Greenwich meridian

The Greenwich meridian, also known as the zero meridian, base meridian, or prime meridian, is the meridian from which longitudes are measured. Substitute for the Paris meridian, it corresponds to the imaginary circumference that joins the poles and gets its name from crossing the London district of Greenwich, specifically because of its old astronomical observatory.

History

The meridian was adopted as a reference at an international conference held in October 1884 in Washington D.C., sponsored by the president of the United States and attended by delegates from 25 countries. The following agreements were adopted at said conference:

1. It is desirable to adopt a single reference meridian to replace the numerous existing ones.
2. The meridian through the Greenwich Royal Observatory will be the prime meridian.
3. The longitudes around the globe to the east and west will be taken up to 180° from the initial meridian.
4. All countries will adopt the universal day.
5. The universal day begins at midnight (solar time) in Greenwich and will last twenty-four hours.
6. Nautical and astronomical days will also start at midnight.
7. All technical studies for the regulation and dissemination of the application of the decimal metric system to the division of time and space will be promoted.

The second resolution was approved with the opposition of Santo Domingo (now the Dominican Republic) and the abstentions of France (whose maps continued to use the Paris meridian for a few more decades) and Brazil.

A time zone extends over fifteen degrees in longitude (because 360 ​​degrees correspond to 24 hours and 360/24 = 15).

The line opposite the Greenwich meridian, that is, the semicircle that completes a trip around the world, corresponds to the international date line, which crosses the Pacific Ocean. For practical reasons —fundamentally, not having several time zones in some archipelagos— this line has been adapted to the geography (it is no longer straight on the surface of the globe), as well as others that limit time zones, so they do not coincide with the meridians.

In the past, most of the marinas in continental Europe used the El Hierro meridian, which passed through Punta de la Orchilla, in the west of this Canary Island. However, there were many other references.

There is an angular difference of five and three seconds between the Greenwich meridian and the reference meridian used by the WGS84 GPS system (called IRM). It is a consequence of the procedure used for the start-up in 1958 of the first Global Positioning System by satellite, when the coordinates in the NAD27 system of the satellite observation station located in the vicinity of Baltimore. The greater precision of the new satellite method translated into a displacement of the 0º Meridian of the GPS System (using the longitude of Baltimore as a starting reference), being located about 102 meters to the east of the Greenwich meridian materialized in the Observatory.This is due to the correction of various concordance errors between the European and American cartographic systems, hardly appreciable by classical geodesic methods. When this difference was found in 1969, the possibility of readjusting the entire GPS system to eliminate this gap was ruled out. For more details, see the IERS International Reference Meridian article.