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Re: Any "on line" Star Position Computation available ?
From: Paul Hirose
Date: 2011 Jun 21, 16:27 -0700
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: http://www.iausofa.org/2010_1201_F/ERA_ST.html 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. --