(Another) GCRS angular separation behavior #8505
Description
This issue is to report another counter-intuitive behavior of angular separations in the GCRS frame. It follows on similar reports in #5185 and #6633 (and may be of interest to @StuartLittlefair and @adrn).
The basic issue is that an object at fixed ICRS position observed at two different times will have different GCRS coordinates. However, the separation between these GCRS coordinates is calculated to be zero. Here is a MWE:
from astropy.coordinates import SkyCoord, GCRS, ICRS
from astropy.time import Time
obj = SkyCoord(ra=37.0*u.degree,dec=-4.5*u.degree, distance=3*u.au)
obj_t0 = obj.transform_to(GCRS(obstime=Time(57370.0, format='mjd')))
obj_t1 = obj.transform_to(GCRS(obstime=Time(57380.0, format='mjd')))
print(obj_t0)
print(obj_t1)
print("Obj(t0) RA,DEC: ", obj_t0.ra.deg,obj_t0.dec.deg)
print("Obj(t1) RA,DEC: ",obj_t1.ra.deg,obj_t1.dec.deg)
print(obj_t0.separation(obj_t1))
print(obj_t1.separation(obj_t0))
and the output:
<SkyCoord (GCRS: obstime=57370.0, obsgeoloc=(0., 0., 0.) m, obsgeovel=(0., 0., 0.) m / s): (ra, dec, distance) in (deg, deg, AU)
(22.02874564, -14.46782793, 2.49351509)>
<SkyCoord (GCRS: obstime=57380.0, obsgeoloc=(0., 0., 0.) m, obsgeovel=(0., 0., 0.) m / s): (ra, dec, distance) in (deg, deg, AU)
(20.38720594, -13.68925892, 2.64894343)>
Obj(t0) RA,DEC: 22.028745635550386 -14.467827929473025
Obj(t1) RA,DEC: 20.387205935596295 -13.689258920287072
0d00m00s
0d00m00s
My understanding is that the frames of obj_t0 and obj_t1 differ (both are GCRS, but at different obstime). Thus, separation is converting the two coordinates to the same obstime, and thus the angular separation is calculated to be zero.
print("Equivalent frame?",obj_t0.is_equivalent_frame(obj_t1))
While I'm guessing this is the "correct" behavior, it is definitely counter-intuitive (i.e., when trying to compute parallax corrections for solar system objects).