Longitude is defined as 0° at the prime meridian, also known as the Greenwich Meridian.
The Prime Meridian and the 180th meridian at 180° longitude on the other side of the earth, form a giant circle that divides the Earth into the Eastern and Western Hemispheres.
Going from north to south, the countries located at the prime meridian are the United Kingdom, France, Spain, Algeria, Mali, Burkina Faso, and Ghana.
The modern Greenwich Meridian, based at the Royal Observatory, Greenwich, was established by Sir George Airy in 1851, and is now marked at night by a laser beam emitted northwards from the observatory.
By 1884, over two-thirds of all ships used the Greenwich Meridian as the reference meridian on their maps.