What is zero degrees longitude?

Zero degrees longitude is the Prime Meridian, also known as the Greenwich Meridian. The home of Greenwich Mean Time and the Prime Meridian of the World is located at the Royal Observatory in Greenwich, United Kingdom.

The Greenwich Meridian was chosen in 1884 as the Prime Meridian in a landslide vote after 25 countries met in Washington D.C. for the International Meridian Conference. The meridian was defined by the position of a transit circle telescope built by Sir George Biddell Airy. The exact position of the Prime Meridian is always changing as the Earth's crust is always moving, but the original reference to the meridian remains the Airy Transit Circle at the Royal Observatory.