Gnomon of Saint-Sulpice
Encyclopedia
The Gnomon of Saint-Sulpice is an astronomical
measurement device located in the Church of Saint-Sulpice (Église Saint-Sulpice) in Paris
, France. 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 astrological events. Those churches are Santa Maria del Fiore
in Florence
, San Petronio
in Bologna
, and the Church of the Certosa in Rome
. These gnomons ultimately fell into disuse with the advent of powerful telescope
s.
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 meters to the east and goes through the Observatory of Paris.
The sun
light passes through a small round opening in the southern stained-glass window of the transept, at a height of 25 meters, forming a small light disk on the floor; this disk will cross the meridian each time the sun reaches its apogee 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. The middle of the meridian is marked with a gold disk which shows the position of the sun during the 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
on 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 meters beyond the wall of the church.
, the parish priest at Saint-Sulpice from 1714 to 1748. Languet de Gergy initially wished to establish the exact astrological time in order to ring the bells at the most appropriate time of day. For this, he commissioned the English clockmaker Henry Sully
to build the gnomon.
The gnomon could also have been used to properly time clocks by properly defining mean time. In effect mean time (the time used in clocks) in only an average of true time, in which true noon deviates from the mechanical average of a clock by as much +/- 16 minutes throughout the year. This is due to the motion of the earth along an ellipse
around the sun in one year, which adds or subtracts to the earth natural rotation the moment
of the elliptical movement. These variations are codified in the equation of time
. Henry Sully however died in 1728 without being able to accomplish this larger project. He was only able to set the meridian line in the floor of the Church. The project was completed by the nearby Observatory of Paris a year later.
Alternatively, a simpler solution would be to consult an almanac
giving the time of sunrise and sundown, calculate the middle point of that time-span, corresponding to the maximum elevation of the sun. This gives the official time at which the sun reaches it maximum elevation, and therefore the time the sun disk crosses the Saint-Sulpice meridian.
Equinox
. The task was given to Pierre-Charles Le Monnier, member of the French Academy of Sciences
.
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 AEquinoctii Explorationem" ("To determine precisely the Paschal Equinox").
Easter
corresponds to the Jewish Passover
, which marks the liberation of the Jews from Egypt, and traditionally falls in the Jewish lunar calendar on the 14th of the month of Nisan
, the day of the first full moon after the Spring equinox. Roman Christians however had a twelve month calendar, first the Julian calendar
until 1582 and then the Gregorian calendar
, which were unrelated to the lunar calendar. Since the Council of Nicaea
in 325, the date of Easter had been decided to be the first full moon after March 21, which at that time corresponded indeed to the spring equinox. The Julian calendar being imprecise however, in the 16th century March 21 fell about 10 days after the Spring equinox, a problem which was solved by the Gregorian calendar. Languet de Gergy, however, wished to verify independently the exact date of the Spring equinox through the gnomon in order to ascertain the date of Easter.
, or the variations in the obliquity of the Earth's axis
. The endeavor is recorded on the plaque at the southern end of the meridian, in the South transept: "Pro nutatione axios terren. obliquitate eclipticae" ("for the nutation
of the earth's axis and the obliquity of the ecliptic").
As mentioned on a brass plaque that covered the stone plaque, the maximum obliquity of the ecliptic was 23°28'40".69 in 1744. From 1745 to 1791, Le Monnier visited Saint-Suplice at the moment of the summer solstice and, focusing the light with a lens fixed to the opening in the stained-glass window, noted when the light fell at noon. The measurements allowed to confirm a variation of 45' per century (the exact figure is 46".85 per century).
