ET UTCM
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Contents: Description, Arguments, Usage, Examples, Sub-Functions, Related Functions, Source Supported Under Version: > 1.15
Description Find ET-UTC offset, given UTC Modified Julian Date
Arguments and Return Values Parameters: Numeric array specifying the UTC Modified Julian Dates for which to find the ET-UTC offsets. Return Value: Double-precision ET-UTC offsets, in seconds
Usage Syntax: ET_UTCM(MJD) Coordinated Universal Time (UTC) is the basis for the time system used in everyday life. In order to keep UTC consistent with the Earth's slightly variable rate of rotation, leap seconds are occasionally inserted into UTC. Terrestrial Time (TT) runs at the same rate but has no leap seconds. Both are referenced to the rotating Earth. Ephemeris Time (ET) is the number of seconds since noon (ET) on January 1, 2000. ET is referenced to the solar system's barycenter (center of mass). It runs at the same rate as Barycentric Time (TB, a.k.a. Barycentric Dynamical Time or TDB). This function is accurate within about 0.000030 seconds after 1972, if the input is that accurate. Before 1972, the accuracy may be further limited by uncertainty in Delta-T. See the function DeltaT for details. This function finds the difference between ET and UTC for the user-specified dates/times. Note that what it really returns is equivalent to the difference (ET-UTC, in seconds) between the input UTC Modified Julian Date (converted from days to seconds) and the corresponding Ephemeris Time (in seconds): MJD2ET(input MJD) - 86400*((input MJD) - MJD(2000, 1, 1, 12, 0, 0, 0)). Since the MJD and ET have different starting points, taking their difference without first subtracting the ET's starting point from the MJD would be rather pointless (and susceptible to roundoff error). For more information, see ftp://naif.jpl.nasa.gov/pub/naif/pds/data/ody-m-spice-6-v1.0/odsp_1000/data/lsk/naif0008.tls and http://heasarc.gsfc.nasa.gov/docs/xte/abc/time_tutorial.html. When ET_UTCM() is entered without any arguments, it prints its description, as shown below.
Examples dv> ET_UTCM() Find ET-UTC offset, given UTC Modified Julian Date Works for array inputs Returns double-precision ET-UTC offset, in seconds Ephemeris Time (ET) is the number of seconds since noon (ET) on January 1, 2000. ET is referenced to the solar system's barycenter (center of mass). It runs at the same rate as Barycentric Time (TB, a.k.a. TDB). This function finds the difference between ET and UTC for the user-specified dates/times. Note that what it really returns is equivalent to the difference (ET-UTC, in seconds) between the input UTC Modified Julian Date (converted from days to seconds) and the corresponding Ephemeris Time (in seconds): MJD2ET(input MJD) - 86400*((input MJD) - MJD(2000,1,1,12,0,0)) Since the MJD and ET have different starting points, taking their difference without first subtracting the ET's starting point from the MJD would be rather pointless (and susceptible to roundoff error). Accurate within about 0.000030 seconds after 1972 (if the input is that accurate). Before 1972, the accuracy may be further limited by uncertainty in Delta-T. See the function DeltaT for details. See ftp://naif.jpl.nasa.gov/pub/naif/pds/data/ody-m-spice-6-v1.0/ odsp_1000/data/lsk/naif0008.tls, the function DeltaT, and http://heasarc.gsfc.nasa.gov/docs/xte/abc/time_tutorial.html S.Marshall 01-23-2010 0 dv> ET_UTCM(MJD(2010, 1, 1, 12, 0, 0)) 66.1839389709322 dv> a = MJD(2010, 1, 1, 12, 0, 0) 55197.5000000000 dv> ET_UTCM(a) 66.1839389709322 dv> MJD2ET(a) - 86400*(a - MJD(2000, 1, 1, 12, 0, 0)) 66.1839389801025 dv> # So ET-UTC as found in those two ways is equal within stated level of accuracy |
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Recent Library Changes Created On: 11-18-2009 |
