Why NCP is not on 0h 90 position (RA/Dec)?

[promm1983]

Hi I'm noobie in astronomy.
I'm just wondering why NCP in Stellarium is not on 0h 90 position and is drifting thru the time. Shouldn't it be ideally on 0h 90 degrees?

[gzotti]

https://en.wikipedia.org/wiki/Precession

[alex-w]

You're get the NCP from SIMBAD - https://simbad.u-strasbg.fr/simbad/sim-basic?Ident=NCP&submit=SIMBAD+search - with aberrated coordinates + Stellarium added another aberration shift.

@gzotti the second aberration is a bug and we should add antiaberration shift for objects from SIMBAD or remove aberration correction to custom objects.

[alex-w]

Done!

[gzotti]

What coordinates does SIMBAD deliver for physical objects? I had expected "Mean J2000" (catalog values without aberration). Is that in contrast coordinates including nutation, aberration (, light deflection, ...)? Is NCP or other non-physical entities handled in the same way?

[alex-w]

Good question and probably anwser in documents for SIMBAD