The prime meridian runs through Greenwich, England, and marks zero degrees of longitude.

The longer version

Zero longitude had to be fixed somewhere, and an international conference in the nineteenth century settled on the line running through the observatory at Greenwich. From that line longitude is counted east and west to 180 degrees, and the world's time offsets are reckoned. The choice was made by vote rather than by any natural feature.

Other countries had proposed their own meridians, and rival maps persisted for decades. Choosing one line simplified navigation enormously, because a ship's chronometer could then be set against a single published reference. The decision was as much commercial as scientific, since shipping charts already used it widely. Today the line is a tourist curiosity, marked in brass and stone.

Where this entry sits

These facts wander through clocks, calendars, time zones, and the long human effort to measure passing hours. You will find leap years, odd month lengths, ancient sundials, and the reasons modern timekeeping works the way it does. The full list sits on the Time and Calendars page, where every entry is listed in order.