Meridians and Paris Time

Did you know that for a while, official Paris time was 9 minutes and 21 seconds ahead of Greenwich Mean Time (GMT)? Let me tell you how that happened.

First of all, I assume you know a meridian is a line that runs from the North Pole to the South Pole. It is used to indicate the longitude of a location.

The Royal Observatory, Greenwich

The Royal Observatory, Greenwich (a town just outside London), is the reference point for both the Prime Meridian, putting Greenwich at 0° longitude, and Greenwich Mean Time. Both these references were progressively adopted around the world over the second half of the 19th century, some international conferences helping the process along.

The Prime Meridian at the Royal Observatory, Greenwich

Paris is in the GMT+1 time zone and at a longitude of ~2°20’E. New York is in the GMT-5 time zone and at ~74’W.

Back in the 17th century, Sun King Louis XIV authorized the building of the royal observatory of Paris (today the Paris Observatory). On Midsummer’s Day 1667, scientists traced the outline of the building in such a way that the Paris Meridian bisected the site from north to south. Meridians were really important in that time as scientists attempted to determine the size and the figure of the earth.

The Paris Observatory

So we now have the Paris Meridian. What about Paris time?

Once upon a time and ironically long before the Sun King came along, people told time by the sun. Later, church bells calling to prayer marked time during the day. Each village, each town lived by the time of their local clocks.

It was only with the arrival of the train that France started to feel the need to unify time across its territory. In order to assure a regular train service, the train companies used Paris time. Progressively, over the second half of the 19th century, French towns adopted double display, local time and Paris time, and in 1891, Paris time was enforced by law across the country.

However, in the meantime, the Greenwich Meridian and GMT had been widely adopted around the world, first by navies, then by train companies and then (nearly) everyone else.

Finally, with the law of March 9, 1911, France adopted the time of the Greenwich Meridian and the division of the day into 24 hours instead of two times 12 hours. However, the text of the law never mentioned Greenwich. Indeed, the reference time remained “Paris time” which is 9 minutes and 21 seconds ahead of GMT. This corresponds to the distance between the Paris Meridian and the Greenwich Meridian.

La Mire du Sud – a sight that originally stood on the Paris Meridian in the gardens of the Paris Observatory, allowing to focus on the Meridian (today it can be found in the public Parc Montsouris)
Paris Observatory and Paris Meridian

In Paris, the Meridian is materialized both inside and outside the Observatory, as well as across town by the Arago medallions, named for Astronomer François Arago (1786-1853) whose statue near the observatory was destroyed by the Vichy Regime in 1942.

Many of the Arago medallions have gone missing since their installation in 1994, not least due to the Da Vinci Code, which has their location all wrong, so don’t go looking for one at the Louvre Pyramid! (Please don’t take any of the remaining medallions. Eiffel Tower key chains only cost 0.50€.)

And if you go to Villers-sur-Mer  in Normandy, you will find a marker materializing the Greenwich Meridian on the beach wall. That’s how much 2°20’ are in distance!

Location of the Paris Observatory on a map of Paris
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The Paris Observatory

The Paris Observatory was founded in 1667 under Sun King Louis XIV and is the oldest observatory in the world still in operation.

It was to be situated on the Paris Meridian (today at 2°20′ East),  which was for a long time in competition with the Greenwich meridian. On Solstice Day 1667, mathematicians traced the lines on the ground where the observatory was to be built. The Paris meridian bisected the site. Today, it is traced on the ground inside the observatory, and the Avenue de l’Observatoire runs along its axis.

The Paris Meridian can also be traced in the city of Paris by the Arago medallions. 135 originally, some of them have disappeared since 1994. They are named after François Arago, director of the Observatory in the 19th century.

The first directors of the Observatory were four generations of Cassinis: Giovanni Domenico Cassini, Jacques Cassini, César François Cassini de Thury and Jean Dominique Comte de Cassini. They held the office until the French Revolution.

The copula of the observatory houses the Arago telescope. Completed in 1854, it was the biggest of its time, and remained in use for over a hundred years. In the 1880, it conducted photometry measurements of Jupiter’s moons (only four were known at the time, though).

Location of the Paris observatory
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Cupolas of Paris

There are a number of cupolas in the Paris skyline. In this new series, Cupolas of Paris, we will have a look at the most prominent ones and what is hidden below them:

  • The Dôme des Invalides – 7th arrondissement – built under Sun King Louis XIV in the 17th century
  • The Panthéon – 5th arrondissement – built in the 18th century
  • The Sorbonne – 5th arrondissement – built in the 17th century
  • The Observatory – 14th arrondissement – built under Sun King Louis XIV in the 17th century
  • The Val de Grâce – 5th arrondissement – built under Sun King Louis XIV in the 17th century
  • The Institut de France – 6th arrondissement – built under Sun King Louis XIV in the 17th century
1 Invalides – 2 Panthéon – 3 Sorbonne – 4 Observatory – 5 Val de Grâce – 6 Institut de France
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