.. include:: references.txt

.. _astropy-coordinates-high-level:

Using the SkyCoord High-level Class
-----------------------------------

The |SkyCoord| class provides a simple and flexible user interface for
celestial coordinate representation, manipulation, and transformation between
coordinate frames.  This is a high-level class that serves as a wrapper
around the low-level coordinate frame classes like `~astropy.coordinates.ICRS`
and `~astropy.coordinates.FK5` which do most of the heavy lifting.

The key distinctions between |SkyCoord| and the low-level classes
(:doc:`frames`) are as follows:

- The |SkyCoord| object can maintain the union of frame attributes for all
  built-in and user-defined coordinate frames in the
  ``astropy.coordinates.frame_transform_graph``.  Individual frame classes hold
  only the required attributes (e.g. equinox, observation time or observer
  location) for that frame.  This means that a transformation from
  `~astropy.coordinates.FK4` (with equinox and observation time) to
  `~astropy.coordinates.ICRS` (with neither) and back to
  `~astropy.coordinates.FK4` via the low-level classes would not remember the
  original equinox and observation time.  Since the |SkyCoord| object stores
  all attributes, such a round-trip transformation will return to the same
  coordinate object.

- The |SkyCoord| class is more flexible with inputs to accomodate a wide
  variety of user preferences and available data formats.

- The |SkyCoord| class has a number of convenience methods that are useful
  in typical analysis.

- At present, |SkyCoord| objects can use only coordinate frames that have
  transformations defined in the ``astropy.coordinates.frame_transform_graph``
  transform graph object.

Creating SkyCoord objects
^^^^^^^^^^^^^^^^^^^^^^^^^^^

The |SkyCoord| class accepts a wide variety of inputs for initialization.
At a minimum these must provide one or more celestial coordinate values
with unambiguous units.  Typically one also specifies the coordinate
frame, though this is not required.

Common patterns are shown below.  In this description the values in upper
case like ``COORD`` or ``FRAME`` represent inputs which are described in detail
in the `Initialization Syntax`_ section.  Elements in square brackets like
``[unit=UNIT]`` are optional.
::

  SkyCoord(COORD, [FRAME], keyword_args ...)
  SkyCoord(LON, LAT, [frame=FRAME], [unit=UNIT], keyword_args ...)
  SkyCoord([FRAME], <lon_attr>=LON, <lat_attr>=LAT, keyword_args ...)

The examples below illustrate common ways of initializing a |SkyCoord| object.
These all reflect initializing using spherical coordinates, which is the
default for all built-in frames.  In order to understand working with coordinates
using a different representation such as cartesian or cylindrical, see the
section on `Representations`_.  First some imports::

  >>> from astropy.coordinates import SkyCoord  # High-level coordinates
  >>> from astropy.coordinates import ICRS, Galactic, FK4, FK5  # Low-level frames
  >>> from astropy.coordinates import Angle, Latitude, Longitude  # Angles
  >>> import astropy.units as u
  >>> import numpy as np

The coordinate values and frame specification can now be provided using
positional and keyword arguments.  First we show positional arguments for
RA and Dec::

  >>> SkyCoord(10, 20, unit="deg")  # Defaults to ICRS
  <SkyCoord (ICRS): ra=10.0 deg, dec=20.0 deg>

  >>> SkyCoord([1, 2, 3], [-30, 45, 8], "icrs", unit="deg")
  <SkyCoord (ICRS): (ra, dec) in deg
      [(1.0, -30.0), (2.0, 45.0), (3.0, 8.0)]>

Notice that the first example above does not explicitly give a frame.  In
this case, the default is taken to be the ICRS system (approximately
correct for "J2000" equatorial coordinates).  It is always better to
explicitly specify the frame when it is known to be ICRS, however, as
anyone reading the code will be better able to understand the intent.

