When visiting the Greenwich Observatory in London, you can stand on the historical prime meridian. I find this endlessly novel!
The prime meridian is a longitude line arbitrarily chosen to represent 0º. The International Meridian Conference in 1884 established the Greenwich meridian as the prime meridian of the world, beating out the Paris meridian, its primary competitor, in a vote from which the French abstained.
Both meridians are sporadically commemorated across Paris and London. For the would-be longitude tourist, London has many Greenwich meridian markers embedded in the pavement. Like this one, laid in the 1960s:
Paris, too, has physical meridian markers dotted throughout the city. A series of “Arago” medallions memorialize the span of the Paris meridian and the work of François Arago in its measurement. (The path of the meridian also paves the floor of the Paris Observatory, if you can negotiate a way in.)
Meridian lines are human-defined, invisible mathematical entities. To mark one on the surface of the earth makes it visible and tangible, bringing you face to face with a point of reference in a global system used mainly by machines.
Meridian markers sit within a broader field of landmarks positioned in reference to celestial objects — of which earthworks in the North American deserts form a striking subset. You can climb inside land artist Nancy Holt’s Sun Tunnels, below, to see the rotation of the earth relative to the constellations. The monumental Star Axis by Charles Ross lays bare the geometry of the stars on a human scale. By way of these pieces, Ross believes, you can “personally interface with the larger order,” and internalize for a moment your place on a rotating planet.
Now, in the age of satellite positioning, the International Reference Meridian (IRM) marks 0º. The line sits about 102 meters east of the Greenwich meridian, due to updated calculations that more precisely locate the center of the earth. The Highams Park neighborhood in London has what is perhaps the only reference slab for the IRM, laid in 2019:
Unlike markers for the earth-based Greenwich and Paris meridians, the IRM plaque does not remain perfectly accurate over time. Tectonic shift moves it east of 0º by about 18 millimeters per year, with every rotation of the earth around the sun.