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Prime Meridian Map

Longitude had to start somewhere, and in 1884 the world agreed on Greenwich. The line itself has since moved.

How to use it

  1. Follow the red line down the map from the North Sea to Antarctica; the shaded countries are the eight it crosses.
  2. Press Zoom to Greenwich to see the historic Airy line and the GPS zero line 102 m apart, marked in purple.
  3. Click a white marker to read where the line runs in each country.
  4. Type a place, or use your location, and press Find nearest point for your distance east or west of zero longitude.

How it works

The map draws the WGS84 zero meridian, the one GPS and every online map use, as a vertical line on the Mercator projection. The Airy transit circle at Greenwich is placed at its published WGS84 position, 5.3 arcseconds west of zero, and the gap of about 102 m at that latitude is computed from those coordinates. The offset exists because the 1884 line was set by a plumb line and the local pull of gravity, while the satellite frame is centred on the Earth's centre of mass.

The nearest point on the meridian is an exact projection of your position onto the plane of the Greenwich great circle, so the distance shown is the true shortest distance to the 0° line, not just the east-west separation. Country outlines are Natural Earth data served from this site. The list of countries was checked one by one; the Togo crossing is a short clip in the far north-west and is marked as such.

Questions

How many countries does the Prime Meridian pass through?
Eight, from north to south: the United Kingdom, France, Spain, Algeria, Mali, Burkina Faso, Togo (briefly) and Ghana. It then crosses the Atlantic and reaches Antarctica in Queen Maud Land. It touches three continents: Europe, Africa and Antarctica.
Why does GPS show 0.0015° W at the Greenwich line?
The brass line in the observatory courtyard marks the Airy transit circle, the meridian adopted in 1884. It was aligned with a plumb line, and gravity at Greenwich pulls very slightly sideways because the Earth's mass is uneven. The WGS84 frame used by GPS is centred on the Earth's centre of mass instead, and its zero meridian runs about 102 m (335 ft) east of the courtyard, across the park.
Why was Greenwich chosen as the Prime Meridian?
By 1884 most of the world's shipping already used charts based on Greenwich, and the United States had adopted it for its time zones the year before. At the International Meridian Conference in Washington 22 countries voted for it; France abstained and kept the Paris meridian on its maps until 1911.
Is the Prime Meridian the same as the International Date Line?
No. They are opposite halves of the same great circle. The Prime Meridian is 0° longitude and passes through Greenwich; the Date Line roughly follows 180°, on the far side of the Earth in the Pacific, and it is where the calendar date changes.

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