String inputs in common formats are acceptable, and the frame can be supplied
as either a class type like `~astropy.coordinates.FK4` or the lower-case
version of the name as a string, e.g. ``"fk4"``::

  >>> coords = ["1:12:43.2 +1:12:43", "1 12 43.2 +1 12 43"]
  >>> sc = SkyCoord(coords, FK4, unit=(u.hourangle, u.deg), obstime="J1992.21")
  >>> sc = SkyCoord(coords, 'fk4', unit='hourangle,deg', obstime="J1992.21")

  >>> sc = SkyCoord("1h12m43.2s", "+1d12m43s", Galactic)  # Units from strings
  >>> sc = SkyCoord("1h12m43.2s +1d12m43s", Galactic)  # Units from string
  >>> sc = SkyCoord("galactic", l="1h12m43.2s", b="+1d12m43s")

Astropy `~astropy.units.Quantity`-type objects are acceptable and encouraged
as a form of input::

  >>> ra = Longitude([1, 2, 3], unit=u.deg)  # Could also use Angle
  >>> dec = np.array([4.5, 5.2, 6.3]) * u.deg  # Astropy Quantity
  >>> sc = SkyCoord(ra, dec, frame='icrs')
  >>> sc = SkyCoord(ICRS, ra=ra, dec=dec, obstime='2001-01-02T12:34:56')

Finally it is possible to initialize from a low-level coordinate frame object.

  >>> c = FK4(1 * u.deg, 2 * u.deg)
  >>> sc = SkyCoord(c, obstime='J2010.11', equinox='B1965')  # Override defaults

A key subtlety highlighted here is that when low-level objects are created they have
certain default attribute values.  For instance the `~astropy.coordinates.FK4`
frame uses ``equinox='B1950.0`` and ``obstime=equinox`` as defaults.  If
this object is used to initialize a |SkyCoord| it is possible to override
the low-level object attributes that were not explicitly set.  If the
coordinate above were created with
``c = FK4(1 * u.deg, 2 * u.deg, equinox='B1960')`` then creating a |SkyCoord|
with a different ``equinox`` would raise an exception.

Initialization Syntax
"""""""""""""""""""""

For spherical representations, which are the most common and are the default
input format for all built-in frames, the syntax for |SkyCoord| is given
below::

  SkyCoord(COORD, [FRAME | frame=FRAME], [unit=UNIT], keyword_args ...)
  SkyCoord(LON, LAT, [DISTANCE], [FRAME | frame=FRAME], [unit=UNIT], keyword_args ...)
  SkyCoord([FRAME | frame=FRAME], <lon_name>=LON, <lat_name>=LAT, [unit=UNIT],
           keyword_args ...)

In the above description, elements in all capital letters (e.g. ``FRAME``)
describes a user input of that element type.  Elements in square brackets are
optional.  For non-spherical inputs see the `Representations`_ section.


**LON**, **LAT**

Longitude and latitude value can be specified as separate positional arguments.
The following options are available for longitude and latitude:

- Single angle value:

  - |Quantity| object
  - Plain numeric value with ``unit`` keyword specifying the unit
  - Angle string which is formatted for :ref:`angle-creation` of
    |Longitude| or |Latitude| objects

- List or |Quantity| array or numpy array of angle values
- |Angle|, |Longitude|, or |Latitude| object, which can be scalar or
  array-valued

**DISTANCE**

The distance to the object from the frame center can be optionally specified:

- Single distance value:

  - |Quantity| or `~astropy.coordinates.Distance` object
  - Plain numeric value for a dimensionless distance
  - Plain numeric value with ``unit`` keyword specifying the unit

- List or |Quantity| or `~astropy.coordinates.Distance` array or numpy array of
  angle values

**COORD**

This input form uses a single object to supply coordinate data.  For the case
of spherical coordinate frames, the coordinate can include one or more
longitude and latitude pairs in one of the following ways:

- Single coordinate string with a LON and LAT value separated by a space.  The
  respective values can be any string which is formatted for
  :ref:`angle-creation` of |Longitude| or |Latitude| objects, respectively.
- List or numpy array of such coordinate strings
- List of (LON, LAT) tuples, where each LON and LAT are scalars (not arrays)
- ``N x 2`` numpy or |Quantity| array of values where the first column is
  longitude and the second column is latitude, e.g.
  ``[[270, -30], [355, +85]] * u.deg``
- List of (LON, LAT, DISTANCE) tuples
- ``N x 3`` numpy or |Quantity| array of values where columns are
  longitude, latitude, and distance respectively.

The input can also be more generalized objects that are not necessarily
represented in the standard spherical coordinates:

- Coordinate frame object, e.g. ``FK4(1*u.deg, 2*u.deg, obstime='J2012.2')``
- |SkyCoord| object (which just makes a copy of the object)
- `~astropy.coordinates.BaseRepresentation` subclass object like
  `~astropy.coordinates.SphericalRepresentation`,
  `~astropy.coordinates.CylindricalRepresentation`,  or
  `~astropy.coordinates.CartesianRepresentation`.

