Unlike the parallels of latitude, which are defined
by the rotational axis of the Earth (the poles being 90° and the Equator,
0°), the prime meridian is arbitrary, and multiple meridians have been
used through history as the prime meridians of various mapmaking systems
(including four different Greenwich meridians). The Greenwich Meridian
established by Sir George Airy in 1851 was agreed upon as the
international standard in October 1884. The zero meridian used by the
Ordnance Survey is about six meters to the west of the line marked at
Greenwich. The Greenwich Meridian is now marked at night by a laser beam
emitted from the observatory.