Here’s what they are, why they matter, and where things get weird. What it is: The starting point for measuring longitude. It runs through Greenwich, London , UK, and divides Earth into Eastern Hemisphere (0° to 180° east) and Western Hemisphere (0° to 180° west).
In 1884, 25 nations voted to make the Greenwich Meridian the world’s prime meridian. Why? Britain was the world’s leading maritime power, and most ships already used Greenwich charts. France abstained (they preferred Paris), but eventually adopted it too. antimeridian and prime meridian
Today, the true zero-longitude line is actually about 100 meters east of the original Greenwich telescope, due to modern GPS using the International Reference Meridian (IRM), which aligns with satellite measurements. But the historic line still draws tourists. 2. The Antimeridian (180° longitude) What it is: Exactly halfway around the world from the Prime Meridian: 180° east / 180° west — they’re the same line. Here’s what they are, why they matter, and
(e.g., plotting earthquakes or shipping routes), if you center a map on the Atlantic, the Pacific gets split — but if you center on the Pacific, the Atlantic gets split. No perfect flat map avoids the antimeridian problem. In 1884, 25 nations voted to make the
This is why Pacific island nations and airlines pay close attention to where the IDL is drawn. GIS / mapping software often struggles with the antimeridian. A shape that crosses 180° longitude (e.g., Russia’s far east) will wrap around the map incorrectly if not handled with antimeridian splitting — dividing the geometry into two pieces, one on each side.
Mostly through the Pacific Ocean , avoiding most land. It passes between Russia and Alaska (through the Bering Strait), then near Fiji, and down between New Zealand’s main islands.