**FRAME**

This can be a `~astropy.coordinates.BaseCoordinateFrame` frame
class or the corresponding string alias.  The frame classes that are built in
to astropy are `~astropy.coordinates.ICRS`, `~astropy.coordinates.FK5`,
`~astropy.coordinates.FK4`, `~astropy.coordinates.FK4NoETerms`,
`~astropy.coordinates.Galactic`, and `~astropy.coordinates.AltAz`.
The string aliases are simply lower-case versions of the class name.

If the frame is not supplied then you will see a special ``ICRS``
identifer.  This indicates that the frame is unspecified and operations
that require comparing coordinates (even within that object) are not allowed.

**unit=UNIT**

The unit specifier can be one of the following:

- `~astropy.units.Unit` object which is an angular unit that is equivalent to
  ``Unit('radian')``
- Single string with a valid angular unit name
- 2-tuple of `~astropy.units.Unit` objects or string unit names specifying the
  LON and LAT unit respectively, e.g. ``('hourangle', 'degree')``
- Single string with two unit names separated by a comma, e.g. ``'hourangle,degree'``

If only a single unit is provided then it applies to both LON and LAT.

**Other keyword arguments**

In lieu of positional arguments to specify the longitude and latitude, the
frame-specific names can be used as keyword arguments:

*ra*, *dec*: **LON**, **LAT** values, optional
    RA and Dec for frames where these are representation, including [FIXME]
    `~astropy.coordinates.ICRS`, `~astropy.coordinates.FK5`,
    `~astropy.coordinates.FK4`, and `~astropy.coordinates.FK4NoETerms`.

*l*, *b*:  **LON**, **LAT** values, optional
    Galactic ``l`` and ``b`` for the `~astropy.coordinates.Galactic` frame.

The following keywords can be specified for any frame:

*distance*: valid `~astropy.coordinates.Distance` initializer, optional
    Distance from reference from center to source.

*obstime*: valid `~astropy.time.Time` initializer, optional
    Time of observation

*equinox*: valid `~astropy.time.Time` initializer, optional
    Coordinate frame equinox

If custom user-defined frames are included in the transform graph and they
have additional frame attributes, then those attributes can also be
set via corresponding keyword args in the |SkyCoord| initialization.

Array operations
^^^^^^^^^^^^^^^^^

It is possible to store arrays of coordinates in a |SkyCoord| object, and
manipulations done in this way will be orders of magnitude faster than
looping over a list of individual |SkyCoord| objects::

  >>> ra = np.random.uniform(0, 360, size=1000) * u.deg
  >>> dec = np.random.uniform(-90, 90, size=1000) * u.deg

  >>> sc_list = [SkyCoord(r, d, 'icrs') for r, d in zip(ra, dec)]
  >>> timeit sc_gal_list = [c.galactic for c in sc_list]  # doctest: +SKIP
  1 loops, best of 3: 7.66 s per loop

  >>> sc = SkyCoord(ra, dec, 'icrs')
  >>> timeit sc_gal = sc.galactic  # doctest: +SKIP
  100 loops, best of 3: 8.92 ms per loop

In addition to vectorized transformations, you can do the usual array
slicing, dicing, and selection::

  >>> north_mask = sc.dec > 0
  >>> sc_north = sc[north_mask]
  >>> len(sc_north)  # doctest: +SKIP
  504
  >>> sc[2:4]  # doctest: +SKIP
  <SkyCoord (ICRS): (ra, dec) in deg
      [(304.304015..., 6.900282...),
       (322.560148..., 34.872244...)]>
  >>> sc[2]  # doctest: +SKIP
  <SkyCoord (ICRS): ra=304.304015... deg, dec=6.900282... deg>


Attributes
^^^^^^^^^^^

The |SkyCoord| object has a number of useful attributes which come in handy.
By digging through these we'll learn a little bit about |SkyCoord| and how it
works.

