The Prime Meridian at Greenwich Observatory (wikipedia)We are used today to give the coordinates of a place on Earth using latitude and longitude, indicating longitude in degrees east or west, respectively, of the Greenwich Prime Meridian.
Thus, for example, the small amateur observatory Sternwarte Peterberg near the place where I did grow up is located exactly 7 degrees east of Greenwich.
However, looking up the location on historical maps, I don't find this longitude. Actually, the French engineers who around 1800 drew the first detailed topographic maps of the region did measure longitude with respect to the Paris Observatory. Their Prussian successors used the El Heirro Meridian, which goes back to Ptolemy in the 2nd century, and later switched to coordinates centered at Berlin.
Actually, in the second half of the 19th century, more than a dozen "Prime Meridians" were in use, creating increasing confusion for transport, trade, and communication around the globe.
Thus, in October 1884, delegates from 25 nations met in Washington, DC, at a conference to determine a prime meridian, which should be used as a universal reference for measuring longitude, and for a universal time. 125 years ago, on October 13, 1884, the "International Meridian Conference held at Washington for the purpose of fixing a Prime Meridian and a Universal Day" resolved
"That a meridian proper, to be employed as a common zero in the reckoning of longitude and the regulation of time throughout the world, should be a great circle passing through the poles and the centre of the transit instrument at the Observatory of Greenwich."and
"That the Conference proposes to the Governments here represented the adoption of the meridian passing through the transit instrument at the Observatory of Greenwich as the initial meridian for longitude."
There was one negative vote, and the delegations from Brazil and France abstained from voting. The French delegation led by astronomer Pierre Jules Janssen, the discoverer of helium, had pleaded for keeping the El Heirro Meridian, but it seems that long tables of data, from tonnages of ships to sales figures of nautical charts and almanachs, all using Greenwich as their reference point, convinced most delegates to officially adopt the de-facto standard.
In 1911, also the French switched to Greenwich longitude and Greenwich time.
The complete PROTOCOLS OF THE PROCEEDINGS of the International Meridian Conference are available via the Project Gutenberg. The vote on the adoption of Greenwich meridian is reported on page 99.