Which line of longitude is internationally defined as 0 degrees?

The Prime Meridian is the internationally defined line of longitude at 0 degrees.

The modern Prime Meridian passes through the Royal Observatory at Greenwich in London, England. It divides measurements of longitude into east and west and serves as the reference from which the world’s time zones were historically organized.

The Greenwich meridian was adopted internationally at the International Meridian Conference in Washington, D.C., in 1884. Twenty-five countries attended, and the conference selected Greenwich as the origin for longitude because it was already widely used by maritime charts and navigation.

The Prime Meridian is different from the Equator. The Equator is a line of latitude at 0 degrees that circles Earth halfway between the poles. Modern satellite-based geodesy places the precise reference meridian slightly east of the historic brass line at Greenwich, but Greenwich remains the familiar cultural and historical location.