When
synonym GMT generally refers to the worldSource
Greenwich in south London Jiaoyuan British Royal Greenwich Observatory is located, the demarcation of the Earth's prime meridian, the starting point for the world's computing time and longitude. Its maritime history, as a standard point of the prime meridian, and Greenwich time its name known. Here difficult terrain, beautiful scenery, both local history and customs, also of East London in the Thames Gateway.
Not only astronomers use GMT, the term is also often appear in newspapers and on the news. We know the local time in all parts of the country. If a major thing on the international, with local time to record, will feel complicated inconvenience. And the future is easy to mistake a long day. Therefore, astronomers proposed acceptable to all and because a convenient method of recording, it is a place where time is GMT standard.
to starting of the prime meridian mean midnight sun when flat. Also known as Greenwich Mean Time or GMT. Places around the world usually equal to the difference between the time of the geographical longitude of the ground. Served as the basic time measurement system has been widely used before 1960. As the Earth's rotation rate was considered to be uniform, so in 1960, when the world is considered to be a homogeneous. As the influence of the earth's rotation rate changes, it is not a uniform time system, atom or when it does not matter any theoretical mechanics, and only compare them by observation skills. Later, when the atom has been replaced and when the world almanac, but still essential in everyday life, astronomy, navigation, geodesy and space flight and so on; at the same time, reflect changes in the Earth's rotation rate of the world, is one of the Earth's rotation parameters, basic information remains astronomy and geophysics.
Basic Theory
When the world is spinning motion of the Earth as a standard time measurement system. Earth rotation angle where the available radial movement relative to the base reference point on Earth to measure. To measure the rotation of the earth, the people on the celestial sphere selected two basic reference points: the vernal equinox the sun of peace.
to the sun as a basic level of a reference point defined by the mean solar time Sun apparent motion, referred to as the mean solar (referred to as MT). Ping Sun is an American astronomer Newcomb (S.Newcomb, 1835 Nian Zhi 1909) a hypothetical reference point in the late nineteenth century caused. It is for the celestial equator at uniform motion, the speed and the average speed is consistent with the true apparent motion of the sun, which ascension of:
a☉ = 18h38m45s.836 + 8640184s.542T + 0S.0929T2
where T is counted from the time of the January 1900 Julian Day 0 12 centuries. When
As a start to mean midnight Greenwich mean solar time of 0:00, when called world (referred to as UT).
world are derived from the star, which conversion formula is:
UT0 = ST-aΦ-λ + 12h
λ is the observation sites of longitude (east longitude) takes the value.
observatories stars obtained by observing the initial value referred to as world UT0. Observers in different locations at the same instant obtained UT0 is different. In UT0 caused by the changes caused by the shift longitude correction Δλ, get a unified global UT1 world. I.e.
UT1 = UT0 + Δλ
Δλ = (xsinλ-ycosλ) tgφ
x, y is momentarily polar coordinates. It is the same as λ, are to CIO as a standard. Φ is the latitude of the observation site. UT1 is the basis for civil time worldwide; It also represents the instantaneous rotation angle of the Earth's spin axis, it is also a fundamental parameter is the study of the Earth's rotation movement. UT2 when added to the Earth's rotation speed UT1 seasonal variations in the correct ΔTs, you can get a smooth year in the world. That
ΔTS expression UT2 = UT1 + ΔTS = UT0 + Δλ + ΔTS
Since 1962, the international uniform application of:
ΔTS = 0s.022sin2πt-0s.012cos2πt-0s.006sin4πt + 0s.007cos4πt
t annual basis, starting from the beginning of the year Bessel.
Δλ and ΔTS value is calculated by the International Time Bureau (BIH) and inform the countries.
to vernal equinox as a basic reference point, determined by the vernal equinox Sun apparent motion time, sometimes referred to as the star (abbreviation ST). Local sidereal time a location is equal in value to the vernal equinox angle with respect to the local meridian. Due to precession and nutation, the vernal equinox is not fixed on the celestial sphere, corresponding to the same epoch, there is not really peace of the vernal equinox equinox. Accordingly, there are points when the true peace of stars when the star of stars. The reason
specified
Before the introduction of standard time, the civilized world each city set its clock according to local official position of the sun (see Sun Times) . This situation changed only after the UK until the introduction of rail travel, because of the emergence of the railway makes it possible to travel fast enough on long distances, requiring continuous re-set the clock, because the train will run daily through several towns. To solve this problem, the establishment of Greenwich Mean Time (Greenwich Mean Time), the practice is to put all the clocks Britain set to the same time while using a timer or a telegram to synchronize the clocks.
If you travel around the world from west to east, across every time zone, will put aside your watch forward an hour, so that when you cross 24 time zones back to put your watch also happens forward set aside for 24 hours, that is, the next day the same time already; the contrary, when you travel around the world from east to west circle, your table is indicated the day before the same hour. To avoid this "date disorder" phenomenon, uniform regulations on international longitude 180 ° "International Date Line." When you cross the international date line from west to east, must be subtracted from the day your timing system; on the contrary, from east to west across the international date line, it must be added one day.
defined
world, i.e., when the Greenwich mean sun, is a form of the Earth's rotation rate. As the Earth's rotation rate was considered to be uniform, so in 1960, when the world is considered to be a homogeneous. It is defined as the basis for the world Newcomb raised. It has been proved, in fact, the Earth's rotation is not uniform, so the world is when a non-uniform, atom or when it does not matter any theoretical mechanics, and only compare them by observation skills. In this way, when the definition of major world should be represented its relationship with the Earth's rotation rate.
