|
Az SI másodpercet 1967-ben határozták meg, mint a céziumatom adott számú rezgésszámát, ezt 1971-ben átnevezték nemzetközi atomidőre.(TAI).<ref>McCarthy & Seidelmann 2009, 202–4.</ref>
Az [[UTC]]-t 1961-ben vezették be. Az UTC nem egyenletesen haladó skála, mivel időnként a Föld egyenetlen forgásához igazítják [[szökőmásodperc]] beiktatásával. Az UTC 1971 végéig körülbelül megegyezett az [[UT2]]-vel, utána pedig az [[UT1]]-el.<ref>McCarthy & Seidelmann 2009, 227–9.</ref>
-----
In the 1970s, it became clear that the clocks participating in TAI were ticking at different rates due to [[gravitational time dilation]], and the combined TAI scale therefore corresponded to an average of the [[altitude]]s of the various clocks. Starting from Julian Date 2443144.5 (1 January 1977 00:00:00), corrections were applied to the output of all participating clocks, so that TAI would correspond to proper time at [[mean sea level]] (the geoid). Because the clocks had been on average well above sea level, this meant that TAI slowed down, by about 10<sup>−12</sup>. The former uncorrected time scale continues to be published, under the name ''EAL'' (''Echelle Atomique Libre'', meaning ''Free Atomic Scale'').<ref>McCarthy & Seidelmann, 215.</ref>
The instant that the gravitational correction started to be applied serves as the epoch for [[Barycentric Coordinate Time]] (TCB), [[Geocentric Coordinate Time]] (TCG), and [[Terrestrial Time]] (TT). All three of these time scales were defined to read JD 2443144.5003725 (1 January 1977 00:00:32.184) exactly at that instant. (The offset is to provide continuity with the older [[Ephemeris Time]].) TAI was henceforth a realisation of TT, with the equation TT(TAI) = TAI + 32.184 s.<ref>McCarthy & Seidelmann, 218–9.</ref>
The continued existence of TAI was questioned in a 2007 letter from the BIPM to the ITU-R which stated "In the case of a redefinition of UTC without leap seconds, the CCTF would consider discussing the possibility of suppressing TAI, as it would remain parallel to the continuous UTC."<ref>CCTF 09-27</ref>
==Jegyzetek==
|
