Skyfield: HomeTable of ContentsChangelogAPI Reference

API Reference

Quick links to the sections below:

Version

Skyfield offers a tuple skyfield.VERSION that lets your code determine the installed version of Skyfield.

import skyfield
print(skyfield.VERSION)

See Checking your Skyfield version.

Opening files

# File you already have.

from skyfield.api import load_file
planets = load_file('~/Downloads/de405.bsp')
load_file(path) Open a file on your local drive, using its extension to guess its type.
# File you want Skyfield to download automatically.

from skyfield.api import load
ts = load.timescale()
planets = load('de405.bsp')
Loader(directory[, verbose, expire]) A tool for downloading and opening astronomical data files.
Loader.build_url(filename) Return the URL Skyfield will try downloading for a given filename.
Loader.days_old(filename) Return how recently filename was modified, measured in days.
Loader.download(url[, filename, backup]) Download a file, even if it’s already on disk; return its path.
Loader.path_to(filename) Return the path to filename in this loader’s directory.
Loader.timescale([delta_t, builtin]) Return a Timescale built using official Earth rotation data.
Loader.tle_file(url[, reload, filename, ts, …]) Load and parse a TLE file, returning a list of Earth satellites.

Time scales

A script will typically start by building a single Skyfield Timescale to use for all date and time conversions:

from skyfield import api
ts = api.load.timescale()

Its methods are:

Timescale.now() Return the current date and time as a Time object.
Timescale.from_datetime(datetime) Return a Time for a Python datetime.
Timescale.from_datetimes(datetime_list) Return a Time for a list of Python datetime objects.
Timescale.utc(year[, month, day, hour, …]) Build a Time from a UTC Calendar date.
Timescale.tai([year, month, day, hour, …]) Build a Time from an International Atomic Time Calendar date.
Timescale.tai_jd(jd[, fraction]) Build a Time from an International Atomic Time Julian date.
Timescale.tt([year, month, day, hour, …]) Build a Time from a Terrestrial Time Calendar date.
Timescale.tt_jd(jd[, fraction]) Build a Time from a Terrestrial Time Julian date.
Timescale.J(year) Build a Time from a Terrestrial Time Julian year or array.
Timescale.tdb([year, month, day, hour, …]) Build a Time from a Barycentric Dynamical Time Calendar date.
Timescale.tdb_jd(jd[, fraction]) Build a Time from a Barycentric Dynamical Time Julian date.
Timescale.ut1([year, month, day, hour, …]) Build a Time from a UT1 Universal Time Calendar date.
Timescale.ut1_jd(jd) Build a Time from a UT1 Universal Time Julian date.
Timescale.from_astropy(t) Build a Skyfield Time from an AstroPy time object.
Timescale.linspace(t0, t1[, num]) Return num times spaced uniformly between t0 to t1.

Time objects

The Time class is Skyfield’s way of representing either a single time, or a whole array of times. The same time can be represented in several different time scales.

t.tai International Atomic Time (TAI) as a Julian date.
t.tt Terrestrial Time (TT) as a Julian date.
t.J Terrestrial Time (TT) as floating point Julian years.
t.tdb Barycentric Dynamical Time (TDB) as a Julian date.
t.ut1 Universal Time (UT1) as a Julian date.

A couple of offsets between time scales are also available.

t.delta_t Difference TT − UT1 in seconds.
t.dut1 Difference UT1 − UTC in seconds.

Other time scales and conversions are available through its methods.