To begin (if you don't know already) one of the most important tools for
learning about attributes and methods of objects is "TAB-discovery".  From
within IPython you can type an object name, the period, and then the <TAB> key
to see what's available.  This can often be faster than reading the
documentation::

  >>> sc = SkyCoord(1, 2, 'icrs', unit='deg', obstime='2013-01-02 14:25:36')
  >>> sc.<TAB>  # doctest: +SKIP
  sc.cartesian                           sc.match_to_catalog_3d
  sc.data                                sc.match_to_catalog_sky
  sc.dec                                 sc.name
  sc.default_representation              sc.obstime
  sc.distance                            sc.position_angle
  sc.equinox                             sc.ra
  sc.fk4                                 sc.realize_frame
  sc.fk4noeterms                         sc.represent_as
  sc.fk5                                 sc.representation
  sc.frame                               sc.representation_component_names
  sc.frame_attr_names                    sc.representation_component_units
  sc.frame_specific_representation_info  sc.representation_info
  sc.from_name                           sc.separation
  sc.galactic                            sc.separation_3d
  sc.get_frame_attr_names                sc.shape
  sc.has_data                            sc.spherical
  sc.icrs                                sc.time_attr_names
  sc.is_frame_attr_default               sc.to_string
  sc.is_transformable_to                 sc.transform_to
  sc.isscalar

Here we see a bunch of stuff there but much of it should be recognizable or
easily guessed.  The most obvious may be the longitude and latitude attributes
which are named ``ra`` and ``dec`` for the ``ICRS`` frame::

  >>> sc.ra
  <Longitude 1.0 deg>
  >>> sc.dec
  <Latitude 2.0 deg>

Next notice that all the built-in frame names ``icrs``, ``galactic``, ``fk5``
``fk4``, and ``fk4noeterms`` are there.  Through the magic of Python
properties, accessing these attributes calls the object
`~astropy.coordinates.SkyCoord.transform_to` method appropriately and returns a
new |SkyCoord| object in the requested frame::

  >>> sc_gal = sc.galactic
  >>> sc_gal  # doctest: +FLOAT_CMP
  <SkyCoord (Galactic): l=99.6379436471 deg, b=-58.7096055983 deg>

Other attributes you should recognize are ``distance``, ``equinox``,
``obstime``, ``shape``.

Digger deeper
"""""""""""""""
*[Casual users can skip this section]*

After transforming to Galactic the longitude and latitude values are now
labeled ``l`` and ``b``, following the normal convention for Galactic
coordinates.  How does the object know what to call its values?  The answer
lies in some less-obvious attributes::

  >>> sc_gal.representation_component_names
  OrderedDict([(u'l', u'lon'), (u'b', u'lat'), (u'distance', u'distance')])

  >>> sc_gal.representation_component_units
  OrderedDict([(u'l', Unit("deg")), (u'b', Unit("deg"))])

  >>> sc_gal.representation
  <class 'astropy.coordinates.representation.SphericalRepresentation'>

Together these tell the object that ``l`` and ``b`` are the longitude and
latitude, and that they should both be displayed in units of degrees as
a spherical-type coordinate (and not, e.g. a cartesian coordinate).
Furthermore the frame's ``representation_component_names`` attribute defines
the coordinate keyword arguments that |SkyCoord| will accept.

Another important attribute is ``frame_attr_names``, which defines the
additional attributes that are required to fully define the frame::

  >>> sc_fk4 = SkyCoord(1, 2, 'fk4', unit='deg')
  >>> sc_fk4.get_frame_attr_names()
  {u'equinox': <Time object: scale='tai' format='byear_str' value=B1950.000>,
   u'obstime': None}

The key values correspond to the defaults if no explicit value is provide by
the user.  This example shows that the `~astropy.coordinates.FK4` frame has two
attributes ``equinox`` and ``obstime`` that are required to fully define the
frame.

Some trickery is happening here because many of these attributes are
actually owned by the underlying coordinate ``frame`` object which does much of
the real work.  This is the middle layer in the three-tiered system of objects:
representation (spherical, cartesian, etc.), frame (aka low-level frame class),
and |SkyCoord| (aka high-level class)::

