In 1884, it was internationally agreed that the prime meridian – for longitude zero – 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 – note that a new funny statue was inaugurated in 2017 in the gardens of the Observatory) 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, but about half of them are still on Paris’s pavements or parks. The list of their initial locations can be found on the Dutch embassy’s website (Jan Dibbets, the artist who created the concept, is Dutch). Six of them are inside the Louvre museum and others are close to 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 his Da Vinci Code, and the medallions of the filmed adaptation are fake!