Time.utc_jpl() Convert to a string like A.D.2014-Jan-18 01:35:37.5000 UTC..
Time.utc_iso([delimiter, places]) Convert to an ISO 8601 string like 2014-01-18T01:35:38Z in UTC.
Time.utc_strftime([format]) Format the UTC time using a Python datetime formatting string.
Time.utc_datetime() Convert to a Python datetime in UTC.
Time.utc_datetime_and_leap_second() Convert to a Python datetime in UTC, plus a leap second value.
Time.astimezone(tz) Convert to a Python datetime in a particular timezone tz.
Time.astimezone_and_leap_second(tz) Convert to a Python datetime and leap second in a timezone.
Time.toordinal() Return the proleptic Gregorian ordinal of the UTC date.
Time.tai_calendar() TAI as a (year, month, day, hour, minute, second) Calendar date.
Time.tt_calendar() TT as a (year, month, day, hour, minute, second) Calendar date.
Time.tdb_calendar() TDB as a (year, month, day, hour, minute, second) Calendar date.
Time.ut1_calendar() UT1 as a (year, month, day, hour, minute, second) Calendar date.
Time.tai_strftime([format]) Format TAI with a datetime strftime() format string.
Time.tt_strftime([format]) Format TT with a datetime strftime() format string.
Time.tdb_strftime([format]) Format TDB with a datetime strftime() format string.
Time.ut1_strftime([format]) Format UT1 with a datetime strftime() format string.
Time.M 3×3 rotation matrix: ICRS → equinox of this date.
Time.MT 3×3 rotation matrix: equinox of this date → ICRS.
Time.gmst Greenwich Mean Sidereal Time (GMST) in hours.
Time.gast Greenwich Apparent Sidereal Time (GAST) in hours.
Time.nutation_matrix() Compute the 3×3 nutation matrix N for this date.
Time.precession_matrix() Compute the 3×3 precession matrix P for this date.
Time.to_astropy() Return an AstroPy object representing this time.

Time utilities

compute_calendar_date(jd_integer[, …]) Convert Julian day jd_integer into a calendar (year, month, day).

Vector functions

The common API shared by planets, Earth locations, and Earth satellites.

VectorFunction Given a time, computes a corresponding position.
VectorFunction.at(t) At time t, compute the target’s position relative to the center.

Either adding two vector functions v1 + v2 or subtracting them v1 - v2 produces a new function of time that, when invoked with .at(t), returns the sum or difference of the vectors returned by the two functions.

Planetary ephemerides

By downloading a SpiceKernel file, Skyfield users can build vector functions predicting the positions of the Moon, Sun, and planets. See Planets and their moons: JPL ephemeris files.

SpiceKernel(path) Ephemeris file in NASA .bsp format.
SpiceKernel.close() Close this ephemeris file.
SpiceKernel.comments() Return the comments string of this kernel.
SpiceKernel.names() Return all target names that are valid with this kernel.
SpiceKernel.decode(name) Translate a target name into its integer code.

Kernels also support lookup using the Python kernel['Mars'] syntax, in which case they return a function of time that returns vectors from the Solar System barycenter to the named body.

Planetary magnitudes

skyfield.magnitudelib.planetary_magnitude(position)

Given the position of a planet, return its visual magnitude.

>>> from skyfield.api import load
>>> from skyfield.magnitudelib import planetary_magnitude
>>> ts = load.timescale()
>>> t = ts.utc(2020, 7, 31)
>>> eph = load('de421.bsp')
>>> astrometric = eph['earth'].at(t).observe(eph['jupiter barycenter'])
>>> print('%.2f' % planetary_magnitude(astrometric))
-2.73

The formulae are from Mallama and Hilton “Computing Apparent Planetary Magnitude for the Astronomical Almanac” (2018). Two of the formulae have inherent limits:

  • Saturn’s magnitude is unknown and the function will return nan (the floating-point value “Not a Number”) if the “illumination phase angle” — the angle of the vertex observer-Saturn-Sun — exceeds 6.5°.
  • Neptune’s magnitude is unknown and will return nan if the illumination phase angle exceeds 1.9° and the position’s date is before the year 2000.

And one formula is not fully implemented (though contributions are welcome!):

  • Skyfield does not compute which features on Mars are facing the observer, which can introduce an error of ±0.06 magnitude.

Planetary reference frames

PlanetaryConstants Planetary constants manager.
Frame Planetary constants frame, for building rotation matrices.

