API Reference¶

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.path_to`(filename)	Return the path to `filename` in this loader’s directory.
`Loader.timescale`([delta_t])	Open or download three time scale files, returning a `Timescale`.
`Loader.tle`(url[, reload, filename])	Load and parse a satellite TLE file.

Time scales¶

A Skyfield Timescale object is typically built at the beginning of each program:

from skyfield import api
ts = api.load_timescale()

It downloads and parses the data tables necessary to correctly convert between Universal Time and the more stable time scales used by astronomers.

`Timescale.now`()	Return the current date and time as a `Time` object.
`Timescale.utc`(year[, month, day, hour, …])	Build a `Time` from a UTC calendar date.
`Timescale.tai`([year, month, day, hour, …])	Build a `Time` from a TAI calendar date.
`Timescale.tai_jd`(jd)	Build a `Time` from a TAI Julian date.
`Timescale.tt`([year, month, day, hour, …])	Build a `Time` from a TT calendar date.
`Timescale.tt_jd`(jd)	Build a `Time` from a TT Julian date.
`Timescale.tdb`([year, month, day, hour, …])	Build a `Time` from a TDB calendar date.
`Timescale.tdb_jd`(jd)	Build a `Time` from a TDB Julian date.
`Timescale.ut1`([year, month, day, hour, …])	Build a `Time` from a UT1 calendar date.
`Timescale.ut1_jd`(jd)	Build a `Time` from UT1 a Julian date.
`Timescale.from_astropy`(t)	Build a Skyfield `Time` from an AstroPy time object.

Time objects¶

The Time class is Skyfield’s way of representing either a single time, or a whole array of times. Each object has four floating point attributes that present the time in several basic 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 decimal 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 an `A.D.2014-Jan-18 01:35:37.5000 UT` string..
`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 date 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 `pytz` provided timezone.
`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`()	Return TAI as a tuple (year, month, day, hour, minute, second).
`Time.tt_calendar`()	Return TT as a tuple (year, month, day, hour, minute, second).

Vector Functions¶

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

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

Planetary Ephemerides¶

By downloading a SpiceKernel file, Skyfield users can build vector functions predicting the positions of the Moon, Sun, and planets.

`SpiceKernel`(path)	Ephemeris file in NASA .bsp format.
`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.
`SpiceKernel.__getitem__`(target)	Return a vector function for computing the location of `target`.

Almanac¶

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

`phase_angle`(ephemeris, body, t)	Compute the phase angle of a body viewed from Earth.
`fraction_illuminated`(ephemeris, body, t)	Compute the illuminated fraction of a body viewed from Earth.
`find_discrete`(start_time, end_time, f[, …])	Find the times when a function changes value.
`seasons`(ephemeris)	Build a function of time that returns the quarter of the year.
`sunrise_sunset`(ephemeris, topos)	Build a function of time that returns whether the sun is up.
`moon_phases`(ephemeris)	Build a function of time that returns the moon phase 0 through 3.

Topocentric Locations¶

You can create a vector function that computes the location of any position on the Earth’s surface.

Topos([latitude, longitude, …]) A vector function that knows the position of a place on Earth.

Earth Satellites¶

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

EarthSatellite(line1, line2[, name, ts]) An Earth satellite loaded from a TLE file and propagated with SGP4.

Stars and other distant objects¶

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

Astronomical positions¶

The ICRF class serves as the base for all other positions classes, which each share its axes but have more specific meanings. Positions are usually returned by calling at(t) on a vector function rather than being constructed manually.

`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 geocenter.

Position methods and attributes¶

All position objects have three attributes which provide access to their raw data:

`ICRF.t`	The `Time` of the position.
`ICRF.position`	A `Distance` array giving x, y, z.
`ICRF.velocity`	A `Velocity` array giving ẋ, ẏ, ż.

If a position lacks a velocity, then the attribute is simply the value None.

All positions support a basic set of 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, distance)
`ICRF.separation_from`(another_icrf)	Return the angle between this position and another.
`ICRF.ecliptic_xyz`([epoch])	Compute J2000 ecliptic position vector (x, y, z).
`ICRF.ecliptic_velocity`()	Compute J2000 ecliptic velocity vector (x_dot, y_dot, z_dot)
`ICRF.ecliptic_latlon`([epoch])	Compute J2000 ecliptic coordinates (lat, lon, distance)
`ICRF.galactic_xyz`()	Compute galactic coordinates (x, y, z)
`ICRF.galactic_latlon`()	Compute galactic coordinates (lat, lon, distance)
`ICRF.from_altaz`([alt, az, alt_degrees, …])	Generate an Apparent position from an altitude and azimuth.

Position methods specific to one class¶

`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
`Geocentric.subpoint`()	Return the latitude and longitude directly beneath this position.

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 measure.
`Distance.au`	Astronomical Units.
`Distance.km`	Kilometers.
`Distance.m`	Meters.
`Velocity`	Velocity measure.
`Velocity.au_per_d`	Astronomical Units.
`Velocity.km_per_s`	Kilometers.
`Angle`	Angle measure.
`Angle.degrees`	Degrees of arc (360 in a complete circle).
`Angle.hours`	Hours of arc (24 in a complete circle).
`Angle.radians`	Radians (τ = 2π in a complete circle).

All three kinds of quantity support one or more methods.

`Distance.to`(unit)	Convert this distance to the given AstroPy unit.
`Velocity.to`(unit)	Convert this velocity to the given AstroPy unit.
`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])	Convert to a string like `12h 07m 30.00s`.
`Angle.dms`([warn])	Convert to a tuple (degrees, minutes, seconds).
`Angle.signed_dms`([warn])	Convert to a tuple (degrees, hours, minutes, seconds).
`Angle.dstr`([places, warn])	Convert to a string like `181deg 52' 30.0"`.