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skyfielders / python-skyfield

Elegant astronomy for Python
MIT License
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Diurnal aberration included? #785

Closed zero0cool0 closed 1 year ago

zero0cool0 commented 1 year ago

Firstly, great software @brandon-rhodes and all contributors, it's really a gem.

Not an issue, but a question, does this software include diurnal aberration when transforming from a local apparent to a topocentric coordinate system as shown in figure 2 of the SOFA cookbook for Earth Attitude?

I should probably mention that my Python skills are pretty limited, but from what I was able to learn from looking at the code is that the call to add_aberration() in positionlib.py passes the bcrs_velocity which would suggest annual aberration, but not diurnal aberration?

brandon-rhodes commented 1 year ago

The bcrs_velocity of an observer is their velocity vector with respect to the frame of reference of the Solar System's center of mass. For an observer on a planetary surface, it's the sum of the planet's motion vector with the observatory's motion vector with respect to the planet. Which I think means that all forms of aberration are automatically included?

zero0cool0 commented 1 year ago

Thank you for the quick response! Indeed, if bcrs_velocity does include the angular velocity of an observer on a planetary surface, then diurnal aberration seems accounted for.