Skyfield

Elegant Astronomy for Python

Skyfield computes positions for the stars, planets, and satellites in orbit around the Earth. Its results should agree with the positions generated by the United States Naval Observatory and their Astronomical Almanac to within 0.0005 arcseconds (which equals half a “mas” or milliarcsecond).

Computing the position of Mars in the sky is as easy as:

from skyfield.api import load

planets = load('de421.bsp')
earth, mars = planets['earth'], planets['mars']

ts = load.timescale()
t = ts.now()
astrometric = earth.at(t).observe(mars)
ra, dec, distance = astrometric.radec()

print(ra)
print(dec)
print(distance)
10h 47m 56.24s
+09deg 03' 23.1"
2.33251 au

Skyfield can compute geocentric coordinates, as shown in the example above, or topocentric coordinates specific to your location on the Earth’s surface:

from skyfield.api import Topos

boston = earth + Topos('42.3583 N', '71.0636 W')
astrometric = boston.at(t).observe(mars)
alt, az, d = astrometric.apparent().altaz()

print(alt)
print(az)
25deg 27' 54.0"
101deg 33' 44.0"

Skyfield does not depend on the AstroPy project or its compiled libraries. But it accepts AstroPy time objects as input, and can return results in native AstroPy units:

from astropy import units as u
xyz = astrometric.position.to(u.au)
altitude = alt.to(u.deg)

print(xyz)
print('{0:0.03f}'.format(altitude))
[-2.19049548  0.71236701  0.36712443] AU
25.465 deg

Documenation

Skyfield’s documentation lives here at the main Skyfield web site:

But the source code and issue tracker live on other web sites:

Most recent release: Version 1.3 on 2018 April 15

See the Changelog for all previous versions!