Ephemeris time
The time scale of ephemeris time ( ET , English Ephemeris Time ) is a completely uniform time measure defined by the dynamics of the solar system - in contrast to civil time ( world time , CET , etc.), which is based on the current rotation of the earth .
The ephemeris time was introduced in 1960 by the International Astronomical Union in consultation with other natural sciences and metrology ; In 1984 it was replaced by the Terrestrial Time TT (from French Temps Terrestrique ) and the Barycentric Dynamic Time TDB (from French Temps Dynamique Barycentrique ); see. below "Second Reform ...".
The fundamental unit ephemeris second is a specific definition of the unit second , which, unlike the second of the solar day, has a constant length. The ephemeris second is based on the mean Earth's rotation 1900–1905 and was replaced in 1967 by the atomic second, today's second.
definition
From 1960 the 31,556,925.9747th part of the tropical year served as the ephemeris second , as it existed on January 0, 1900 (= December 31, 1899) 12:00 UT .
At this point, i. H. 1899/1900, voted world time (the time UT , Universal Time ) and ET match almost. The Ephemeris Time was an ideally uniform time, on the basis of which the calculation of astronomical events over long periods of time (millennia) was reliably possible.
The deviation Delta T of the ephemeris time from other time systems could indeed be predicted with a certain probability, but could only be precisely determined afterwards using astronomical measurements (mainly measurements of the course of the moon ).
Basics
Around 1900 the interdisciplinary definition of the fundamental parameters of science and technology began . With regard to the time system, it was carried out on the basis of years of internationally coordinated measurement campaigns that ran from 1900 to 1905. Our time system is based on the results from that time - see also the definition of the Earth's rotation pole CIO and the geodetic reference system GRS80 .
From 1900 to 1905, however, the rotation of the earth was slower than the long-term average , as could only be demonstrated later through increased measuring accuracy . As a result, civil time, which is linked to this rotation, and the more precise time scale defined by the orbits of the earth and planets diverged increasingly, which had been assumed since around 1890.
This time correction Δ T had added up to several tens of seconds until the middle of the 20th century; in 1964 it was about 40 seconds (?). As a result, there was an urgent need for a time reform. By 1950 the accuracy of the quartz watches, invented a decade earlier, had risen to such an extent that a flawless reform of the time system could be achieved.
The above Time deviation was explained physically reproducible in 1960 by the definition of the ephemeris time . The second, defined 60 years earlier, was still used, but was given the name ephemeris second to distinguish it from other fundamental measures of time . In addition, because of the uneven rotation of the earth, a leap second was introduced into the time every 1–2 years .
etymology
The name ephemeris time comes from ephemeris , by which astronomy understands a mathematical system of formulas and tables with which the times of astronomical events are calculated in advance. The first ephemeris probably go back to ancient astronomers and were significantly refined from around 1300 for increasing shipping and more precise navigation (see Alfonsine tables , lunar orbit , Jupiter's moons and length problem ).
1984: Second reform of the time system
When, at the end of the 1970s, technical progress made the methods of time measurement more accurate by more than 1 power of ten, the International Astronomical Union decided on January 1, 1984 to carefully redefine some time scales:
- The Ephemeris Time (ET) was replaced by the Terrestrial Time (TT), which refers to the geocenter and whose basis is the second of the International System of Units .
- At the same time, due to the relativity of time, the Barycentric Dynamic Time (TDB) valid for the dynamics of the solar system was introduced , which refers to the center of mass of the system. The barycentre is not exactly in the center of the sun , but rather close to the edge of the sun because of the mass of the planets - depending on the position of Jupiter and Saturn either just inside or just outside the sun.
- Finally, the exact relation to atomic time was defined, which is more essential for physical laboratories than the earth's rotation (UT) or its orbit around the sun (ET).