Unexpected behavior with SkyCoord.separation method

Dear Astropy,

Thanks for such a useful project!

I noticed an unexpected behavior when computing separations between two sets of SkyCoords. When one of the SkyCoods is an array and the other is a single value, array.separation(value) produces the correct result but value.separation(array) does not. Here’s a simple example involving computing the distance between the Kepler field and the moon.

Cheers,

Erik

%pylab inline
from astropy.time import Time
import astropy.coordinates
from astropy.coordinates import SkyCoord, EarthLocation
from astropy import units as u

loc = EarthLocation.of_site('KPNO')

t0 = Time('2017-01-01 00:00:00', scale='utc')
t = t0 + np.arange(0.,365,1) * u.day
moon = astropy.coordinates.get_moon(t,location=loc)
kepler = SkyCoord('19h22m', '+44d30m', frame='icrs')

sep = moon.separation(kepler)
plot(t.datetime, sep.degree)

sep = kepler.separation(moon)
plot(t.datetime, sep.degree)

[petigura]

Thanks for looking into this! If this is the expected behavior than that’s great. It might help others if the docs were a little more explicit about this.

• The get_moon docstring doesn’t say anything about the coordinates being in the GCRS frame.
• I imagine that calculating target/moon and target/sun separations is a pretty common use-case of the astropy package, so if the separation method is counter intuitive, this should be stated explicitly.

Cheers,

Erik

On Sep 30, 2017, at 11:39 PM, StuartLittlefair [e-mail] wrote:

@adrn is right. It may not be particularly intuitive - I’ve been bitten by this myself in the past - but this is the expected, and I think correct, behaviour.

posA.separation(posB) calculates the separation of A and B, in the frame of object A. The mistake is to think that separation gives the separation on the sky, when it does not.

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[ricozan]

I just want to clarify that when dealing with the moon and a target, e.g for preparing observations one should use:

moon.separation( target )

thanks! I was also puzzled by this and I’m glad I found this post.