The Prime Meridian is the meridian (line of longitude) passing through Royal Greenwich Observatory, Greenwich, England; it is the meridian at which the longitude is 0 degrees. It is sometimes referred to as the Greenwich Meridian.

The meridian was agreed upon in October 1884. At the behest of U.S. President Chester A. Arthur, 41 delegates from 25 nations met in Washington, D.C., USA for the International Meridian Conference.

At the conference the following important principles were established:

1. It was desirable to adopt a single world meridian to replace the numerous ones already in existence.
2. The meridian passing through the principal transit instrument at the Observatory at Greenwich was to be the "initial meridian".
3. That all longitude would be calculated both east and west from this meridian up to 180°.
4. All countries would adopt a universal day.
5. The universal day would be a mean solar day, beginning at the mean midnight at Greenwich and counted on a 24-hour clock.
6. That nautical and astronomical days everywhere would begin at mean midnight.
7. All technical studies to regulate and extend the application of the decimal system to the division of time and space would be supported.

Resolution 2, fixing the meridian at Greenwich, was passed 22-1 (San Domingo, now Haiti, voted against); France and Brazil abstained. The French did not adopt the Greenwich meridian until 1911.

The international date line (the jagged red line down the right side of the image to the right) is on the opposite side of the world from the Prime Meridian.

Previously, the meridian of the Canarian island El Hierro was used by most of the navies of continental Europe. It was conventionally defined as the meridian 20 degrees west from Paris (17° 39' 46" west of Greenwich).

Other prime meridians used previously include Rome (12° 27' 08.04" east of Greenwich), Copenhagen, Jerusalem, Saint Petersburg (Pulkovo meridian, 30° 19'; 42.09" east of Greenwich), Pisa, Paris (2° 20' 14" east of Greenwich), and Philadelphia.

With respect to other solid celestial bodies, prime meridians are likewise arbitrarily defined. The prime meridian of Earth's moon lies directly in the middle of the face of the moon visible from earth and passes near the crater Bruce. The prime meridian of the planet Mars is defined by the crater Airy-0.

External links

• scanned TIFFs of the conference proceedings http://www.ucolick.org/~sla/leapsecs/scans-meridian.html