  >>> sc.frame
  <ICRS Coordinate: ra=1.0 deg, dec=2.0 deg>

  >>> sc.has_data is sc.frame.has_data
  True

  >>> sc.frame.<TAB>  # doctest: +SKIP
  sc.frame.cartesian                           sc.frame.ra
  sc.frame.data                                sc.frame.realize_frame
  sc.frame.dec                                 sc.frame.represent_as
  sc.frame.default_representation              sc.frame.representation
  sc.frame.distance                            sc.frame.representation_component_names
  sc.frame.frame_attr_names                    sc.frame.representation_component_units
  sc.frame.frame_specific_representation_info  sc.frame.representation_info
  sc.frame.get_frame_attr_names                sc.frame.separation
  sc.frame.has_data                            sc.frame.separation_3d
  sc.frame.is_frame_attr_default               sc.frame.shape
  sc.frame.is_transformable_to                 sc.frame.spherical
  sc.frame.isscalar                            sc.frame.time_attr_names
  sc.frame.name                                sc.frame.transform_to
  >>> sc.frame.name
  'icrs'

The |SkyCoord| object exposes the ``frame`` object attributes as its own.  Though
it might seem a tad confusing at first, this a good thing because it makes
|SkyCoord| objects and `~astropy.coordinates.BaseCoordinateFrame` objects
behave very similarly and most routines can accept either one as input without
much bother (duck typing!).

The lowest layer in the stack is the abstract
`~astropy.coordinates.UnitSphericalRepresentation` object:

  >>> sc_gal.frame.data
  <UnitSphericalRepresentation lon=1.739010... rad, lat=-1.024675... rad>

Transformations
^^^^^^^^^^^^^^^^^

The topic of transformations is covered in detail in the section on
:ref:`astropy-coordinates-transforming`.

For completeness here we will give some simple examples.  Once you've defined
your coordinates and the reference frame, you can transform from that frame to
another frame.  You can do this a few different ways: if you just want the
default version of that frame, you can use attribute-style access (as mentioned
previously).  For more control, you can use the
`~astropy.coordinates.SkyCoord.transform_to` method, which accepts a frame
name, frame class, frame instance, or |SkyCoord|::

  >>> from astropy.coordinates import FK5
  >>> sc = SkyCoord(1, 2, 'icrs', unit='deg')
  >>> sc.galactic  # doctest: +FLOAT_CMP
  <SkyCoord (Galactic): l=99.6379436471 deg, b=-58.7096055983 deg>

  >>> sc.transform_to('fk5')  # Same as sc.fk5 and sc.transform_to(FK5)  # doctest: +FLOAT_CMP
  <SkyCoord (FK5): equinox=J2000.000, ra=1.00000655566 deg, dec=2.00000243092 deg>

  >>> sc.transform_to(FK5(equinox='J1975'))  # Transform to FK5 with a different equinox  # doctest: +FLOAT_CMP
  <SkyCoord (FK5): equinox=J1975.000, ra=0.679672818323 deg, dec=1.86083014099 deg>

Transforming to a |SkyCoord| instance is an easy way of ensuring that two
coordinates are in the exact same reference frame::

  >>> sc2 = SkyCoord(3, 4, 'fk4', unit='deg', obstime='J1978.123', equinox='B1960.0')
  >>> sc.transform_to(sc2)
  <SkyCoord (FK4): equinox=B1960.000, obstime=J1978.123, ra=0.48726331438 deg, dec=1.77731617297 deg>

.. _astropy-skycoord-representations:

Representations
^^^^^^^^^^^^^^^^

So far we have been using a spherical coordinate representation in the all the
examples, and this is the default for the built-in frames.  Frequently it is
convenient to initialize or work with a coordinate using a different
representation such as cartesian or cylindrical.  In this section we discuss
how to initialize an object using a different representation and how to
change the representation of an object.  For more information about
representation objects themselves see :ref:`astropy-coordinates-representations`.

Initialization
"""""""""""""""

Most of what you need to know can be inferred from the examples below and
by extrapolating the previous documentation for spherical representations.
Initialization just requires setting the ``representation`` keyword and
supplying the corresponding components for that representation::

    >>> c = SkyCoord(x=1, y=2, z=3, unit='kpc', representation='cartesian')
    >>> c
    <SkyCoord (ICRS): x=1.0 kpc, y=2.0 kpc, z=3.0 kpc>
    >>> c.x, c.y, c.z
    (<Quantity 1.0 kpc>, <Quantity 2.0 kpc>, <Quantity 3.0 kpc>)

Other variations include::

    >>> SkyCoord(1, 2*u.deg, 3, representation='cylindrical')
    <SkyCoord (ICRS): rho=1.0 , phi=2.0 deg, z=3.0 >

