地球時
维库，知识与思想的自由文库
地球時（TT）是現代在地球表面上的時間標準，他是位於大地水準面體驗的原時。在天文學，他為侷限在地球上的觀測者顯示星曆表的時間座標。他與地心座標時（Geocentric Coordinate Time）有直接的關聯，是天文的地球系統時間標準。由於重力時間擴張，地球時的滴答聲會以恒定的比例比地心座標時緩慢些。
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[编辑] 歷史
地球時的近似概念是由國際天文聯合會逐步規範出來的，在1976年的第16屆會員大會上，以地球力學時（Terrestrial Dynamical Time）的名義被提出，以相對於以太陽系星曆表為標準的質心動力時（Barycentric Dynamical Time）。這兩種時間標準在定義上只有少許的差異，而且在動力學上也不致對地球力學時造成誤解。
在1991年第21屆會員大會的第四號議案，國際天文聯合會重新更精確的定義，並改名為地球時。在同一次的會員大會上，地球時被定義為等同於地心座標時，地球時與地心座標時的關係是線性的變化，也就是與大地水準面是適當時間，這使得地球時與地心座標時的轉換可以經由驗證來決定。測定大地水準面的重力位能則是物理測量學的工作。
在2000年第24次會員大會的B1.9提案，國際天文聯合會又再次定義地球時，對地球時和地心座標時的比率給了明確的定義：6.969290134 × 10−10。這新的解釋重新精確定義了大地水準面的重力位能，因而消除了校正鐘錶對於海平面的需求。
[编辑] 定義
地球時和地心座標時間的差異只是比率上的常數，他由下列的等式定義：
- TT = (1 − LG) TCG + E
公式中的TT（地球時）和TCG（地心座標時）的關係是線性的，並且都以國際單位制的秒為計數的單位；LG是兩種時間之間的比例常數，E是為解決因為曆元不同造成差異的常數。LG 的精確數值是6.969290134 × 10−10。在1991年，TT初次被定義時，LG是經由實驗測量出來的，當時最佳的估計直是6.969291 × 10−10。
地球時和地心座標時較常見的關係式如下：
- TT = TCG − LG × (JDTCG − 2443144.5003725) × 86400
此處，JDTCG 是地心座標時的儒略日。這是會變動的地心座標時未經修正的計數所產生的秒數，所以這個等式的形是看起來有些複雜，對使用的曆元給了明確的儒略日。但是（參考下一篇論文），上述公式中的曆元曾經錯誤的被給為儒略日2443144.5，現在呈現的才是正確的數值。
Time coordinates on the TT and TCG scales are conventionally specified using traditional means of specifying days, carried over from non-uniform time standards based on the rotation of the Earth. Specifically, both Julian Dates and the Gregorian calendar are used. For continuity with their predecessor Ephemeris Time, TT and TCG were set to match ET at around Julian Date 2443144.5 (1977-01-01T00Z). More precisely, it was defined that TT instant 1977-01-01T00:00:32.184 exactly and TCG instant 1977-01-01T00:00:32.184 exactly correspond to the International Atomic Time (TAI) instant 1977-01-01T00:00:00.000 exactly. This is also the instant at which TAI introduced corrections for gravitational time dilation.
TT and TCG expressed as Julian Dates can be related precisely and most simply by the equation
- JDTT = EJD + (JDTCG − EJD) (1 − LG)
where EJD is 2443144.5003725 exactly.
[编辑] Realisation
TT is a Platonic time scale: a theoretical ideal, not dependent on a particular realisation. For practical purposes, TT must be realised by actual clocks in the Earth system.
The main realisation of TT is supplied by TAI. The TAI service, running since 1958, attempts to match the rate of proper time on the geoid, using an ensemble of atomic clocks spread over the surface and low orbital space of the Earth. TAI is canonically defined retrospectively, in monthly bulletins, in relation to the readings that particular groups of atomic clocks showed at the time. Estimates of TAI are also provided in real time by the institutions that operate the participating clocks. Because of the historical difference between TAI and ET when TT was introduced, the TAI realisation of TT is defined thus:
- TT(TAI) = TAI + 32.184 s
Because TAI is never revised once published, it is possible for errors in it to become known and remain uncorrected. It is thus possible to produce a better realisation of TT based on reanalysis of historical TAI data. The BIPM has done this approximately annually since 1992. These realisations of TT are named in the form "TT(BIPM05)", with the digits indicate the year of publication. They are published in the form of table of differences from TT(TAI). The latest as of April 2006 is TT(BIPM05).
The international communities of precision timekeeping, astronomy, and radio broadcasts are preparing to create a new precision time scale based on observations of an ensemble of pulsars. This new pulsar time scale will serve as an independent means of computing TT, and it may eventually be useful to identify defects in TAI.
