Ephemeris time

The introduced in 1960 time scale of Ephemeris Time (. ET, English Ephemeris Time) is defined by the dynamics of the solar system, completely uniform measure of time - as opposed to civil time ( Universal Time, GMT, etc.) depends on the current rotation of the earth. The introduction of the ET was carried out by the International Astronomical Union in consultation with other natural sciences and metrology.

1984, the Ephemeris time has been through the Dynamic Time (DT ), internationally abbreviated TD (Fr. temps dynamique ) replaced. It operates in two versions, the TDT, Terrestrial Dynamic Time ( geocentric) and the TDB, Barycentric Dynamic Time (relative to the center of mass of the solar system ).

The fundamental unit ephemeris is a specific definition of the unit of seconds, in contrast to the second of the solar day has a constant length. It is based on the mean rotation of the earth 1900-1905 and in 1967 replaced by the atomic second, now the SI second.

Basics

The small irregularities of the earth's rotation, were suspected since about 1890 and there are a few milliseconds in the area, had already summed up to the mid-20th century to several tens of seconds, providing urgent need existed for a time reform. Around 1950, the accuracy of a decade earlier invented quartz watches had increased so much that a proper reform of the system time could be accomplished.

The noticeable until today monotonically increasing time difference of ephemeris and civil time, which makes every 1-2 years need a leap second will depend, inter alia, begun in 1900 with the interdisciplinary down the fundamental quantities of science and technology together. It took place regarding the timing system on the basis of years of internationally coordinated measurement campaigns, which ran from 1900 to 1905. In the former results of our time system is based to this day - see the definition of Erdrotationspols CIO and the Geodetic Reference System GRS80.

Now, however, the Earth's rotation was 1900-1905 - how to only able to prove through increased measurement accuracy later - a bit slower than the long term average. This ran the ground-based and defined by earth and planetary orbits, accurate time scale increasingly apart, which has been fixed by the definition of Ephemeris physically reproducible. While it remained in 1960 continued in the second 60 years previously defined, gave her to distinguish it from other fundamental measures of time the name ephemeris.

The name comes from Ephemeris Ephemeris, of which astronomy a mathematical system of formulas and tables understands, be calculated in advance with the times of astronomical events. The first ephemeris probably go back to ancient astronomers and were from about 1300 significantly refined for the increasing shipping and accurate navigation (see Alfonsinische panels, lunar orbit, Jupiter 's moons and length problem).

Precise definition

As ephemeris served from 1960 31,556,925.9747 ste part of the tropical year, as the 0th January 1900 ( = December 31, 1899 ) was 12:00 UT.

At this time, certain universal time ( the time UT, Universal Time) and ET match almost. The Ephemeris was an ideal uniform passage of time, based on which the calculation of astronomical events over long periods of time ( thousands of years ) was reliable. Although the deviation of the Ephemeris of different time systems (symbol AT) could be predicted with a certain probability, but on the basis of astronomical measurements (mainly surveys of the lunar race ) are always only subsequently determined accurately.

In the ET- introduction in 1964 this was Zeitkorrektion? T = TDT - UTC about 40 seconds while it had grown to the beginning of 2006 by the required due to the non-uniform rotation of the earth leap seconds to 65.184 s (?).

1984: Second reform of the system time

In the late 1970s, technological progress, the methods of measuring time again made ​​more accurate by more than 1 order of magnitude, the International Astronomical Union decided to date, January 1, 1984 to a careful redefinition of some time scales:

• The Ephemeris Time (ET ) was established by Terrestrial Dynamic Time DT ( correctly TDT Terrestrial Dynamical Time = ) superseded whose base is the SI second.
• At the same time led (because of the relativity of time ) is valid for the dynamics of the solar system time scale TDB, which at its center of gravity refers ( the barycenter ). The barycenter is not exactly in the center of the sun, but because of the planetary masses near the solar limb - depending on the position of Jupiter and Saturn either just inside or just outside the sun.