[Orekit Users] Right ascension and declination

[Philippe Demoulin <Philippe.Demoulin@aeronomie.be>]

Hi everybody,

I am completely new to Orekit, and I followed the 3-day course at CS 
last week (thanks Luc !).  I just started with a small program to 
compute the right ascension and declination of Jupiter :

           AbsoluteDate date = new AbsoluteDate(2017, 12, 6, 10, 17, 0.0,
                   TimeScalesFactory.getUTC());

    	   CelestialBody body   = CelestialBodyFactory.getJupiter() ;
           Frame eme2000 = FramesFactory.getEME2000();

           Vector3D bodyInEME2000  = body.getPVCoordinates(date, 
eme2000).getPosition();

           double rightAscension = bodyInEME2000.getAlpha() ;
           if (rightAscension < 0) rightAscension = rightAscension + 2. 
* FastMath.PI ;
           double declination = bodyInEME2000.getDelta() ;
           System.out.format(Locale.US, "At %s UTC, %s RA = %10.5f°, 
DEC = %10.5f° %n",
                   date, body.getName(), 
FastMath.toDegrees(rightAscension),
                   FastMath.toDegrees(declination)) ;

The output of this program is :
"At 2017-12-06T10:17:00.000 UTC, Jupiter RA =  219.81633°, DEC = 
-14.44226°"

Computing the same parameters with the JPL Horizons system 
(https://ssd.jpl.nasa.gov/horizons.cgi) gives RA = 219.81392° and DEC = 
-14.44186°, i.e. a difference of 8.7" in RA and -1.4" in DEC.

Do you have any idea of the reason of these different answers ?

Thank you for your help.

Philippe Demoulin
Institut royal d'Aéronomie Spatiale de Belgique
Avenue circulaire 3, 1180 Uccle