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- Using the SkyCoord High-level Class

# 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 `ICRS` and `FK5` which do most of the heavy lifting.

The key distinctions between `SkyCoord` and the low-level classes (Using and Designing Coordinate 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 `FK4` (with equinox and observation time) to `ICRS` (with neither) and back to `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 accommodate 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, dec) in deg
        (10.0, 20.0)>

    >>> SkyCoord([1, 2, 3], [-30, 45, 8], frame='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 `FK4`, an instance of a frame class, a `SkyCoord` instance (from which the frame will be extracted), or the lower-case version of a frame 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, frame=FK4, unit=(u.hourangle, u.deg), obstime="J1992.21")
    >>> sc = SkyCoord(coords, frame=FK4(obstime="J1992.21"), unit=(u.hourangle, u.deg))
    >>> sc = SkyCoord(coords, frame='fk4', unit='hourangle,deg', obstime="J1992.21")

    >>> sc = SkyCoord("1h12m43.2s", "+1d12m43s", frame=Galactic)  # Units from strings
    >>> sc = SkyCoord("1h12m43.2s +1d12m43s", frame=Galactic)  # Units from string
    >>> sc = SkyCoord(l="1h12m43.2s", b="+1d12m43s", frame='galactic')
    >>> sc = SkyCoord("1h12.72m +1d12.71m", frame='galactic')

Note that frame instances with data and `SkyCoord` instances can only be passed as frames using the `frame=` keyword argument and not as positional arguments.

For representations that have `ra` and `dec` attributes one can supply a coordinate string in a number of other common formats. Examples include:

    >>> sc = SkyCoord("15h17+89d15")
    >>> sc = SkyCoord("275d11m15.6954s+17d59m59.876s")
    >>> sc = SkyCoord("8 00 -5 00.6", unit=(u.hour, u.deg))
    >>> sc = SkyCoord("J080000.00-050036.00", unit=(u.hour, u.deg))
    >>> sc = SkyCoord("J1874221.31+122328.03", unit=u.deg)

Astropy `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(ra=ra, dec=dec, frame=ICRS, 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 `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 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 `Distance` object
  - Plain numeric value for a dimensionless distance
  - Plain numeric value with `unit` keyword specifying the unit
- List or `Quantity` or `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 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)
- `BaseRepresentation` subclass object like `SphericalRepresentation`, `CylindricalRepresentation`, or `CartesianRepresentation`.

FRAME

This can be a `BaseCoordinateFrame` frame class, an instance of such a class, or the corresponding string alias. The frame classes that are built in to astropy are `ICRS`, `FK5`, `FK4`, `FK4NoETerms`, `Galactic`, and `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` identifier. 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:

- `Unit` object which is an angular unit that is equivalent to `Unit('radian')`
- Single string with a valid angular unit name
- 2-tuple of `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] `ICRS`, `FK5`, `FK4`, and `FK4NoETerms`.
l, b: LON, LAT values, optional
  Galactic `l` and `b` for the `Galactic` frame.

The following keywords can be specified for any frame:

distance: valid `Distance` initializer, optional
  Distance from reference from center to source.
obstime: valid `Time` initializer, optional
  Time of observation
equinox: valid `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, frame='icrs') for r, d in zip(ra, dec)]
    >>> timeit sc_gal_list = [c.galactic for c in sc_list]
    1 loops, best of 3: 7.66 s per loop

    >>> sc = SkyCoord(ra, dec, frame='icrs')
    >>> timeit sc_gal = sc.galactic
    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)
    504
    >>> sc[2:4]
    <SkyCoord (ICRS): (ra, dec) in deg
        [(304.304015..., 6.900282...),
         (322.560148..., 34.872244...)]>
    >>> sc[2]
    <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, frame='icrs', unit='deg', obstime='2013-01-02 14:25:36')
    >>> sc.<TAB>
    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 `transform_to` method appropriately and returns a new `SkyCoord` object in the requested frame:

    >>> sc_gal = sc.galactic
    >>> sc_gal
    <SkyCoord (Galactic): (l, b) in deg
        (99.6378552814, -58.7096929334)>

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, frame='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 `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, dec) in deg
        (1.0, 2.0)>

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

    >>> sc.frame.<TAB>
    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 `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 `UnitSphericalRepresentation` object:

    >>> sc_gal.frame.data
    <UnitSphericalRepresentation (lon, lat) in rad
        (1.73900863429, -1.02467744452)>

## Transformations

The topic of transformations is covered in detail in the section on Transforming Between Systems.

