Prime meridian

Time and longitude

The Earth turns 360° in 24 hours, so every 15° of longitude is one hour of solar time and every degree is 4 minutes. A place on the 180° meridian is 12 hours from Greenwich in solar time, whether you count east or west. That is the root of the date line.

Civil time zones round these solar offsets to whole or half hours and follow borders, so clocks and the sun agree only roughly.

How to use it

1. The line is drawn in orange across the whole map as soon as the page opens.
2. Pan and zoom to follow it. On the web map's Mercator projection, parallels are straight horizontal lines and meridians straight vertical ones.
3. Under the map, read the list of countries whose land the line crosses and the notes about the line.
4. Use the other line pages linked below to compare positions, or the miles per degree calculator for the length of a degree at this latitude.

Why the Prime meridian is there

Unlike the equator, the prime meridian is a choice, not a fact of nature. Any meridian could serve. The International Meridian Conference of 1884 chose the one through the Royal Observatory at Greenwich, and it became the zero for longitude and for the world's time zones.

GPS and modern maps use the IERS Reference Meridian, which runs about 102 meters east of the historic transit circle at Greenwich. A GPS receiver standing on the brass line there reads a small west longitude.

How long a meridian is

A meridian runs from pole to pole along the ellipsoid. Because the Earth is flattened, a degree of latitude is shorter near the equator than near the poles. Summing the meridional radius of curvature from 90° S to 90° N on WGS84 gives 20,003.9 km, or 12,430 miles, for one meridian.

The equator is longer, 20,037.5 km for half of it, because the equatorial radius exceeds the polar one. The miles in a degree guide has the full table.

What the map shows

Testing a point every 0.02° along the line against the Natural Earth 1:50m outlines finds land in 8 countries: United Kingdom, France, Spain, Algeria, Mali, Burkina Faso, Togo, Ghana. At that scale small islands drop out, so the list covers the mainland and larger islands.

• The IERS reference meridian is about 102 m east of the historic Airy transit circle.
• All time zones are numbered from this meridian.

The United States and this line

Longitude is also time. The Earth turns 15° an hour, so Denver's capitol, at -104.985°, sees the sun cross its meridian 7.00 hours after it crosses Greenwich, on average. That is close to the 7 hours between Greenwich Mean Time and Mountain Standard Time.