System
basic unit is the mean solar mean solar day (see Japan), a flat 24 comprises a flat solar day sun hours (86,400 seconds flat sun). When to mean midnight as the start of 0:00 Greenwich mean solar time, sometimes called the world, referred to as UT. There are strict conversion formula between the star and the world. When the world is when the turned earth from the foundation, also known as the Earth's rotation. Each observatory observations of stars is obtained when the initial value UT0 world, it was suggested that the earth movement and non-uniformity of rotation of the earth, and was later confirmed by observation, but for a long time, although UT0 been as early as 200 years ago It is applied as a uniform time measurement system. It was introduced since 1956 in the polar motion UT0 △ λ seasonal variation and rotation speed experience correction △ T S, corresponding world is obtained UT1 and UT2. The relationship between them is:
UT1 = UT0 + △ λ,
UT2 = UT1 + △ TS = UT0 + △ λ + △ TS < / i>.
where △ λ = ( x sinλ- y cosλ) tg zi , which geographical latitude and longitude of the observation spot (λ, zi ) and the polar coordinates ( x , y ) related. Earth rotation unevenness having complex forms, comprising a cycle, long-term variations, short-term variations and irregularities variety of factors. The large number of people astronomical observation data obtained by the cycle (also called seasonal variation) experiences correction △ T S. It is a periodic function, while the annual rotation of the earth is not exactly the same, but little change in amplitude and phase, substantially stable in a certain range. Since 1962, △ International Time Bureau use of T S is:
△ T S = 0.022sin (2 πt) < /i>-0.012cos(2πt) -0.006sin (4 πt) +0.007 (cos4 πt) ,
< p> where t annual basis, starting from the beginning of the year Bessel. 1970 International Time Bureau from 1967 to 1969, according to the time measurement data around the world, we provide for short-term changes in the Earth's rotation correction value, in 1972 formally adopted.Determination
world by stellar observation, the projections of the star. Common measurement method and corresponding apparatus are: ① method ── transit transit instrument, photoelectric transit instrument, photographic zenith tube; ② Supreme Court ── Rider et ASTROLABE difference prism, photoelectric ASTROLABE. These observations using instruments, a night observation mean square error of about ± 5 ms. According to astronomical observations a year around the world, when the world after a comprehensive treatment of the resulting accuracy of about ± 1 ms. Due to various factors (mainly environmental factors) affect a long time, the measurement accuracy of a world without significant improvement. Measurement methods and techniques are facing an innovation. The new test method is mainly radio interferometry, satellite laser ranging and Doppler LLR and satellite observations like. The accuracy of the measurement is expected to increase by orders of magnitude.
world when the earth
UT1 (Universal Time, world) conversion between (for Terrestrial Time, be
ball) to TT celestial a fundamental problem is often encountered when dealing with the subject system measuring time, satellite navigation and positioning, precise orbit, monitoring and control communication, space geodetic, geophysics, etc., to solve this problem by calculating the difference ΔT between the two achieved:
(ΔT = TTT-TUT1)
TT is essentially based on the system defined by the atomic physics, is a high degree of stability, a uniform system time , and UT1 is a world defined by the Earth's rotation system based on. Since long-term slow earth rotation, seasonal variations and irregular change characteristics, thus UT1 is not a uniform system time, resulting in the value of ΔT is not uniform. The value of ΔT can not be accurately predicted, only after the data processing, and then give a specific numerical astronomical observation. ΔT the current data are available at certain time intervals gives a discrete number of specific epoch, need to know if any other value of the epoch, in general, can be obtained by calculation based on empirical formula ΔT fit real data. Related scholars made a lot of empirical formula for ΔT, a common form of trigonometric functions generally, low-order polynomial, high-order polynomial.
In general, the smaller the scope of empirical formula, or a limited precision calculation. Some scholars derived based on the least square method a new set of empirical formula, not only for a large range, in the form of simple, easy programming, and high accuracy results.
Application
1960 years ago, the world had as a basic time measurement systems are widely used. Due to changes in the speed of rotation of the earth, it is not a uniform system time. However, after as it relates to the earth's rotation angle, it was replaced when the almanac even in 1960 as a function of time measurement standard, world of everyday life, astronomy, navigation, geodesy and space vehicle tracking is still required. At the same time, precisely when the world is one of the basic data of the Earth's rotation, can provide the necessary base for the study of relevant disciplines of the Earth's rotation theory, the Earth's internal structure, plate motion, earthquake prediction and the Earth, Earth-Moon system, the origin and evolution of the solar system, etc. material.
Info
sidereal
with equinox as the base reference point, Sun apparent motion is determined by the vernal equinox time, Acronym ST. Local sidereal time a location is equal in value to the vernal equinox with respect to the angle where the local meridian.
mean solar
when the sun and the earth rotation corresponding to the angle, which, though in line to earth from the turned-based time measurement standards, but We can not meet the needs of daily life and scientific applications. Therefore, when they chose an average speed really apparent motion of the sun on Sunday based on mean solar time. Because the earth orbit is elliptical, so that the apparent motion of the sun really is not uniform (or true when the sun is not uniform). In order to obtain a real time measurement systems based on solar Sun apparent motion, while unrelated to its unevenness, introduced in the late nineteenth Newcomb an imaginary reference point - mean sun. It is for the celestial equator at uniform motion, the speed and the average speed of the sun true coincides with a convention which ascension expression determined, and a predetermined level sun Ascension of the Sun and the mean longitude difference should be as small as possible. With a predetermined level sun imaginary point as the time base reference point, called the mean solar time. Flat Ascension of the Sun numerical expression is:
wherein T is omitted from centuries 1900.0 counting Ru (Julian century equal to a mean solar day 36525). Newcomb proposed this expression, not only to the mean solar nowadays a precise definition, but also when the star between mean solar time and established a relationship of mutual conversion.