Some interpretations of the gnomon at Saint-Sulpice give it an occult
meaning. The author Dan Brown
in The Da Vinci Code
describes it as "a pagan astronomical instrument (...) an ancient sundial of sorts, vestige of the pagan temple that had once stood on this very spot", despite an early modern building date of 1714, and the fact that it is an astronomical device with nothing especially pagan about it. Brown also qualifies the obelisk as "Egyptian" despite its recent date of manufacture in 1743: "a most unexpected structure, a colossal Egyptian obelisk". He also equates the Saint-Sulpice meridian with the Paris Meridian
, although they are different, being several hundred meters apart: "Long before the establishment of Greenwhich as the prime meridian, the zero longitude had passed through Paris and through the Church of Saint-Sulpice".
The building of the Gnomon inside the Church of Saint-Sulpice occurred at a time when Rome was relaxing its stance against the theories of Galileo Galilei
, as his works were being printed in Rome with the agreement of the Holy See
, and in 1757 the Pope removed the works of Galileo from the Index Librorum Prohibitorum
.
. Commissioned by Pope Clement XI
, it was designed by
Francesco Bianchini
and completed in 1702.
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
measurement device located in the Church of Saint-Sulpice (Église Saint-Sulpice) in Paris
Paris
Paris is the capital and largest city in France, situated on the river Seine, in northern France, at the heart of the Île-de-France region...
, France. It is a gnomon
Gnomon
The gnomon is the part of a sundial that casts the shadow. Gnomon is an ancient Greek word meaning "indicator", "one who discerns," or "that which reveals."It has come to be used for a variety of purposes in mathematics and other fields....
, 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 astrological events. Those churches are Santa Maria del Fiore
Santa Maria del Fiore
The Basilica di Santa Maria del Fiore is the cathedral church of Florence, Italy. The Duomo, as it is ordinarily called, was begun in 1296 in the Gothic style to the design of Arnolfo di Cambio and completed structurally in 1436 with the dome engineered by Filippo Brunelleschi...
in Florence
Florence
Florence is the capital city of the Italian region of Tuscany and of the province of Florence. It is the most populous city in Tuscany, with approximately 370,000 inhabitants, expanding to over 1.5 million in the metropolitan area....
, San Petronio
San Petronio Basilica
The Basilica of San Petronio is the main church of Bologna, Emilia Romagna, northern Italy. It dominates the Piazza Maggiore. It is the fifth largest church in the world, stretching for 132 meters in length and 60 meters in width, while the vault reaches 45 meters inside and 51 meters in the facade...
in Bologna
Bologna
Bologna is the capital city of Emilia-Romagna, in the Po Valley of Northern Italy. The city lies between the Po River and the Apennine Mountains, more specifically, between the Reno River and the Savena River. Bologna is a lively and cosmopolitan Italian college city, with spectacular history,...
, and the Church of the Certosa in Rome
Rome
Rome is the capital of Italy and the country's largest and most populated city and comune, with over 2.7 million residents in . The city is located in the central-western portion of the Italian Peninsula, on the Tiber River within the Lazio region of Italy.Rome's history spans two and a half...
. These gnomons ultimately fell into disuse with the advent of powerful telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
s.
Structure
The gnomon of Saint-Sulpice is composed of different parts that span the breadth of the transeptTransept
For the periodical go to The Transept.A transept is a transverse section, of any building, which lies across the main body of the building. In Christian churches, a transept is an area set crosswise to the nave in a cruciform building in Romanesque and Gothic Christian church architecture...
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
Meridian (geography)
A meridian is an imaginary line on the Earth's surface from the North Pole to the South Pole that connects all locations along it with a given longitude. The position of a point along the meridian is given by its latitude. Each meridian is perpendicular to all circles of latitude...
, 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
Paris Meridian
The Paris Meridian is a meridian line running through the Paris Observatory in Paris, France—now longitude 2°20′14.025″ east. It was a long-standing rival to Greenwich as the prime meridian of the world, as was the Meridian of Antwerp in Antwerp, Belgium....
, established by Louis XIV in 1667, which is located a few hundred meters to the east and goes through the Observatory of Paris.
The sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
light passes through a small round opening in the southern stained-glass window of the transept, at a height of 25 meters, forming a small light disk on the floor; this disk will cross the meridian each time the sun reaches its apogee 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. The middle of the meridian is marked with a gold disk which shows the position of the sun during the equinox
Equinox
An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator...
. It is located right in front of the altar
Altar
An altar is any structure upon which offerings such as sacrifices are made for religious purposes. Altars are usually found at shrines, and they can be located in temples, churches and other places of worship...
.
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
Summer solstice
The summer solstice occurs exactly when the axial tilt of a planet's semi-axis in a given hemisphere is most inclined towards the star that it orbits. Earth's maximum axial tilt to our star, the Sun, during a solstice is 23° 26'. Though the summer solstice is an instant in time, the term is also...
on 21 June.
At the other end is an obelisk
Obelisk
An obelisk is a tall, four-sided, narrow tapering monument which ends in a pyramid-like shape at the top, and is said to resemble a petrified ray of the sun-disk. A pair of obelisks usually stood in front of a pylon...
, 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 meters beyond the wall of the church.
Usage of the Gnomon
The gnomon was built at the initiative of Jean-Baptiste Languet de GergyJean-Baptiste Languet de Gergy
Jean-Baptiste Languet de Gergy was parish priest at Eglise Saint-Sulpice in Paris from 1714 to 1748. He was the initiator of the construction of the Gnomon of Saint-Sulpice. Languet de Gergy initially wished to establish the exact astrological time in order to ring the bells at the most...
, the parish priest at Saint-Sulpice from 1714 to 1748. Languet de Gergy initially wished to establish the exact astrological time in order to ring the bells at the most appropriate time of day. For this, he commissioned the English clockmaker Henry Sully
Henry Sully
Henry Sully was an English clockmaker. He lived for many years in France.-Marine clock:He invented a marine clock to determine longitude accurately, a sophisticated pendulum clock. He presented a first Montre de la Mer in 1716 to the French Académie des Sciences. He was the first to develop...
to build the gnomon.
Ellipse
In geometry, an ellipse is a plane curve that results from the intersection of a cone by a plane in a way that produces a closed curve. Circles are special cases of ellipses, obtained when the cutting plane is orthogonal to the cone's axis...
around the sun in one year, which adds or subtracts to the earth natural rotation the moment
Moment
- Science, engineering, and mathematics :* Moment , used in probability theory and statistics* Moment , several related concepts, including:** Angular momentum or moment of momentum, the rotational analog of momentum...
of the elliptical movement. These variations are codified in the equation of time
Equation of time
The equation of time is the difference between apparent solar time and mean solar time. At any given instant, this difference will be the same for every observer...
. Henry Sully however died in 1728 without being able to accomplish this larger project. He was only able to set the meridian line in the floor of the Church. The project was completed by the nearby Observatory of Paris a year later.
Computation of official time
The time the sun disk crosses the Saint-Sulpice meridian gives the "true" local midday at that place. In order to compute the official French time from this, it is necessary to:- add or subtract the deviation given by the equation of timeEquation of timeThe equation of time is the difference between apparent solar time and mean solar time. At any given instant, this difference will be the same for every observer...
. - add half a second to have the mean Paris time.
- add 50 minutes 39 seconds to obtain Central European TimeCentral European TimeCentral European Time , used in most parts of the European Union, is a standard time that is 1 hour ahead of Coordinated Universal Time . The time offset from UTC can be written as +01:00...
. - add one hour in summer to take into account daylight savings time.
Alternatively, a simpler solution would be to consult an almanac
Almanac
An almanac is an annual publication that includes information such as weather forecasts, farmers' planting dates, and tide tables, containing tabular information in a particular field or fields often arranged according to the calendar etc...
giving the time of sunrise and sundown, calculate the middle point of that time-span, corresponding to the maximum elevation of the sun. This gives the official time at which the sun reaches it maximum elevation, and therefore the time the sun disk crosses the Saint-Sulpice meridian.