Almanac

Routines to search for events like sunrise, sunset, and Moon phase.

seasons(ephemeris) Build a function of time that returns the quarter of the year.
moon_phase(ephemeris, t) Return the Moon phase 0°–360° at time t, where 180° is Full Moon.
moon_phases(ephemeris) Build a function of time that returns the moon phase 0 through 3.
moon_nodes(ephemeris) Build a function of time that identifies lunar nodes.
oppositions_conjunctions(ephemeris, target) Build a function to find oppositions and conjunctions with the Sun.
meridian_transits(ephemeris, target, topos) Build a function of time for finding when a body transits the meridian.
sunrise_sunset(ephemeris, topos) Build a function of time that returns whether the Sun is up.
dark_twilight_day(ephemeris, topos) Build a function of time returning whether it is dark, twilight, or day.
risings_and_settings(ephemeris, target, topos) Build a function of time that returns whether a body is up.
lunar_eclipses(start_time, end_time, eph) Return the lunar eclipses between start_time and end_time.

Geographic locations

Skyfield supports two Earth datums for translating between latitude/longitude and Cartesian coordinates. They each use a slightly different estimate of the Earth’s oblateness. The most popular is WGS84, which is used by the world’s GPS devices:

Each datum offers a method for taking a latitude and longitude and returning a GeographicPosition that knows its position in space:

Geoid.latlon(latitude_degrees, longitude_degrees) Return a GeographicPosition for a given latitude and longitude.

Going in the other direction, there are several methods for converting an existing Skyfield position into latitude, longitude, and height:

Geoid.latlon_of(position) Return the latitude and longitude of a geocentric position.
Geoid.height_of(position) Return height above the ellipsoid for a geocentric position.
Geoid.geographic_position_of(position) Return the GeographicPosition of a geocentric position.
Geoid.subpoint_of(position) Return the point on the ellipsoid directly below a position.

Once you have used either of the above approaches to build a GeographicPosition, it offers several methods:

GeographicPosition.at(t) At time t, compute the target’s position relative to the center.
GeographicPosition.lst_hours_at(t) Return the Local Apparent Sidereal Time, in hours, at time t.
GeographicPosition.refract(altitude_degrees, …) Predict how the atmosphere will refract a position.
GeographicPosition.rotation_at(t) Compute rotation from GCRS to this location’s altazimuth system.

Kepler orbits

See Kepler Orbits for computing the positions of comets, asteroids, and other minor planets.

Earth satellites

By downloading TLE satellite element sets, Skyfield users can build vector functions that predict their positions. See Earth Satellites.

EarthSatellite(line1, line2[, name, ts]) An Earth satellite loaded from a TLE file and propagated with SGP4.
EarthSatellite.from_satrec(satrec, ts) Build an EarthSatellite from a raw sgp4 Satrec object.
TEME The SGP4-specific True Equator Mean Equinox frame of reference.

Stars and other distant objects

Star The position in the sky of a star or other fixed object.

Astronomical positions

The ICRF three-dimensional position vector serves as the base class for all of the following position classes. Each class represents an (x,y,z) .position and .velocity vector oriented to the axes of the International Celestial Reference System (ICRS), an inertial system that’s an update to J2000 and that does not rotate with respect to the universe.

ICRF An (x,y,z) position and velocity oriented to the ICRF axes.
Barycentric An (x,y,z) position measured from the Solar System barycenter.
Astrometric An astrometric (x,y,z) position relative to a particular observer.
Apparent An apparent (x,y,z) position relative to a particular observer.
Geocentric An (x,y,z) position measured from the center of the Earth.

Positions are usually generated by the at(t) method of a vector function, rather than being constructed manually. But you can also build a position directly from a raw vector, or from right ascension and declination coordinates with position_of_radec().

position_of_radec(ra_hours, dec_degrees[, …]) Build a position object from a right ascension and declination.

All position objects offer five basic attributes:

.position An (x,y,z) Distance.
.velocity An (x,y,z) Velocity, or None.
.t The Time of the position, or None.
.center Body the vector is measured from.
.target Body the vector is measured to.

