In 1884, it was internationally agreed that the prime meridian – used as the origin of the longitudes – would be the Greenwich meridian, crossing the Royal Observatory of England. But before that, especially under Louis XIV (when France was a leading country in arts and science), the Paris meridian was commonly used, at least by French explorers.
Paris meridian is also connected with the birth of the metric system, since the French revolutionaries decided that 1 meter would equal 1/40,000,000 of a meridian’s length. A famous astronomer involved in the campaigns to measure it as accurately as possible was François Arago, who later became the director of the Paris Observatory.
In the 1990s, it was decided to honor Arago (whose statue, on “his” boulevard, just behind the Observatory, was destroyed during WW2) by spreading bronze medallions on the Parisian section of the meridian, which crosses the most historical observation room of the building.
Many of these medallions disappeared or were actually stolen, but about 60 of them can still be seen on Paris’s pavements or parks. The full list of their initial locations can be found on the Dutch embassy’s website (Jan Dibbets, the artist who created the concept, is Dutch). 6 of them are inside the Louvre museum and others cross the Cour Napoléon, behind the main pyramid. But Dan Brown didn’t respect the path of the meridian when he assimilated it to a so-called Rose-Line in the last chapter of his Da Vinci Code, and the medallions of the filmed adaptation are fake!