Conversion from Latitude, Longitude, Altitude to ICRS

[troyrock]

Is there a way to convert a location from Latitude, Longitude, Altitude to ICRS? Thank you.

[lulunac27a]

ICRS uses coordinates with right ascension and declination, so it needs latitude, longitude, altitude, and azimuth at a specified date and time.

[brandon-rhodes]

Happily, yes! If you build a geographic location object, its .at() method should compute its location in ICRS coordinates:

from skyfield.api import load, wgs84

ts = load.timescale()
t = ts.utc(2020, 4, 23)

lowell = wgs84.latlon(35.2029, -111.6646)

print(lowell.at(t).xyz.km)

Could you suggest how the documentation of Skyfield might be expanded or improved to make this easier to discover? That would guide me in getting this better explained, for other folks who might have the same question.

[troyrock]

I did find this information but I don't see a way to define the altitude in this case. I admit that defining a location in the sky above the Earth may not be commonly desired and that may be the reason the altitude is not included in the definition here.

[brandon-rhodes]

Oh, drat, I forgot the altitude in my example code! The latlon() method lets you specify a distance above the geoid:

https://rhodesmill.org/skyfield/api-topos.html#skyfield.toposlib.Geoid.latlon

latlon(latitude_degrees, longitude_degrees, elevation_m=0.0)

So providing that third argument should give you the position you need.