As the United Kingdom grew into an advanced
maritime nation, British mariners kept at least one
chronometer on GMT to calculate their
longitude from the
Greenwich meridian, which was by convention considered to have longitude zero degrees, internationally adopted in the
International Meridian Conference of 1884. Synchronisation of the chronometer on GMT did not affect shipboard time, which was still solar time. But this practice, combined with mariners from other nations drawing from
Nevil Maskelyne's method of
lunar distances based on observations at Greenwich, led to GMT being used worldwide as a standard time independent of location. Most
time zones were based upon GMT, as an offset of a number of hours (and possibly a half-hour) "ahead of GMT" or "behind GMT".
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, from which the term "
railway time" is derived. It was gradually adopted for other purposes, but a legal case in 1858 held "
local mean time" to be the official time.
 On 14 May 1880, a letter signed by 'Clerk to Justices' appeared in 'The Times', stating that 'Greenwich time is now kept almost throughout England, but it appears that Greenwich time is not legal time. For example, our polling booths were opened, say, at 8 13 and closed at 4 13 PM.'
 This was changed later in 1880, when Greenwich Mean Time 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 GMT in 1916, supplanting
Dublin Mean Time.
time signals from Greenwich Observatory were first broadcast on 5 February 1924, rendering the
time ball at the observatory redundant in the process.
The daily rotation of the Earth is irregular (see
ΔT) and constantly slows; therefore the
atomic clocks constitute a much more stable timebase. On 1 January 1972, GMT was superseded as the international civil time standard by Coordinated Universal Time, maintained by an ensemble of atomic clocks around the world.
Universal Time (UT), a term introduced in 1928, initially represented mean time at Greenwich determined in the traditional way to accord with the originally defined
universal day; from 1 January 1956 (as decided by the
IAU at Dublin, 1955, at the initiative of
William Markowitz) this "raw" form of UT was re-labeled UT0 and effectively superseded by refined forms UT1 (UT0 equalised for the effects of polar wandering)
 and UT2 (UT1 further equalised for annual seasonal variations in earth rotation rate).
Indeed, even the Greenwich meridian itself is not quite what it used to be—defined by "the centre of the transit instrument at the Observatory at Greenwich". Although that instrument still survives in working order, it is no longer in use and now the meridian of origin of the world's longitude and time is not strictly defined in material form but from a statistical solution resulting from observations of all time-determination stations which the
BIPM takes into account when co-ordinating the world's time signals. Nevertheless, the line in the old observatory's courtyard today differs no more than a few metres from that imaginary line which is now the prime meridian of the world.
— Howse, D. (1997). Greenwich time and the longitude. London: Philip Wilson.