How to use it
- Look at the purple line on the globe: the Prime Meridian, and its other half at 180 degrees.
- Type a place or press My location to see how far you are from it.
- Read the countries it crosses and the facts.
5 practical uses
- Plan a visit to Greenwich or another marker on the line.
- Teach longitude and time zones.
- Find how far east or west a place is from the meridian.
- Understand the GPS offset at Greenwich.
- See which countries the line crosses.
The maths behind it
The meridian is the half great circle at λ = 0 from pole to pole, 20,004 km long, continued by the 180th meridian on the far side. The nearest point on it from a place is found by searching the latitude that minimises the haversine distance, since the perpendicular from a point to a great circle is itself a great-circle arc. The countries crossed are found by testing every edge of every outline for a change of sign of the longitude.
What it is
Longitude needs a starting line, and in 1884 an international conference chose the meridian through the Airy transit circle at the Royal Observatory in Greenwich, London, because most of the world's shipping charts already used it. Time zones count from it too: Greenwich Mean Time is noon when the mean sun crosses it.
The line your GPS calls 0° runs 102 m east of the brass strip in the observatory courtyard. The old meridian was defined by the local vertical, which is deflected slightly by the mass of the land around; satellite positioning uses the centre of the Earth instead, and the difference shows up as that gap.
Where it runs
From the North Pole through the Greenwich meridian in England, France, Spain, Algeria, Mali, Burkina Faso, Togo and Ghana, then across the Gulf of Guinea and the Atlantic to Antarctica. Its other half, the 180th meridian, is the basis of the International Date Line.