Eratosthenes sketched the first geographic grid
A geographic coordinate system pins any spot on Earth to a pair of angles, latitude and longitude, measured on a chosen model of the planet. Eratosthenes of Cyrene is generally credited with the idea at Alexandria, and an 1884 conference hosted by the United States made Greenwich the world's zero line.
Eratosthenes composed his Geography, since lost, while working at Alexandria's great library during the 3rd century BC. Roughly a hundred years on, Hipparchus improved the scheme, fixing latitude from the stars instead of the Sun's height and longitude from the timing of lunar eclipses rather than dead reckoning. Ptolemy's Geography in the 2nd century counted latitude from the Equator. Once translated into Arabic in the 9th century, these works were corrected by Al-Khwārizmī, who fixed their overlong Mediterranean and shifted the Arabic prime meridian roughly 10 degrees east of Ptolemy's.
At the International Meridian Conference of 1884, twenty-two of twenty-five nations voted to place zero longitude at the Royal Observatory in Greenwich. The Dominican Republic voted no, and France and Brazil abstained; France kept Paris time until switching to Greenwich Mean Time in 1911. The point where the Equator meets the Greenwich meridian lies in the Gulf of Guinea, about 625 kilometres south of Tema in Ghana, and is jokingly called Null Island.
Latitude can be defined three ways, from a plumb line, from a line perpendicular to the ellipsoid, or from Earth's centre, which is why a full specification must name a datum. Different datums can put the same spot hundreds of metres apart without anything moving. In Britain, WGS 84 and the OSGB36 grid on printed maps differ by about 112 metres at Greenwich, and ED50 by 120 to 180. The ground itself shifts too: tides raised by the Moon and Sun move points up to a metre daily, continents drift up to 10 centimetres a year, and Scandinavia is still rebounding about a centimetre a year after the last ice sheets melted.
Scale varies with position. At the Equator, one degree of latitude spans about 110.6 kilometres and one degree of longitude about 111.3, but meridians converge at the poles, so each degree of longitude shrinks steadily toward them.
Source: Geographic coordinate system
