Answer - Warning Science and History Geekery Below

In 1666, Louis the 14th authorized the building of the Paris Observatory with the Paris Meridian bisecting the building. In the early 19th century, astronomer Francois Arago calculated the meridian with greater accuracy. Paris lost out to Greenwich as the location for the Prime Meridian in the International Meridian Conference in 1884 when a standard meridian was chosen.

In 1995, the Arago Society of Paris wanted to do something commemorate him so they commissioned Dutch artist Jan Dibbets to come up with an idea. He decided to put down a series of medallions bearing his name on the line of the Paris Meridian in the city. This is one of them. Some geography geeks try to walk the entire line of medallions although some are missing or have been covered over.

You may also remember that the Paris Meridian was a key plot tool in the DaVinci Code as the line runs through St Sulpice Church.

For more Arago trivia...There was a statue of him near the Paris Observatory. All that stands there now is an empty granite stand. The Germans took the statue and melted it down during WW II for the war effort.