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Which London observatory marks the historic Prime Meridian?
In 1884 an international conference chose the Greenwich meridian as the world's 0 degrees longitude.
The Prime Meridian passes through the Royal Observatory in Greenwich, London, and the line is defined as zero degrees longitude. Before 1884, ships and nations used a variety of meridians centered on their own observatories. At the International Meridian Conference in Washington, D.C., delegates voted to adopt the Greenwich meridian as the world standard, partly because most sea charts and shipping already used it.
The line was physically defined by Airy's Transit Circle, a telescope built in 1850 under the Astronomer Royal, George Airy, and used for observations from 1851. Royal Museums Greenwich says it made around 600,000 observations over more than a century. Visitors can stand on the brass line in the observatory courtyard with one foot in each hemisphere.
Today's global positioning relies on the IERS Reference Meridian, which lies about 102 meters east of the historic line, because satellite systems tie coordinates to the Earth's center of mass instead of a local plumb line. Consequently, a GPS receiver standing on the brass line does not read exactly zero degrees longitude. The conference vote was not unanimous, and some delegations, including France, did not support or abstained, which I am noting from general recollection rather than a fetched page.