Greenwich became longitude zero partly because most of the world's sea charts already used it
In 1884, delegates from twenty-six countries met in Washington, D.C., to settle on a single starting line for longitude. Greenwich won, largely because the United States already based its time zones on it and 72 percent of world commerce relied on sea charts drawn from it.
A meridian joins every point sharing the same longitude, running from pole to pole; on a perfectly round Earth it would be half of a great circle. Measured on the modern WGS 84 ellipsoid, one stretches 20,003.93144 km. The word comes from the Latin meridies, midday, because the Sun crosses any given meridian at local solar noon, halfway between sunrise and sunset. The same root supplies the a.m. and p.m. of the 12-hour clock.
Eratosthenes set the first prime meridian in 200 BCE, though without good latitude measurements it caused trouble. By the early modern era countries each used their own favourite, most often Paris or Greenwich, and the muddle became intolerable once railways had to run timetables across towns that each kept their own time. The 1884 conference agreed on one prime meridian through Greenwich, longitudes counted up to 180 degrees east and west, and a universal day starting at midnight there. Most countries now keep standard time from one of 24 meridians, although India, Nepal, South Australia and the Chatham Islands are offset by 30 or 45 minutes.
The line itself has wandered. New transit instruments were built beside older ones, nudging the zero point each time, and the switch to WGS 84 for satellite positioning pushed the geodetic zero line 102.478 metres to the east of where astronomers had last fixed it. Nothing at Greenwich marks where it now runs; finding it takes a GPS receiver.
A compass needle follows a different line, the magnetic meridian through the magnetic poles, and the angle between it and the true meridian is the magnetic declination. Henry David Thoreau marked a true meridian sight line from his family's house to the depot so he could check his compass drift before and after a day's surveying. Navigators used meridians to fix latitude as well: take the Sun's altitude as it crosses the local meridian, subtract it from 90 degrees and add the Sun's declination.
Source: Meridian (geography)
