# API Reference — Reference Frames

The `skyfield.framelib` module provides a number of standard reference frames, whose use is described in Coordinates in other reference frames.

class skyfield.framelib.mean_equator_and_equinox_of_date

  The coordinate frame of Earth’s mean equator and equinox.

  This frame is used for measuring right ascension and declination. It tracks the Earth’s ‘mean’ equator and equinox which shift slowly across the sky due to precession, but ignores the smaller effects of nutation.

class skyfield.framelib.true_equator_and_equinox_of_date

  The dynamical frame of Earth’s true equator and true equinox of date.

  This frame is used for measuring right ascension and declination. Unlike the fixed reference frames J2000 and the ICRS, this ‘TETE’ frame rotates slowly as the Earth’s precession and nutation shift the equinox point. Unlike the `TEME` frame, this frame doesn’t ignore nutation.

  This is supplied as an explicit reference frame in case you want (x,y,z) coordinates; if you want angles, it’s better to use the standard position method `radec(epoch='date')` since that will return the conventional units of hours-of-right-ascension instead of the degrees-of-longitude that `frame_latlon()` would return.

  This reference frame combines current theories of the Earth’s precession and nutation with a small offset between the ITRS and J2000 systems to produce right ascension and declination for a given date relative to the Earth’s axis and equator of rotation.

skyfield.framelib.itrs = <skyfield.framelib.itrs object>

  The International Terrestrial Reference System (ITRS).

  This is the IAU standard for an Earth-centered Earth-fixed (ECEF) coordinate system, anchored to the Earth’s crust and continents. This reference frame combines three other reference frames: the Earth’s true equator and equinox of date, the Earth’s rotation with respect to the stars, and (if your `Timescale` has polar offsets loaded) the polar wobble of the crust with respect to the Earth’s pole of rotation.

  Added in version 1.34.

skyfield.framelib.ecliptic_frame = <skyfield.framelib.ecliptic_frame object>

  Reference frame of the true ecliptic and equinox of date.

skyfield.framelib.ecliptic_J2000_frame = <skyfield.framelib.InertialFrame object>

  Reference frame of the true ecliptic and equinox at J2000.

skyfield.framelib.galactic_frame = <skyfield.framelib.InertialFrame object>

  Galactic System II reference frame.