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    Re: Any "on line" Star Position Computation available ?
    From: Paul Hirose
    Date: 2011 Jun 21, 16:27 -0700

    Antoine Couette wrote:
     > Yes, Andr�s, it will happen that with the quite dramatic recent changes
     > in Astronomical Computations precepts, some software might be lagging
     > behind in terms of being adequately maintained.
    One example is the "new paradigm" for coordinates with respect to the
    celestial equator. I think the Almanac introduced it in 2006. Instead of
    the equinox as the reference system's "Greenwich", now it's the CIO
    (celestial intermediate origin). The CIO is a point which is always on
    the celestial equator but moves as little as possible. So it doesn't
    make a full circle of the sky in 26000 years like the equinox. It just
    wobbles around the same general area of the celestial sphere.
    Declination is the same as before, since the old and new paradigm both
    use the equator as the reference plane. The new right ascension is
    called "intermediate RA" to avoid confusion with the old "equinox RA".
    The angle that expresses the Earth's rotation with respect to the CIO is
    ERA (Earth rotation angle).
    In the new paradigm, you transform a body's ICRS coordinates to the
    intermediate frame, then rotate the frame by the ERA (Earth rotation
    angle) to obtain coordinates in a terrestrial system (it rotates with 
    the Earth). This does the same thing as transforming from the ICRS to a 
    true equator / true equinox system, then applying Greenwich apparent 
    sidereal time. Both methods yield the same result.
    The main advantage of the new paradigm is software reliability. ERA is
    insensitive to precession. In the old paradigm, the correct Greenwich
    apparent sidereal time routine depends on the precession / nutation
    model. For example, at the SOFA site there are five different GAST routines:
    Andres Ruiz wrote:
    > With this program based in NOVAS 3.1 I have obtained : GHA_ARIES = 211º 
    > 40.8'
    > date UTC UT1 TT UTCjd TTjd UT1jd dT objt GHA
    > 24/04/2011 0:00:00 _00:00:0.23339 _00:01:6.18400 2455675.5 2455675.501 
    > 2455675.5 66.41739 Aries 211º 40.8'
    > With http://aa.usno.navy.mil/cgi-bin/aa_flamenav.pl and with my own 
    > calculations based in Meeus´s "Astronomical algorithms" GHA_ARIES = 211º 
    > 40.9'
    > GAST = 14:06:43.49917
    > Why this 0.1' error?
    Perhaps a sign error?
    If I use -.23339 for UT1-UTC, GHA Aries = 211°40.82'.
    If I use +.23339 for UT1-UTC, GHA Aries = 211°40.93'.
    IERS Bulletin B No. 280
    (http://hpiers.obspm.fr/eop-pc/products/bulletins/bulletins.html) says
    UT1-UTC = -237.7005 ms at 2011-04-24 0 h UTC.
    With that value I get 14h06m43.2605s Greenwich apparent sidereal time at
    0 h UTC, with a sidereal time model compatible with IAU 2006 precession 
    and 2000A nutation.
    JPL HORIZONS says 14 06 43.2569
    The USNO MICA 2.0 program says 14 06 43.2990. However, MICA's time input
    only allows .1 s precision, so it's impossible to input a precise UT1
    corresponding to 0 h UTC. The best you can do is 23:59:59.8 UT1. But I
    can input that UT1 into my program, which says 14h06m43.2983s, only
    .0007 s less than the MICA.
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