At different times in the past, it has been calculated in different ways, including being calculated from noon;  as a consequence, it cannot be used to specify a precise time unless a context is given.
The term GMT should not thus be used for technical purposes. Because of Earth's uneven speed in its elliptical orbit and its axial tilt , noon This event may occur up to 16 minutes before or after noon GMT, a discrepancy calculated by the equation of time.
Noon GMT is the annual average i. Originally, astronomers considered a GMT day to start at noon while for almost everyone else it started at midnight.
It is a term commonly used in the United Kingdom and countries of the Commonwealth , including Australia, New Zealand, South Africa, India, Pakistan, Bangladesh and Malaysia; and in many other countries of the eastern hemisphere.
In some countries the United Kingdom for example Greenwich Mean Time is the legal time in the winter and the population uses the term.
See GMT in legislation, below, for further explanation. 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 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.
Greenwich Mean Time was adopted across the island of Great Britain by the Railway Clearing House in , 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 held " local mean time " to be the official time. For example, our polling booths were opened, say, at 8 13 and closed at 4 13 p.
On 1 January , GMT was superseded as the international civil time standard by Coordinated Universal Time, maintained by an ensemble of atomic clocks around the world.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 , initially represented mean time at Greenwich determined in the traditional way to accord with the originally defined universal day ; from 1 January as decided by the IAU at Dublin, , at the initiative of William Markowitz this "raw" form of UT was re-labelled 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.
Historically GMT has been used with two different conventions for numbering hours. The long-standing astronomical convention dating from the work of Ptolemy , was to refer to noon as zero hours see Julian day.
This contrasted with the civil convention of referring to midnight as zero hours dating from the Romans. The latter convention was adopted on and after 1 January for astronomical purposes, resulting in a discontinuity of 12 hours, or half a day.
The instant that was designated 'December Legally, the civil time used in the UK is called still "Greenwich mean time" without capitalisation , according to the Interpretation Act , with an exception made for those periods when the Summer Time Act orders an hour's shift for daylight saving.
The Interpretation Act , section 9, provides that whenever an expression of time occurs in an Act, the time referred to shall unless otherwise specifically stated be held to be Greenwich mean time.
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