    >>> SkyCoord(rho=1*u.km, phi=2*u.deg, z=3*u.m, representation='cylindrical')
    <SkyCoord (ICRS): rho=1.0 km, phi=2.0 deg, z=3.0 m>

    >>> SkyCoord(rho=1, phi=2, z=3, unit=(u.km, u.deg, u.m), representation='cylindrical')
    <SkyCoord (ICRS): rho=1.0 km, phi=2.0 deg, z=3.0 m>

    >>> SkyCoord(1, 2, 3, unit=(None, u.deg, None), representation='cylindrical')
    <SkyCoord (ICRS): rho=1.0 , phi=2.0 deg, z=3.0 >

In general terms, the allowed syntax is as follows::

  SkyCoord(COORD, [FRAME | frame=FRAME], [unit=UNIT], [representation=REPRESENTATION],
           keyword_args ...)
  SkyCoord(COMP1, COMP2, [COMP3], [FRAME | frame=FRAME], [unit=UNIT],
           [representation=REPRESENTATION], keyword_args ...)
  SkyCoord([FRAME | frame=FRAME], <comp1_name>=COMP1, <comp2_name>=COMP2,
           <comp3_name>=COMP3, [representation=REPRESENTATION], [unit=UNIT],
           keyword_args ...)

In this case the ``keyword_args`` now includes the element
``representation=REPRESENTATION``.  In the above description, elements in all
capital letters (e.g. ``FRAME``) describes a user input of that element type.
Elements in square brackets are optional.

**COMP1**, **COMP2**, **COMP3**

Component values can be specified as separate positional arguments or as
keyword arguments.  In this formalism the exact types of allowed input depend
on the details of the representation.  In general the following input forms
are supported:

- Single value:

  - Component class object
  - Plain numeric value with ``unit`` keyword specifying the unit

- List or component class array or numpy array of values

Each representation component has a specified class (the "component class")
which is used to convert generic input data into a pre-defined object
class with a certain unit.  These component classes are expected to be
subclasses of the `~astropy.units.Quantity` class.

**COORD**

This input form uses a single object to supply coordinate data.  The coordinate
can specify one or more coordinate positions as follows:

- List of ``(COMP1, .., COMP<M>)`` tuples, where each component is a scalar (not
  array) and there are ``M`` components in the representation.  Typically
  there are 3 components, but some
  (e.g. `~astropy.coordinates.UnitSphericalRepresentation`)
  can have fewer.
- ``N x M`` numpy or |Quantity| array of values, where ``N`` is the number
  of coordinates and ``M`` is the number of components.

**REPRESENTATION**

The representation can be supplied either as a
`~astropy.coordinates.representation.BaseRepresentation` class (e.g.
`~astropy.coordinates.CartesianRepresentation` or as a string name which is
simply the class name in lower case and without the final ``representation``
(e.g. ``'cartesian'``).

The rest of the inputs for creating a |SkyCoord| object in the general case are
the same as for spherical.

Details
"""""""""

The available set of representations is dynamic and may change depending what
representation classes have been defined.  The built-in representations are:

=====================  =======================================================
  Name                   Class
=====================  =======================================================
``spherical``          `~astropy.coordinates.SphericalRepresentation`
``unitspherical``      `~astropy.coordinates.UnitSphericalRepresentation`
``physicsspherical``   `~astropy.coordinates.PhysicsSphericalRepresentation`
``cartesian``          `~astropy.coordinates.CartesianRepresentation`
``cylindrical``        `~astropy.coordinates.CylindricalRepresentation`
=====================  =======================================================

Each frame knows about all the available representations, but different
frames may use different names for the same components.  A common example
is that the `~astropy.coordinates.Galactic` frame uses ``l`` and ``b``
instead of ``ra`` and ``dec`` for the ``lon`` and ``lat`` components of
the `~astropy.coordinates.SphericalRepresentation`.

