Using Skyfield with AstroPy¶

The AstroPy Project is a sprawling community effort to bring together a complete toolkit for working astronomers and astrophysicists. It can convert between dozens of different units, wrap numeric vectors in its own array and table data types, translate between a number of timescales and coordinate frames, run image processing algorithms on astronomical images, and query online sky catalogs.

Skyfield does not depend upon AstroPy, but can represent its results using AstroPy units. Here are the points of connection that Skyfield provides between the two libraries:

1. You can provide an AstroPy Time value and Skyfield will translate it into its own representation.

   from astropy.time import Time
   from skyfield.api import load
   
   atime = Time('2010-01-01T00:00:00', scale='utc')
   print(atime)
   
   ts = load.timescale()
   t = ts.from_astropy(atime)
   print(t.utc_jpl())
   

   2010-01-01T00:00:00.000
   A.D. 2010-Jan-01 00:00:00.0000 UTC
   

2. A skyfield Barycentric, Astrometric, or Apparent position and velocity can convert themselves into an AstroPy quantity using any linear and velocity units you specify.

   import astropy.units as u
   from skyfield.api import load
   
   planets = load('de421.bsp')
   earth = planets['earth']
   
   ts = load.timescale()
   t = ts.utc(1980, 1, 1)
   barycentric = earth.at(t)
   
   print(barycentric.position.to(u.au))
   print(barycentric.velocity.to(u.au / u.day))
   

   [-0.16287311  0.88787399  0.38473904] AU
   [-0.01721258 -0.00279426 -0.0012121 ] AU / d
   

3. A Skyfield position can also return a complete AstroPy SkyCoord object that couples the position vector with its reference frame.

   from astropy.coordinates import ICRS
   sc = barycentric.to_skycoord()
   print(sc)
   

   <SkyCoord (ICRS): (x, y, z) in AU
       (-0.16287311, 0.88787399, 0.38473904)>
   

4. A Skyfield angle can express itself as an AstroPy quantity in any requested unit of angular measure.

   ra, dec, distance = barycentric.radec()
   declination = dec.to(u.deg)
   print('{0:0.03f}'.format(declination))
   

   23.084 deg