As the United Kingdom grew into an advanced maritime nation,
British mariners kept at least one timepiece on GMT in order
to calculate their longitude from the Greenwich meridian,
which was by convention considered to have longitude zero
degrees. This did not affect shipboard time itself, which
was still solar time. This, combined with mariners from other
nations drawing from Nevil Maskelyne's method of lunar distances
based on observations at Greenwich, eventually led to GMT
being used world-wide as a reference time independent of location.
Most time zones were based upon this reference as a number
of hours and half-hours "ahead of GMT" or "behind
Greenwich Mean Time was adopted across the island of Great
Britain by the Railway Clearing House in 1847, and by almost
all railway companies by the following year. It was gradually
adopted for other purposes, but a legal case in 1858 held
"local mean time" to be the official time. This
changed in 1880, when GMT was legally adopted throughout the
island of Great Britain. GMT was adopted on the Isle of Man
in 1883, Jersey in 1898 and Guernsey in 1913. Ireland adopted
Greenwich Mean Time in 1916, supplanting Dublin Mean Time.
Hourly time signals from Greenwich Observatory were first
broadcast on 5 February 1924.
The daily rotation of the Earth is somewhat irregular and
is slowing down slightly. Atomic clocks constitute a much
more stable timebase. On 1 January 1972, GMT was replaced
as the international time reference by Coordinated Universal
Time, maintained by an ensemble of atomic clocks around the
world. UT1, introduced in 1928, represents earth rotation
time. Leap seconds are added to or subtracted from UTC to
keep it within 0.9 seconds of UT1.
The international prime meridian is no longer precisely the
Greenwich meridian, but remains close to it (5.31"E).