How to query a region on the sky when given only ecliptic coordinates?
This is because the ICRS has an origin at the barycenter and this is not the same as the center of the Sun (the heliocenter). The heliocenter is offset from the barycenter (although it's still within the radius of the Sun) and varies with time, primarily due to the perturbations of Jupiter and Saturn.

If you use BarycentricTrueEcliptic it will work fine:

from astropy.coordinates import BarycentricTrueEcliptic
from astropy import units as u

cvz = BarycentricTrueEcliptic(lon=0*u.deg, lat=-90*u.deg)
cvz.transform_to(ICRS)
Out[9]:
  &lt;ICRS Coordinate: (ra, dec) in deg
  (89.9999853, -66.56071866)&gt;


Which agrees with the Coordinate conversions using the LAMBDA website at NASA's HEASARC