All positions support these methods:

ICRF.distance() Compute the distance from the origin to this position.
ICRF.speed() Compute the magnitude of the velocity vector.
ICRF.radec([epoch]) Compute equatorial RA, declination, and distance.
ICRF.hadec() Compute hour angle, declination, and distance.
ICRF.separation_from(another_icrf) Return the angle between this position and another.
ICRF.frame_xyz(frame) Return this position as an (x,y,z) vector in a reference frame.
ICRF.frame_xyz_and_velocity(frame) Return (x,y,z) position and velocity vectors in a reference frame.
ICRF.frame_latlon(frame) Return longitude, latitude, and distance in the given frame.
ICRF.frame_latlon_and_rates(frame) Return a reference frame longitude, latitude, range, and rates.
ICRF.from_time_and_frame_vectors(t, frame, …) Constructor: build a position from two vectors in a reference frame.
ICRF.to_skycoord([unit]) Convert this distance to an AstroPy SkyCoord object.
ICRF.phase_angle(sun) Return this position’s phase angle: the angle Sun-target-observer.
ICRF.fraction_illuminated(sun) Return the fraction of the target’s disc that is illuminated.
ICRF.is_sunlit(ephemeris) Return whether a position in Earth orbit is in sunlight.
ICRF.from_altaz([alt, az, alt_degrees, …]) Generate an Apparent position from an altitude and azimuth.

In addition to the methods above, several subclasses of the base position class provide unique methods of their own:

Barycentric.observe(body) Compute the Astrometric position of a body from this location.
Astrometric.apparent() Compute an Apparent position for this body.
Apparent.altaz([temperature_C, pressure_mbar]) Compute (alt, az, distance) relative to the observer’s horizon

Reference frames

skyfield.framelib.true_equator_and_equinox_of_date The dynamical frame of the Earth’s true equator and equinox of date.
skyfield.framelib.itrs The International Terrestrial Reference System (ITRS).
skyfield.framelib.ecliptic_frame Reference frame of the true ecliptic and equinox of date.
skyfield.framelib.ecliptic_J2000_frame Reference frame of the true ecliptic and equinox at J2000.
skyfield.framelib.galactic_frame Galactic System II reference frame.

Constellations

skyfield.api.load_constellation_map()

Load Skyfield’s constellation boundaries and return a lookup function.

Skyfield carries an internal map of constellation boundaries that is optimized for quick position lookup. Call this function to load the map and return a function mapping position to constellation name.

>>> from skyfield.api import position_of_radec, load_constellation_map
>>> constellation_at = load_constellation_map()
>>> north_pole = position_of_radec(0, 90)
>>> constellation_at(north_pole)
'UMi'

If you pass an array of positions, you’ll receive an array of names.

skyfield.api.load_constellation_names()

Return a list of abbreviation-name tuples, like ('Aql', 'Aquila').

You can pass the list to Python’s dict() to build a dictionary that turns a constellation abbreviation into a full name:

>>> from skyfield.api import load_constellation_names
>>> d = dict(load_constellation_names())
>>> d['UMa']
'Ursa Major'

By swapping the order of the two items, you can map the other way, from a full name back to an abbreviation:

>>> f = dict(reversed(item) for item in load_constellation_names())
>>> f['Ursa Major']
'UMa'
skyfield.data.stellarium.parse_constellations(lines)

Return a list of constellation outlines.

Each constellation outline is a list of edges, each of which is drawn between a pair of specific stars:

[
    (name, [(star1, star2), (star3, star4), ...]),
    (name, [(star1, star2), (star3, star4), ...]),
    ...
]

Each name is a 3-letter constellation abbreviation; each star is an integer Hipparcos catalog number. See Drawing a finder chart for comet NEOWISE for an example of how to combine this data with the Hipparcos star catalog to draw constellation lines on a chart.

skyfield.data.stellarium.parse_star_names(lines)

Return the names in a Stellarium star_names.fab file.

Returns a list of named tuples, each of which offers a .hip attribute with a Hipparcos catalog number and a .name attribute with the star name. Do not depend on the tuple having only length two; additional fields may be added in the future.

