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# ICRS

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

  Bases: `BaseRADecFrame`

  A coordinate or frame in the ICRS system.

  If you’re looking for “J2000” coordinates, and aren’t sure if you want to use this or `FK5`, you probably want to use ICRS. It’s more well-defined as a catalog coordinate and is an inertial system, and is very close (within tens of milliarcseconds) to J2000 equatorial.

  For more background on the ICRS and related coordinate transformations, see the references provided in the See Also section of the documentation.

  Parameters:
    data `BaseRepresentation` subclass instance

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

    ra `Angle`, optional, keyword-only

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

    dec `Angle`, optional, keyword-only

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

    distance `Quantity` [:ref: ‘length’], optional, keyword-only

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

    pm_ra_cosdec `Quantity` [: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_dec `Quantity` [:ref: ‘angular speed’], optional, keyword-only

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

    radial_velocity `Quantity` [:ref: ‘speed’], optional, keyword-only

      The radial velocity of this object.

    representation_type `BaseRepresentation` subclass, `python: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_type `BaseDifferential` subclass, `python:str`, `python: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`.

    copy bool, optional

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

  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` |

  Attributes Documentation

  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 = 'icrs'