Longitude divides Earth between east and west. Instead of bisecting the world horizontally at the equator, we draw an imaginary line from the North and South Poles called the prime meridian, which travels through Greenwich, England.
The prime meridian is 0° longitude. (On the opposite side of the globe from the prime meridian is 180° longitude. The international date line roughly follows this longitude.)
Of the three points that determine the longitudinal angle, one is on the prime meridian, one on the imaginary line through Earth between the North and South Poles, and one is the point on the surface whose longitude you want to discover.