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 `transform_to` method, which accepts a frame name, frame class, frame instance, or `SkyCoord`:

    >>> from astropy.coordinates import FK5
    >>> sc = SkyCoord(1, 2, frame='icrs', unit='deg')
    >>> sc.galactic
    <SkyCoord (Galactic): (l, b) in deg
        (99.6378552814, -58.7096929334)>

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

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

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

## 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 Using and Designing Coordinate 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, y, z) in kpc
        (1.0, 2.0, 3.0)>
    >>> 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, phi, z) in (, deg, )
        (1.0, 2.0, 3.0)>

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

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

    >>> SkyCoord(1, 2, 3, unit=(None, u.deg, None), representation='cylindrical')
    <SkyCoord (ICRS): (rho, phi, z) in (, deg, )
        (1.0, 2.0, 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 `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. `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 `BaseRepresentation` class (e.g. `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` | `SphericalRepresentation`
`unitspherical` | `UnitSphericalRepresentation`
`physicsspherical` | `PhysicsSphericalRepresentation`
`cartesian` | `CartesianRepresentation`
`cylindrical` | `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 `Galactic` frame uses `l` and `b` instead of `ra` and `dec` for the `lon` and `lat` components of the `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
    {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, y, z) in kpc
        (1.0, 2.0, 3.0)>

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

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

    >>> c.representation = 'unitspherical'
    >>> c
    <SkyCoord (ICRS): (ra, dec) in deg
        (63.4349488229, 53.3007747995)>

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, y, z) in kpc
        (1.0, 2.0, 3.0)>
    >>> c.representation
    <class 'astropy.coordinates.representation.SphericalRepresentation'>

#### Example 1: Plotting random data in Aitoff projection

This is an example how to make a plot in the Aitoff projection using data in a `SkyCoord` object. Here a randomly generated data set will be used.

First we need to import the required packages. We use matplotlib here for plotting and numpy to get the value of pi and to generate our random data.

    >>> from astropy import units as u
    >>> from astropy.coordinates import SkyCoord
    >>> import numpy as np

We now generate random data for visualisation. For RA this is done in the range of 0 and 360 degrees (`ra_random`), for DEC between -90 and +90 degrees (`dec_random`). Finally, we multiply these values by degrees to get an `Quantity` with units of degrees.

    >>> ra_random = np.random.rand(100)*360.0 * u.degree
    >>> dec_random = (np.random.rand(100)*180.0-90.0) * u.degree

As next step, those coordinates are transformed into an astropy.coordinates `SkyCoord` object.

    >>> c = SkyCoord(ra=ra_random, dec=dec_random, frame='icrs')

Because matplotlib needs the coordinates in radians and between -\pi and \pi, not 0 and 2\pi, we have to convert them. For this purpose the `astropy.coordinates.Angle` object provides a special method, which we use here to wrap at 180:

    >>> ra_rad = c.ra.wrap_at(180 * u.deg).radian
    >>> dec_rad = c.dec.radian

As last step we set up the plotting environment with matplotlib using the Aitoff projection with a specific title, a grid, filled circles as markers with a markersize of 2 and an alpha value of 0.3. We use a figure with an x-y ratio that is well suited for such a projection and we move the title upwards from its usual position to avoid overlap with the axis labels.

    >>> import matplotlib.pyplot as plt
    >>> plt.figure(figsize=(8,4.2))
    >>> plt.subplot(111, projection="aitoff")
    >>> plt.title("Aitoff projection of our random data")
    >>> plt.grid(True)
    >>> plt.plot(ra_rad, dec_rad, 'o', markersize=2, alpha=0.3)
    >>> plt.subplots_adjust(top=0.95,bottom=0.0)
    >>> plt.show()

(Source code, png, hires.png, pdf)

#### Example 2: Plotting star positions in bulge and disk

This is more realistic example how to make a plot in the Aitoff projection using data in a `SkyCoord` object. Here a randomly generated data set (multivariate normal distribution) for both stars in the bulge and in the disk of a galaxy will be used. Both types will be plotted with different number counts.

As in the last example, we first import the required packages.

    >>> from astropy import units as u
    >>> from astropy.coordinates import SkyCoord
    >>> import numpy as np

We now generate random data for visualisation using `numpy.random.multivariate_normal`.

    >>> disk = np.random.multivariate_normal(mean=[0,0,0], cov=np.diag([1,1,0.5]), size=5000)
    >>> bulge = np.random.multivariate_normal(mean=[0,0,0], cov=np.diag([1,1,1]), size=500)
    >>> galaxy = np.concatenate([disk, bulge])

As next step, those coordinates are transformed into an astropy.coordinates `SkyCoord` object.

    >>> c_gal = SkyCoord(galaxy, representation='cartesian', frame='galactic')
    >>> c_gal_icrs = c_gal.icrs

Again, as in the last example, we need to convert the coordinates in radians and make sure they are between -\pi and \pi:

    >>> ra_rad = c_gal_icrs.ra.wrap_at(180 * u.deg).radian
    >>> dec_rad = c_gal_icrs.dec.radian

We use the same plotting setup as in the last example:

    >>> import matplotlib.pyplot as plt
    >>> plt.figure(figsize=(8,4.2))
    >>> plt.subplot(111, projection="aitoff")
    >>> plt.title("Aitoff projection of our random data")
    >>> plt.grid(True)
    >>> plt.plot(ra_rad, dec_rad, 'o', markersize=2, alpha=0.3)
    >>> plt.subplots_adjust(top=0.95,bottom=0.0)
    >>> plt.show()

(Source code, png, hires.png, pdf)

## Convenience methods

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

- `match_to_catalog_sky`,
- `match_to_catalog_3d`,
- `position_angle`,
- `separation`,
- `separation_3d`

Addition information and examples can be found in the section on Separations, Catalog Matching, and Related Functionality.