Computation of the Paschal equinox
After this first attempt, Languet de Gergy resumed the project in 1742, this time with the objective of properly defining the EasterEaster
Easter is the central feast in the Christian liturgical year. According to the Canonical gospels, Jesus rose from the dead on the third day after his crucifixion. His resurrection is celebrated on Easter Day or Easter Sunday...
Equinox
Equinox
An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator...
. The task was given to Pierre-Charles Le Monnier, member of the French Academy of Sciences
French Academy of Sciences
The French Academy of Sciences is a learned society, founded in 1666 by Louis XIV at the suggestion of Jean-Baptiste Colbert, to encourage and protect the spirit of French scientific research...
.
The inscription at the base of the obelisk
Obelisk
An obelisk is a tall, four-sided, narrow tapering monument which ends in a pyramid-like shape at the top, and is said to resemble a petrified ray of the sun-disk. A pair of obelisks usually stood in front of a pylon...
mentions Charles Claude Le Monnier, as well as the mission of the gnomon in Latin: "Ad Certam Paschalis AEquinoctii Explorationem" ("To determine precisely the Paschal Equinox").
Easter
Easter
Easter is the central feast in the Christian liturgical year. According to the Canonical gospels, Jesus rose from the dead on the third day after his crucifixion. His resurrection is celebrated on Easter Day or Easter Sunday...
corresponds to the Jewish Passover
Passover
Passover is a Jewish holiday and festival. It commemorates the story of the Exodus, in which the ancient Israelites were freed from slavery in Egypt...
, which marks the liberation of the Jews from Egypt, and traditionally falls in the Jewish lunar calendar on the 14th of the month of Nisan
Nisan
Nisan is the first month of the ecclesiastical year and the seventh month of the civil year, on the Hebrew calendar. The name of the month is Babylonian; in the Torah it is called the month of the Aviv, referring to the month in which barley was ripe. It is a spring month of 30 days...
, the day of the first full moon after the Spring equinox. Roman Christians however had a twelve month calendar, first the Julian calendar
Julian calendar
The Julian calendar began in 45 BC as a reform of the Roman calendar by Julius Caesar. It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year .The Julian calendar has a regular year of 365 days divided into 12 months...
until 1582 and then the Gregorian calendar
Gregorian calendar
The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter...
, which were unrelated to the lunar calendar. Since the Council of Nicaea
First Council of Nicaea
The First Council of Nicaea was a council of Christian bishops convened in Nicaea in Bithynia by the Roman Emperor Constantine I in AD 325...
in 325, the date of Easter had been decided to be the first full moon after March 21, which at that time corresponded indeed to the spring equinox. The Julian calendar being imprecise however, in the 16th century March 21 fell about 10 days after the Spring equinox, a problem which was solved by the Gregorian calendar. Languet de Gergy, however, wished to verify independently the exact date of the Spring equinox through the gnomon in order to ascertain the date of Easter.
Obliquity of the ecliptic
Le Monnier further used the gnomon from 1744 to establish the variations of the eclipticEcliptic
The ecliptic is the plane of the earth's orbit around the sun. In more accurate terms, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun...
, or the variations in the obliquity of the Earth's axis
Axial tilt
In astronomy, axial tilt is the angle between an object's rotational axis, and a line perpendicular to its orbital plane...
. The endeavor is recorded on the plaque at the southern end of the meridian, in the South transept: "Pro nutatione axios terren. obliquitate eclipticae" ("for the nutation
Nutation
Nutation is a rocking, swaying, or nodding motion in the axis of rotation of a largely axially symmetric object, such as a gyroscope, planet, or bullet in flight, or as an intended behavior of a mechanism...
of the earth's axis and the obliquity of the ecliptic").
