East–west position is measured by longitude. Longitude is the angular distance east or west of the prime meridian, which passes through Greenwich, England. Values range from 0° to 180° east or west.

Unlike latitude lines, meridians of longitude converge at the poles and are farthest apart at the equator. Time zones are based roughly on longitude because Earth rotates 15 degrees per hour. Determining longitude accurately at sea was a major historical challenge solved by precise chronometers.

Together with latitude, longitude defines the geographic coordinate system used worldwide. GPS and modern mapping systems report longitude as a matter of routine. The choice of the Greenwich meridian as zero was formalized at an international conference in 1884 and remains the standard.

• Angular distance east or west of the prime meridian
• Ranges from 0° to 180°
• Meridians converge at the poles
• Basis for time zones