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 (half a “mas” or milliarcsecond).

• Written in pure Python and installs without any compilation.
• Supports Python 2.6, Python 2.7 and Python 3.
• Has NumPy as its only binary dependency, the fundamental package for scientific computing with Python, whose vector operations make Skyfield efficient.

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

from skyfield.api import load

# Create a timescale and ask the current time.
ts = load.timescale()
t = ts.now()

# Load the JPL ephemeris DE421 (covers 1900-2050).
planets = load('de421.bsp')
earth, mars = planets['earth'], planets['mars']

# What's the position of Mars, viewed from Earth?
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 N, W, wgs84

boston = earth + wgs84.latlon(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' 44.1"

While Skyfield itself has no dependency on the AstroPy library, it’s willing to accept AstroPy time objects as input and 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

Academics can cite Skyfield as ascl:1907.024 or 2019ascl.soft07024R (more…)