The Prime Meridian
An imaginary line running from pole to pole through the Royal Observatory in Greenwich, London. Established in 1884, the prime meridian at 0° longitude divides the Eastern and Western hemispheres. Every measurement of position on Earth begins here, at this invisible seam where geography meets time.
24 Meridian Hours
The Earth rotates 360° in 24 hours, which means each time zone spans exactly 15° of longitude. As you travel east from Greenwich, each 15° increment adds one hour. Travel west, and each increment subtracts one. This elegant geometric relationship between angle and time is the foundation of global coordination.
The Longitude Problem
For centuries, sailors could determine latitude by the stars but had no reliable method for longitude at sea. Ships were lost, fleets destroyed. In 1714, the British Parliament offered £20,000 for a practical solution. John Harrison's marine chronometer H4, completed in 1761, finally solved the problem by keeping precise time at sea.
The International Date Line
At 180° longitude, the date changes. Step west across this invisible boundary and it's tomorrow. Step east and it's yesterday. The line zigzags around island nations and political boundaries, a compromise between mathematical precision and human convenience. Here, time folds back on itself.
Satellite Precision
Today, GPS satellites determine longitude to within centimeters. A constellation of 31 satellites orbiting at 20,200 km broadcasts timing signals that receivers triangulate into position. What took Harrison decades to solve with clockwork, we now solve with atomic clocks traveling at 14,000 km/h in medium Earth orbit.
The Geometry of Days
Every day is born at the date line and sweeps westward as the Earth rotates. At any given moment, two calendar dates coexist on the planet, separated by that zigzagging line in the Pacific. Longitude doesn't just measure position — it measures the continuous creation and dissolution of days themselves.