For a particular frame, in order to see the full list of representations
and how it names all the components, first make an instance of that frame
without any data, and then print the ``representation_info`` property::

    >>> ICRS().representation_info  # doctest: +SKIP
    {astropy.coordinates.representation.CartesianRepresentation:
      {u'names': (u'x', u'y', u'z'),
       u'units': (None, None, None)},
     astropy.coordinates.representation.SphericalRepresentation:
      {u'names': (u'ra', u'dec', u'distance'),
       u'units': (Unit("deg"), Unit("deg"), None)},
     astropy.coordinates.representation.UnitSphericalRepresentation:
      {u'names': (u'ra', u'dec'),
       u'units': (Unit("deg"), Unit("deg"))},
     astropy.coordinates.representation.PhysicsSphericalRepresentation:
      {u'names': (u'phi', u'theta', u'r'),
       u'units': (Unit("deg"), Unit("deg"), None)},
     astropy.coordinates.representation.CylindricalRepresentation:
      {u'names': (u'rho', u'phi', u'z'),
       u'units': (None, Unit("deg"), None)}
    }

This is a bit messy but it shows that for each representation there is a
``dict`` with two keys:

- ``names``: defines how each component is named in that frame
- ``units``: defines the units of each component when output, where ``None``
  means to not force a particular unit.

For a particular coordinate instance you can use the ``representation``
attribute in conjunction with the ``representation_component_names`` attribute
to figure out what keywords are accepted by a particular class object.  The
former will be the representation class the system is expressed in (e.g.,
spherical for equatorial frames), and the latter will be a dictionary mapping
names for that frame to the component name on the representation class::

    >>> import astropy.units as u
    >>> icrs = ICRS(1*u.deg, 2*u.deg)
    >>> icrs.representation
    <class 'astropy.coordinates.representation.SphericalRepresentation'>
    >>> icrs.representation_component_names
    OrderedDict([(u'ra', u'lon'), (u'dec', u'lat'), (u'distance', u'distance')])

Changing representation
""""""""""""""""""""""""""

The representation of the coordinate object can be changed, as shown
below.  This actually does *nothing* to the object internal data which
stores the coordinate values, but it changes the external view of that
data in two ways:

- The object prints itself in accord with the new representation.
- The available attributes change to match those of the new representation
  (e.g. from ``ra, dec, distance`` to ``x, y, z``).

Setting the ``representation`` thus changes a *property* of the object (how it
appears) without changing the intrinsic object itself which represents a point
in 3d space.
::

    >>> c = SkyCoord(x=1, y=2, z=3, unit='kpc', representation='cartesian')
    >>> c
    <SkyCoord (ICRS): x=1.0 kpc, y=2.0 kpc, z=3.0 kpc>

    >>> c.representation = 'cylindrical'
    >>> c  # doctest: +FLOAT_CMP
    <SkyCoord (ICRS): rho=2.2360679775 kpc, phi=63.4349488229 deg, z=3.0 kpc>
    >>> c.phi.to(u.deg)  # doctest: +FLOAT_CMP
    <Angle 63.43494882292201 deg>
    >>> c.x  # doctest: +SKIP
    ...
    AttributeError: 'SkyCoord' object has no attribute 'x'

    >>> c.representation = 'spherical'
    >>> c  # doctest: +FLOAT_CMP
    <SkyCoord (ICRS): ra=63.4349488229 deg, dec=53.3007747995 deg, distance=3.74165738677 kpc>

    >>> c.representation = 'unitspherical'
    >>> c  # doctest: +FLOAT_CMP
    <SkyCoord (ICRS): ra=63.4349488229 deg, dec=53.3007747995 deg>

You can also use any representation class to set the representation::

    >>> from astropy.coordinates import CartesianRepresentation
    >>> c.representation = CartesianRepresentation

Note that if all you want is a particular representation without changing the
state of the |SkyCoord| object, you should instead use the
``astropy.coordinates.SkyCoord.represent_as()`` method::

    >>> c.representation = 'spherical'
    >>> cart = c.represent_as(CartesianRepresentation)
    >>> cart
    <CartesianRepresentation x=1.0 kpc, y=2.0 kpc, z=3.0 kpc>
    >>> c.representation
    <class 'astropy.coordinates.representation.SphericalRepresentation'>


Convenience methods
^^^^^^^^^^^^^^^^^^^^

A number of convenience methods are available, and you are encouraged to read
the available docstrings below:

- `~astropy.coordinates.SkyCoord.match_to_catalog_sky`,
- `~astropy.coordinates.SkyCoord.match_to_catalog_3d`,
- `~astropy.coordinates.SkyCoord.position_angle`,
- `~astropy.coordinates.SkyCoord.separation`,
- `~astropy.coordinates.SkyCoord.separation_3d`

Addition information and examples can be found in the section on
:ref:`astropy-coordinates-separations-matching`.