Editing Terrestrial Time (section)

=== Other standards ===

TT is in effect a continuation of (but is more precisely uniform than) the former [[Ephemeris Time]] (ET).  It was designed for continuity with ET,<ref>P K Seidelmann (ed.) (1992), [https://books.google.com/books?id=uJ4JhGJANb4C&pg=PA42 'Explanatory Supplement to the Astronomical Almanac', at p.42]; also IAU Commission 4 (Ephemerides), Recommendations to IAU General Assembly 1976, Notes on Recommendation 5, note 2.</ref> and it runs at the rate of the SI second, which was itself derived from a calibration using the second of ET (see, under Ephemeris time, [[Ephemeris time#Redefinition of the second|Redefinition of the second]] and [[Ephemeris time#Implementations|Implementations]]). The [[Ephemeris time#JPL ephemeris time argument Teph|JPL ephemeris time argument T<sub>eph</sub>]] is within a few milliseconds of TT.

TT is slightly ahead of [[UT1]] (a refined measure of mean solar time at Greenwich) by an amount known as {{nobr|1=[[ΔT (timekeeping)|Δ''T'']] = TT − UT1.}}  Δ''T'' was measured at +67.6439 seconds (TT ahead of UT1) at 0&nbsp;h [[UTC]] on 1&nbsp;January 2015;<ref>[[US Naval Observatory]] ([[USNO]]) data file online at [https://web.archive.org/web/20190808224315/http://maia.usno.navy.mil:80/ser7/deltat.data https://web.archive.org/web/20190808224315/http://maia.usno.navy.mil:80/ser7/deltat.data (accessed 27 October 2015)].</ref> and by retrospective calculation, Δ''T'' was close to zero about the year 1900. Δ''T'' is expected to continue to increase, with UT1 becoming steadily (but irregularly) further behind TT in the future. In fine detail, Δ''T'' is somewhat unpredictable, with 10-year extrapolations diverging by 2-3 seconds from the actual value.<ref>{{cite web |title=Delta T: Past, Present and Future |url=http://asa.hmnao.com/SecK/DeltaT.html |website=The Astronomical Almanac Online |archive-url=https://web.archive.org/web/20220918033245/http://asa.hmnao.com/SecK/DeltaT.html |archive-date=18 September 2022 |date=2020 |url-status=dead}}</ref>