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Improve coordinate transformations for Solar System objects #9362
Description
This question on stack overflow comparing transformations between coordinate systems for asteroids and planets got me thinking that the coordinate transformations astropy assume the motion of the SkyCoord object is negligible.
For example, in the ICRS->GCRS transform we drop any velocity information in the SkyCoord, and use erfa.atciqz instead of erfa.atciq.
As the question on SO shows, the results can be out by a few degrees for nearby and fast moving objects...
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StuartLittlefair commentedon May 5, 2020
Thinking about this issue, I'm not sure we can do the "right thing" for fast moving solar system objects.
What is missing in our transformations is the planetary aberration term, or correcting for the fact that the light that reaches us at
obstimewas actually emitted earlier, when the body had a different position.For solar system objects we have ephemerides for we correct for this since we can predict their location at an arbitrary time. For other fast moving objects, even if the user provides instantaneous velocity information we can't correct unless we assume motion in a straight line.
See #10230 (comment) for the same point being raised in a similar location. Since that is being actively discussed I am flagging this as a duplicate and closing.