# GCRS¶

class astropy.coordinates.builtin_frames.GCRS(*args, copy=True, representation_type=None, differential_type=None, **kwargs)[source]

A coordinate or frame in the Geocentric Celestial Reference System (GCRS).

GCRS is distinct form ICRS mainly in that it is relative to the Earth’s center-of-mass rather than the solar system Barycenter. That means this frame includes the effects of aberration (unlike ICRS). For more background on the GCRS, see the references provided in the See Also section of the documentation. (Of particular note is Section 1.2 of USNO Circular 179)

This frame also includes frames that are defined relative to the Earth, but that are offset (in both position and velocity) from the Earth.

The frame attributes are listed under Other Parameters.

Parameters
dataBaseRepresentation subclass instance

A representation object or None to have no data (or use the coordinate component arguments, see below).

raAngle, optional, must be keyword

The RA for this object (dec must also be given and representation must be None).

decAngle, optional, must be keyword

The Declination for this object (ra must also be given and representation must be None).

distanceQuantity, optional, must be keyword

The Distance for this object along the line-of-sight. (representation must be None).

pm_ra_cosdecQuantity, optional, must be keyword

The proper motion in Right Ascension (including the cos(dec) factor) for this object (pm_dec must also be given).

pm_decQuantity, optional, must be keyword

The proper motion in Declination for this object (pm_ra_cosdec must also be given).

radial_velocityQuantity, optional, must be keyword

The radial velocity of this object.

representation_typeBaseRepresentation subclass, str, optional

A representation class or string name of a representation class. This sets the expected input representation class, thereby changing the expected keyword arguments for the data passed in. For example, passing representation_type='cartesian' will make the classes expect position data with cartesian names, i.e. x, y, z in most cases unless overriden via frame_specific_representation_info. To see this frame’s names, check out <this frame>().representation_info.

differential_typeBaseDifferential subclass, str, dict, optional

A differential class or dictionary of differential classes (currently only a velocity differential with key ‘s’ is supported). This sets the expected input differential class, thereby changing the expected keyword arguments of the data passed in. For example, passing differential_type='cartesian' will make the classes expect velocity data with the argument names v_x, v_y, v_z unless overriden via frame_specific_representation_info. To see this frame’s names, check out <this frame>().representation_info.

copybool, optional

If True (default), make copies of the input coordinate arrays. Can only be passed in as a keyword argument.

Other Parameters
obstimeTime

The time at which the observation is taken. Used for determining the position of the Earth.

obsgeoloc

The position of the observer relative to the center-of-mass of the Earth, oriented the same as BCRS/ICRS. Either [0, 0, 0], CartesianRepresentation, or proper input for one, i.e., a Quantity with shape (3, …) and length units. Defaults to [0, 0, 0], meaning “true” GCRS.

obsgeovel

The velocity of the observer relative to the center-of-mass of the Earth, oriented the same as BCRS/ICRS. Either [0, 0, 0], CartesianRepresentation, or proper input for one, i.e., a Quantity with shape (3, …) and velocity units. Defaults to [0, 0, 0], meaning “true” GCRS.

Attributes Summary

Attributes Documentation

default_differential
default_representation
frame_attributes = {'obsgeoloc': <astropy.coordinates.attributes.CartesianRepresentationAttribute object>, 'obsgeovel': <astropy.coordinates.attributes.CartesianRepresentationAttribute object>, 'obstime': <astropy.coordinates.attributes.TimeAttribute object>}
frame_specific_representation_info
name = 'gcrs'
obsgeoloc = <CartesianRepresentation (x, y, z) in m (0., 0., 0.)>
obsgeovel = <CartesianRepresentation (x, y, z) in m / s (0., 0., 0.)>
obstime = <Time object: scale='tt' format='jyear_str' value=J2000.000>