GMT Time – Greenwich Mean Time

GMT Time – Greenwich Mean Time

What means GMT Time?

Greenwich Mean Time (abbreviated GMT) is the mean solar time at the prime meridian.

The Greenwich Mean Time was 1884-1928 World time, in this capacity, she is now superseded by Coordinated Universal Time UTC.

The term itself is still in the UK for the time zone Greenwich Mean Time (GMT / WET, UTC +0) in use, sometimes for the associated DST Western European Summer Time (WEST / WEST, UTC +1). Since 1925, the designation of different locations is used for different time standards. It is recommended that if you want accurate time recording, do not use this name.
On average, the Sun crosses at 12:00 GMT the lunch circle (meridian passage) of Greenwich and has thereby approximate their highest point in the sky (upper culmination). Due to the uneven speed of the Earth in its elliptical orbit and the tilt of Earth’s axis, the actual meridian passage deviates by up to 16 minutes of it (see equation of time), but evened out over the year. So GMT follows an imaginary mean sun, which at a constant speed in a year along the equator (instead of as in reality along the ecliptic) moves.

A more accurate method of astronomical time measurement is to measure the time intervals between the meridian passages of a fixed star (sidereal time).

History of GMT Time

For the purpose of astronomical navigation at sea the Nautical Almanac of Nevil Maskelyne was first published in 1767 , whose astronomical tables were designed for the local time of the Observatory Greenwich. Navigators that evaluated their astronomical observations with the help of this yearbook , automatically received related to the meridian of Greenwich position , whereas it had formerly been customary to specify the longitude with respect to the origin or destination point. Therefore charts also began increasingly to take over Greenwich as the standard meridian for their coordinate systems . As the International Meridian Conference a prime meridian stipulated internationally binding in 1884 , the choice fell on the already predominantly common Greenwich meridian. For applications requiring uniform time information for different places , local mean time of Greenwich is to use . This period was introduced in 1880 under the name Greenwich Mean Time as the legal standard time in the United Kingdom.

For worldwide coordination of timing the Bureau International de l’ Heure (BIH ) was founded in Paris, which was adopted by the International Astronomical Union, the newly founded (IAU ) in 1920 . Soon after, the IAU changed the definition of GMT . Previously it was in the main the night working astronomers been customary to let the day begin at noon , to not constantly at midnight , in the midst of the work, the need to change the date ( astronomical day , just as in the Julian date scale astronomy yet is common today ) . Accordingly , GMT was originally ” the mean solar time on the meridian of Greenwich , calculated from the average lunch ” . The IAU led to the first January 1925 a new time scale a , the following day at midnight began civic custom and called the new scale back GMT , while the existing scale in Greenwich Mean Astronomical Time ( GMAT) has been renamed. The resulting confusion led the IAU in 1928 , the new GMT in Universal Time (UT ) rename . UT was so ( and is essentially to present) the mean solar time on the meridian of Greenwich, reckoned from midnight.

Since UT is derived from the Earth’s rotation , but which is subject to short-term fluctuations and long-term slowdown , it is not strictly uniform running time scale and for many scientific and technical purposes , some not useful . Therefore, the derived from the uniform planetary motion Ephemeris Time (ET ) was first introduced in 1952 , which at first January 1972 was replaced in turn by the Coordinated Universal Time (UTC). UTC is based on the strictly uniform generated by atomic clocks SI second ; leap seconds where necessary, ensure that UTC is not more than 0.9 s differs from the irregular UT .

GMT term in today time

Today, UTC is still referred to in the UK and in West Africa as GMT , although it (now UT) may differ by up to 0.9 s from the actual GMT . Because of this confusion , it is advisable not to use the term Greenwich Mean Time away from these regional designations of the local time zone .

The term GMT partially used Internet protocols in timestamps (eg for the time settings Date: Sat, 12 Mar 2005 00:30:42 +0000 (GMT) in a mail header) is always meant UTC .
In addition, most electronic devices use with the time and date, to the concept GMT . When setting the appropriate time zone of the user’s location , the input or selection often takes the form of specifying the hours that the local time is ahead (+ ) or – if the standard time ( GMT or UTC ) () .

In Greenwich itself in seven out of twelve months applies to British Summer Time , as well as the prime meridian at the Royal Greenwich Observatory in 1984 was shifted about 100 meters (WGS84 coordinates).

Greenwich Civil Time

In the British seafaring , the term Greenwich Civil Time ( GCT ) of HM Nautical Almanac Office was prescribed , but left open after 1924 /25, whether it was nautical clocks change , astronomical ‘ for lunch or bourgeois ‘ make to midnight , the United States Naval Observatory used the term Greenwich Civil Time for the new regime GMT for lunch change. In 1952 took over the British and U.S. Marine general world time.