As mentioned on a brass plaque that covered the stone plaque, the maximum obliquity of the ecliptic was 23°28'40".69 in 1744. From 1745 to 1791, Le Monnier visited Saint-Suplice at the moment of the summer solstice and, focusing the light with a lens fixed to the opening in the stained-glass window, noted when the light fell at noon. The measurements allowed to confirm a variation of 45' per century (the exact figure is 46".85 per century).
Perihelion
The gnomon also permitted the calculation of the perihelion (the moment the earth is closest to the sun in her elliptical trajectory around the sun), by measuring the size of the image of the sun reflected on the ground and finding the time it was smallest.Interpretations
Occult
The word occult comes from the Latin word occultus , referring to "knowledge of the hidden". In the medical sense it is used to refer to a structure or process that is hidden, e.g...
meaning. The author Dan Brown
Dan Brown
Dan Brown is an American author of thriller fiction, best known for the 2003 bestselling novel, The Da Vinci Code. Brown's novels, which are treasure hunts set in a 24-hour time period, feature the recurring themes of cryptography, keys, symbols, codes, and conspiracy theories...
in The Da Vinci Code
The Da Vinci Code
The Da Vinci Code is a 2003 mystery-detective novel written by Dan Brown. It follows symbologist Robert Langdon and Sophie Neveu as they investigate a murder in Paris's Louvre Museum and discover a battle between the Priory of Sion and Opus Dei over the possibility of Jesus having been married to...
describes it as "a pagan astronomical instrument (...) an ancient sundial of sorts, vestige of the pagan temple that had once stood on this very spot", despite an early modern building date of 1714, and the fact that it is an astronomical device with nothing especially pagan about it. Brown also qualifies the obelisk as "Egyptian" despite its recent date of manufacture in 1743: "a most unexpected structure, a colossal Egyptian obelisk". He also equates the Saint-Sulpice meridian with the Paris Meridian
Paris Meridian
The Paris Meridian is a meridian line running through the Paris Observatory in Paris, France—now longitude 2°20′14.025″ east. It was a long-standing rival to Greenwich as the prime meridian of the world, as was the Meridian of Antwerp in Antwerp, Belgium....
, although they are different, being several hundred meters apart: "Long before the establishment of Greenwhich as the prime meridian, the zero longitude had passed through Paris and through the Church of Saint-Sulpice".
The building of the Gnomon inside the Church of Saint-Sulpice occurred at a time when Rome was relaxing its stance against the theories of Galileo Galilei
Galileo Galilei
Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...
, as his works were being printed in Rome with the agreement of the Holy See
Holy See
The Holy See is the episcopal jurisdiction of the Catholic Church in Rome, in which its Bishop is commonly known as the Pope. It is the preeminent episcopal see of the Catholic Church, forming the central government of the Church. As such, diplomatically, and in other spheres the Holy See acts and...
, and in 1757 the Pope removed the works of Galileo from the Index Librorum Prohibitorum
Index Librorum Prohibitorum
The Index Librorum Prohibitorum was a list of publications prohibited by the Catholic Church. A first version was promulgated by Pope Paul IV in 1559, and a revised and somewhat relaxed form was authorized at the Council of Trent...
.
Basilica of Santa Maria degli Angeli
A similar gnomon built to calculate the exact date of Easter also exists in the Basilica of Santa Maria degli AngeliBasilica of Santa Maria degli Angeli
The Basilica of Santa Maria degli Angeli is a church situated in the plain at the foot of the hill of Assisi, Italy, in the frazione of Santa Maria degli Angeli....
. Commissioned by Pope Clement XI
Pope Clement XI
Pope Clement XI , born Giovanni Francesco Albani, was Pope from 1700 until his death in 1721.-Early life:...
, it was designed by
Francesco Bianchini
Francesco Bianchini
Francesco Bianchini was an Italian philosopher and scientist. He worked for the curia of three popes, including being camiere d`honore of Clement XI, and secretary of the commission for the reform of the calendar, working on the method to calculate the astronomically correct date for Easter in a...
and completed in 1702.