MAPS AND HISTORY

Greenwich and the International Date Line

Greenwich
0°Opposite
180°

A starting point for measurement

A meridian joins the poles. To state a place’s longitude, we need to choose a meridian from which to count east or west. Greenwich is the shared reference now called the prime meridian.

On the other side of the world

The 180° meridian lies opposite Greenwich. It is the reference for locating the Date Line in the Pacific. The Date Line does not follow it exactly: it bends so countries or island groups can share a calendar date.

What happened in Washington in 1884

The conference recommended Greenwich as a reference and defined a universal day. But it did not set the route of the Date Line or introduce twenty-four time zones overnight. Countries adopted their own rules afterwards.

An older question

The first circumnavigation encountered a one-day discrepancy at Cape Verde in 1522. In 1629 Bergier proposed a line where days would begin. His 180° depended on his chosen prime meridian; it should not simply be equated with modern Greenwich.

From calendars to maps

Countries decide which date they use. Cartographers draw the line to reflect those decisions. We therefore check separately when the calendar changed and when the map changed.

Sources and scope

You can consult the sources here. Where a detail still needs checking, we say so in the text.