The international celestial reference frame (J2000.0) is very convenient, because it lets you register, and communicate star and planet positions across borders. But there is something I do not understand. Why can I not - at the same time - see geocentric coordinates for Ra/Dec J2000.0, and topocentric coordinates for Az/Alt of the date? It seems like I in settings must choose either topcentric, or not topocentric coordinates, but that seems illogical to me. I would like to use a setting where I - at the same time, and without first changing any setting - can see topocentric coordinates for Az/Alt, and geocentric coordinates for Ra/Dec. Is that possible?
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This is not possible. You are either on the surface or in the center of the planet you are standing on.
I am of course somewhere on the surface of the planet, and for that place I can always use the Az/Alt coordinates to see where the star or planet is on the sky. But I don't need both the Az/Alt, and the Ra/Dec coordinates to figure that out! And if I want to communicate where I see the star or planet Az/Alt, I think it would be useful if I should only communicate the geocentric coordinates Ra/Dec J2000.0, and then my friend somewhere else on the planet could figure those geocentric coordinates out in his or hers local coordinates Az/Alt.
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