different times.  Returning to Saint Sulpice afforded one of those opportunities.  I have been to Saint Sulpice three times now - once with Ron, once with Beth and this time with Chris and Jeff and each visit was different. For this visit, I decided to focus on the gnomon.

The Gnomon of Saint Sulpice
The gnomon of Saint Sulpice has an air of mystery surrounding it thanks to the author Dan Brown and The DaVinci Code.  The Church of Saint Sulpice and this gnomon are featured in the novel as part of the search for the holy grail.  Due to the popularity of the novel and movie though, the church has posted a sign highlighting the fictional aspects associated with the church and gnomon.  The gnomon of Saint Sulpice is not a pagan astronomical instrument and it is not a vestige of an ancient pagan temple that stood in the same spot as the Church of Saint Sulpice (the pagan temple is also refuted) and the obelisk is not Egyptian.  The Saint Sulpice meridian is not a rose line nor is it the same as the Paris meridian as they are several hundred meters apart, so the zero longitude has never passed through the Church of Saint Sulpice.  That's the fiction.

The facts are:
  1. The gnomon is an astronomical measurement device designed to cast a spot of sunlight on the floor in order to determine the position of the sun in the sky
  2. There are three other gnomons like this one built in Italian churches
  3. Gnomons were replaced with powerful telescopes
  4. The Saint Sulpice meridian is a strict north-south axis marked by a brass line set into marble on the floor of the church
  5. Sunlight passes through a small hole 25 meters high in a window in the south transept
  6. At noon each day, as sunlight hits the hole in the window, a small spot of light is cast on the brass line
  7. The brass line spans the width of the transept, starting at a plaque on the floor on the south side and ending on the gnomon on the north side
  8. The spot of sunlight is cast on the plaque on the south side in summer, then moves along the line to a gold disk in the floor at the altar at the equinox and then continues along the line to a point high on the obelisk in winter
The plaque on the south transept floor and the meridian line leading to the gnomon
for the nutation of the earth's axis and the obliquity of the ecliptic
The decorative rail of the altar, making it difficult to see the equinox marker
A brass disk in the center of a brass semi-circle marks the equinox
The brass line continues from the altar to the north transept
The brass line moves up the wall and is embedded in the midline of the obelisk; text mentioning God, the Kings and his Ministers were deleted during the French Revolution. The top left text says, "To determine precisely the Paschal (Easter) Equinox"

The brass median line embedded in the obelisk

The top of the obelisk
And finally, an alligator detail on the obelisk <shrug>
So it seems that the gnomon is really just a clock.

Jean-Baptiste Languet de Gergy, a parish priest at the Church of Saint Sulpice, wanted to establish the exact astrological time so that the church bells could be rung at the right time of day.  I guess that could be sort of important if the people around the church used the ringing bells to determine what time it was.  I mean, it would be pretty cool to have a much longer lunch hour in the summer and all, but I can see where it would be rather confusing for the bell ringer when there are only 8 hours of daylight in December and more than 16 hours of daylight in June.  So to ensure the church bells rang at the appropriate hour, Languet de Gergy commissioned Henry Sully, an English clockmaker who lived in France, to build the gnomon.  Unfortunately for Sully, he died in 1728, and was only able to complete the meridian line in the floor of the church.

The meridian line and the little hole in the south transept window were enough to allow Languet de Gergy to ring his bells on time.  But he was vexed by another problem.  When in the sam hill is Easter?  Since Easter falls on the first Sunday after the full moon following the vernal equinox in the Northern Hemisphere, Languet de Gergy had another pesky date and time problem  because Roman Christians were using a Julian calendar then, not a lunar calendar.  By the Julian calendar, Languet de Gergy would be celebrating Easter about 10 days late.  What's a priest to do?  It just wouldn't be right for a church that can ring it's bells on time to celebrate Easter on the wrong day.  So In 1742, Languet de Gergy commissioned Pierre Charles Claude Le Monnier, a French astronomer, to complete the gnomon project and include the ability to properly define the Easter Equinox.  Problem solved! 

I find it fascinating that a gnomon was built and all kinds of sciency stuff was carried out in a church at the same time that Gallileo's works were still censored and prohibited by the Pope in Rome.  Being about 700 miles from Rome must have had its privileges, but I would bet that Paris being the center for the Age of Enlightenment had a much bigger influence.

And once again, I couldn't resist the candles!

After our tour of the church, it was time for a coffee break!

We had intended on coffee but ended up with a fabulous hot chocolate instead!
Soaking up the atmosphere at a sidewalk cafe
Chris and Jeff at Cafe Conti