Searching

skyfield.searchlib.find_discrete()

Find the times at which a discrete function of time changes value.

This routine is used to find instantaneous events like sunrise, transits, and the seasons. See Searching for the dates of astronomical events for how to use it yourself.

skyfield.searchlib.find_maxima()

Find the local maxima in the values returned by a function of time.

This routine is used to find events like highest altitude and maximum elongation. See Searching for the dates of astronomical events for how to use it yourself.

skyfield.searchlib.find_minima()

Find the local minima in the values returned by a function of time.

This routine is used to find events like minimum elongation. See Searching for the dates of astronomical events for how to use it yourself.

Osculating orbital elements

This routine returns osculating orbital elements for an object’s instantaneous position and velocity.

osculating_elements_of(position[, …]) Produce the osculating orbital elements for a position.
OsculatingElements.apoapsis_distance Distance object
OsculatingElements.argument_of_latitude Angle object
OsculatingElements.argument_of_periapsis Angle object
OsculatingElements.eccentric_anomaly Angle object
OsculatingElements.eccentricity numpy.ndarray
OsculatingElements.inclination Angle object
OsculatingElements.longitude_of_ascending_node Angle object
OsculatingElements.longitude_of_periapsis Angle object
OsculatingElements.mean_anomaly Angle object
OsculatingElements.mean_longitude Angle object
OsculatingElements.mean_motion_per_day Angle object
OsculatingElements.periapsis_distance Distance object
OsculatingElements.periapsis_time Time object
OsculatingElements.period_in_days numpy.ndarray
OsculatingElements.semi_latus_rectum Distance object
OsculatingElements.semi_major_axis Distance object
OsculatingElements.semi_minor_axis Distance object
OsculatingElements.time Time object
OsculatingElements.true_anomaly Angle object
OsculatingElements.true_longitude Angle object

Units

Distance Distance
Velocity Velocity
Angle Angle
AngleRate Rate at which an angle is changing

All three kinds of quantity support one or more methods.

Distance.au Astronomical units.
Distance.km Kilometers (1,000 meters).
Distance.m Meters.
Distance.length() Compute the length when this is an (x,y,z) vector.
Distance.light_seconds() Return the length of this vector in light seconds.
Distance.to(unit) Convert this distance to the given AstroPy unit.
Velocity.au_per_d Astronomical units per day.
Velocity.km_per_s Kilometers per second.
Velocity.m_per_s Meters per second.
Velocity.to(unit) Convert this velocity to the given AstroPy unit.
Angle.radians Radians (𝜏 = 2𝜋 in a circle).
Angle.hours Hours (24h in a circle).
Angle.degrees Degrees (360° in a circle).
Angle.arcminutes() Return the angle in arcminutes.
Angle.arcseconds() Return the angle in arcseconds.
Angle.mas() Return the angle in milliarcseconds.
Angle.to(unit) Convert this angle to the given AstroPy unit.
Angle.hms([warn]) Convert to a tuple (hours, minutes, seconds).
Angle.signed_hms([warn]) Convert to a tuple (sign, hours, minutes, seconds).
Angle.hstr([places, warn, format]) Return a string like 12h 07m 30.00s; see Formatting angles.
Angle.dms([warn]) Convert to a tuple (degrees, minutes, seconds).
Angle.signed_dms([warn]) Convert to a tuple (sign, degrees, minutes, seconds).
Angle.dstr([places, warn, format]) Return a string like 181deg 52' 30.0"; see Formatting angles.
AngleRate.radians Rate of change in radians.
AngleRate.degrees Rate of change in degrees.
AngleRate.arcminutes Rate of change in arcminutes.
AngleRate.arcseconds Rate of change in arcseconds.
AngleRate.mas Rate of change in milliarcseconds.
Rate.per_day Units per day of Terrestrial Time.
Rate.per_hour Units per hour of Terrestrial Time.
Rate.per_minute Units per minute of Terrestrial Time.
Rate.per_second Units per second of Terrestrial Time.

Trigonometry

position_angle_of(anglepair1, anglepair2) Return the position angle of one position with respect to another.