GCRS

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

Bases: BaseRADecFrame

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, keyword-only

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

decAngle, optional, keyword-only

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

distanceQuantity [:ref: ‘length’], optional, keyword-only

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

pm_ra_cosdecQuantity [:ref: ‘angular speed’], optional, keyword-only

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

pm_decQuantity [:ref: ‘angular speed’], optional, keyword-only

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

radial_velocityQuantity [:ref: ‘speed’], optional, keyword-only

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 overridden 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 overridden 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.

obsgeolocCartesianRepresentation, Quantity

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.

obsgeovelCartesianRepresentation, Quantity

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

default_differential	Default representation for differential data (e.g., velocity)
default_representation	Default representation for position data
frame_attributes
frame_specific_representation_info	Mapping for frame-specific component names
name
obsgeoloc
obsgeovel
obstime

Attributes Documentation

default_differential

Default representation for differential data (e.g., velocity)

default_representation

Default representation for position data

frame_attributes = {'obsgeoloc': <astropy.coordinates.attributes.CartesianRepresentationAttribute object>, 'obsgeovel': <astropy.coordinates.attributes.CartesianRepresentationAttribute object>, 'obstime': <astropy.coordinates.attributes.TimeAttribute object>}

frame_specific_representation_info

Mapping for frame-specific component names

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>