Greenwich Mean Time (GMT)

updated:  20 sept 2015

We all may heard about GMT, but sadly most of us don’t know what is that about, so here I’m going to post that under the category “ONE INFO A DAY” where one interesting topic will be posted daily.

Greenwich Mean Time (GMT) is a time system originally referring to mean solar time at the Royal Observatory in Greenwich, London, which later became adopted as a global time standard. It is arguably the same as Coordinated Universal Time (UTC) and when this is viewed as a time zone the name Greenwich Mean Time is especially used by bodies connected with the United Kingdom, such as the BBC World Service,the Royal Navy, the Met Office and others particularly in Arab countries, such as the Middle East Broadcasting Center and OSN.

Before the introduction of UTC on 1 January 1972 Greenwich Mean Time (also known as Zulu time)
was the same as Universal Time
(UT) which is a standard astronomical concept used in many technical
fields. Astronomers no longer use the term “Greenwich Mean Time”.
In the United Kingdom, GMT is the official time only during winter; during summer British Summer Time is used. GMT is the same as Western European Time.
Noon Greenwich Mean Time is rarely the exact moment when the sun crosses the Greenwich meridian (and reaches its highest point in the sky at Greenwich) because of Earth’s uneven speed in its elliptic orbit and its axial tilt. This event may be up to 16 minutes away from noon GMT (a discrepancy calculated by the equation of time). The fictitious mean sun is the annual average of this nonuniform motion of the true Sun, necessitating the inclusion of mean in Greenwich Mean Time.
Historically the term GMT has been used with two different
conventions, sometimes numbering hours starting at midnight and
sometimes starting at noon. The more specific terms UT and UTC do not
share this ambiguity, always referring to midnight as zero hours.
Astronomers preferred the latter GMT convention in order to simplify
their observational data so that each entire night was logged under a
single calendar date.

As the United Kingdom grew into an advanced maritime nation, British mariners kept at least one chronometer on GMT in order to calculate their longitude from the Greenwich meridian, which was by convention considered to have longitude zero degrees (this convention was internationally adopted in the International Meridian Conference of 1884). Note that the synchronization of the chronometer
on GMT did not affect shipboard time itself, 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, eventually led to GMT being used
worldwide 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 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. 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, rendering the time ball at the observatory obsolete in the process.
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. 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; then 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 equalized for the effects of polar wandering) and UT2 (UT1 further equalized for annual seasonal variations in earth rotation rate). Leap seconds are nowadays added to or subtracted from UTC to keep it within 0.9 seconds of UT1.

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

Ambiguity in the definition of GMT

Historically the term 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  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 1925 for astronomical purposes as well,
resulting in a discontinuity of 12 hours, or half a day earlier. The
instant that was designated December 31.5 GMT in 1924 almanacs became
January 1.0 GMT in 1925 almanacs. The term Greenwich Mean Astronomical
Time (GMAT) was introduced to unambiguously refer to the previous
noon-based astronomical convention for GMT. The more specific terms UT and UTC do not share this ambiguity, always referring to midnight as zero hours.

Greenwich Mean Time in legislation

Several countries throughout the world legislatively define their local time by explicit reference to Greenwich Mean Time Some examples are:

  • United Kingdom: The Interpretation Act 1978, 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. Under subsection 23(3), the same rule applies to deeds and
    other instruments.
  • Belgium: Decrees of 1946 and 1947 set legal time as one hour ahead of GMT.
  • Republic of Ireland: Standard Time (Amendment) Act, 1971, section 1,and Interpretation Act 2005, section 18(i).
  • Canada: Interpretation Act, R.S.C. 1985, c. I-21, section 35(1).

Time zone

Those countries marked in dark blue on the map above use Western European Summer Time and advance their clock one hour in summer. In the United Kingdom, this is known as British Summer Time (BST); in the Republic of Ireland it is called Irish Standard Time (IST)—officially changing to GMT in winter. Those countries marked in light blue keep their clocks on UTC/GMT/WET year round.


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Jeffry is a Mechanical Engineer by education and an aspiring writer and blogger. After working hard for around 12 hours a day on his core job, he spends his remaining time in blogging and reading articles online. And he loves to make poor jokes, so be prepared.

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