I'm trying to calculate the Julian Day, given a UTC year, month and date in the Gregorian calendar. I tried using the formula on Wikipedia, but that doesn't work. Consider 2010-01-31 and 2010-02-01. These dates are exactly one day apart, but their JDNs, according to the formula on Wikipedia, are 2455230 and 2455229, respectively. Predicted values of UT1 - UTC are provided as an Earth Orientation Product. An example showing the variation of the length of the day to late 2008 is shown below. Units are milliseconds. Julian Day Number is a count of days elapsed since Greenwich mean noon on 1 January 4713 B.C., Julian proleptic calendar. New eclipse ide.
Contents:Description, Arguments, Usage, Examples, Sub-Functions, Related Functions, Source
Current .dvrc Version:
Find ET-UTC offset, given UTC Julian Date
Parameters: Numeric array specifying the UTC Julian Dates for which to find the ET-UTC offsets.
Return Value: Double-precision ET-UTC offsets, in seconds
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 Julian Date (converted from days to seconds) and the corresponding Ephemeris Time (in seconds): JD2ET(input JD) - 86400*((input JD) - JD(2000, 1, 1, 12, 0, 0, 0)). Since the JD and ET have different starting points, taking their difference without first subtracting the ET's starting point from the JD 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_UTC() is entered without any arguments, it prints its description, as shown below.
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