# Astronomical Coordinate Systems (astropy.coordinates)¶

## Introduction¶

The coordinates package provides classes for representing a variety of celestial/spatial coordinates and their velocity components, as well as tools for converting between common coordinate systems in a uniform way.

## Getting Started¶

The simplest way to use coordinates is to use the SkyCoord class. SkyCoord objects are instantiated by passing in positions (and optional velocities) with specified units and a coordinate frame. Commonly sky positions are passed in as Quantity objects and the frame is specified with the string name. As an example of creating a SkyCoord to represent an ICRS (Right ascension [RA], Declination [Dec]) sky position:

>>> from astropy import units as u
>>> from astropy.coordinates import SkyCoord
>>> c = SkyCoord(ra=10.625*u.degree, dec=41.2*u.degree, frame='icrs')


The initializer for SkyCoord is very flexible and supports inputs provided in a number of convenient formats. The following ways of initializing a coordinate are all equivalent to the above:

>>> c = SkyCoord(10.625, 41.2, frame='icrs', unit='deg')
>>> c = SkyCoord('00h42m30s', '+41d12m00s', frame='icrs')
>>> c = SkyCoord('00h42.5m', '+41d12m')
>>> c = SkyCoord('00 42 30 +41 12 00', unit=(u.hourangle, u.deg))
>>> c = SkyCoord('00:42.5 +41:12', unit=(u.hourangle, u.deg))
>>> c
<SkyCoord (ICRS): (ra, dec) in deg
(10.625, 41.2)>


The examples above illustrate a few simple rules to follow when creating a coordinate object:

• Coordinate values can be provided either as unnamed positional arguments or via keyword arguments like ra and dec, or l and b (depending on the frame).
• The coordinate frame keyword is optional because it defaults to ICRS.
• Angle units must be specified for all components, either by passing in a Quantity object (e.g., 10.5*u.degree), by including them in the value (e.g., '+41d12m00s'), or via the unit keyword.

SkyCoord and all other coordinates objects also support array coordinates. These work the same as single-value coordinates, but they store multiple coordinates in a single object. When you’re going to apply the same operation to many different coordinates (say, from a catalog), this is a better choice than a list of SkyCoord objects, because it will be much faster than applying the operation to each SkyCoord in a for loop. Like the underlying ndarray instances that contain the data, SkyCoord objects can be sliced, reshaped, etc.:

>>> c = SkyCoord(ra=[10, 11, 12, 13]*u.degree, dec=[41, -5, 42, 0]*u.degree)
>>> c
<SkyCoord (ICRS): (ra, dec) in deg
[(10., 41.), (11., -5.), (12., 42.), (13.,  0.)]>
>>> c[1]
<SkyCoord (ICRS): (ra, dec) in deg
(11., -5.)>
>>> c.reshape(2, 2)
<SkyCoord (ICRS): (ra, dec) in deg
[[(10., 41.), (11., -5.)],
[(12., 42.), (13.,  0.)]]>


### Coordinate access¶

Once you have a coordinate object you can access the components of that coordinate (e.g., RA, Dec) and get string representations of the full coordinate.

The component values are accessed using (typically lower-case) named attributes that depend on the coordinate frame (e.g., ICRS, Galactic, etc.). For the default, ICRS, the coordinate component names are ra and dec:

>>> c = SkyCoord(ra=10.68458*u.degree, dec=41.26917*u.degree)
>>> c.ra
<Longitude 10.68458 deg>
>>> c.ra.hour
0.7123053333333335
>>> c.ra.hms
hms_tuple(h=0.0, m=42.0, s=44.299200000000525)
>>> c.dec
<Latitude 41.26917 deg>
>>> c.dec.degree
41.26917
0.7202828960652683


Coordinates can be converted to strings using the to_string() method:

>>> c = SkyCoord(ra=10.68458*u.degree, dec=41.26917*u.degree)
>>> c.to_string('decimal')
'10.6846 41.2692'
>>> c.to_string('dms')
'10d41m04.488s 41d16m09.012s'
>>> c.to_string('hmsdms')
'00h42m44.2992s +41d16m09.012s'


For additional information see the section on Working with Angles.

### Transformation¶

The simplest way to transform to a new coordinate frame is by accessing the appropriately-named attribute. For instance to get the coordinate in the Galactic frame use:

>>> c_icrs = SkyCoord(ra=10.68458*u.degree, dec=41.26917*u.degree, frame='icrs')
>>> c_icrs.galactic
<SkyCoord (Galactic): (l, b) in deg
(121.17424181, -21.57288557)>


For more control, you can use the transform_to method, which accepts a frame name, frame class, or frame instance:

>>> c_fk5 = c_icrs.transform_to('fk5')  # c_icrs.fk5 does the same thing
>>> c_fk5
<SkyCoord (FK5: equinox=J2000.000): (ra, dec) in deg
(10.68459154, 41.26917146)>

>>> from astropy.coordinates import FK5
>>> c_fk5.transform_to(FK5(equinox='J1975'))  # precess to a different equinox
<SkyCoord (FK5: equinox=J1975.000): (ra, dec) in deg
(10.34209135, 41.13232112)>


This form of transform_to also makes it straightforward to convert from celestial coordinates to AltAz coordinates, allowing the use of SkyCoord as a tool for planning observations. For a more complete example of this, see Determining and plotting the altitude/azimuth of a celestial object.

Some coordinate frames such as AltAz require Earth rotation information (UT1-UTC offset and/or polar motion) when transforming to/from other frames. These Earth rotation values are automatically downloaded from the International Earth Rotation and Reference Systems (IERS) service when required. See IERS data access (astropy.utils.iers) for details of this process.

### Representation¶

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. This can be done by setting the representation for either SkyCoord objects or low-level frame coordinate objects:

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

>>> c.representation = 'cylindrical'
>>> c
<SkyCoord (ICRS): (rho, phi, z) in (kpc, deg, kpc)
(2.23606798, 63.43494882, 3.)>


For all the details see Representations.

### Distance¶

SkyCoord and the individual frame classes also support specifying a distance from the frame origin. The origin depends on the particular coordiante frame; this can be, e.g., centered on the earth, centered on the solar system barycenter, etc. Two angles and a distance specify a unique point in 3D space, which also allows converting the coordinates to a Cartesian representation:

>>> c = SkyCoord(ra=10.68458*u.degree, dec=41.26917*u.degree, distance=770*u.kpc)
>>> c.cartesian.x
<Quantity 568.71286542 kpc>
>>> c.cartesian.y
<Quantity 107.3008974 kpc>
>>> c.cartesian.z
<Quantity 507.88994292 kpc>


With distances assigned, SkyCoord convenience methods are more powerful, as they can make use of the 3D information. For example, to compute the physical, 3D separation between two points in space:

>>> c1 = SkyCoord(ra=10*u.degree, dec=9*u.degree, distance=10*u.pc, frame='icrs')
>>> c2 = SkyCoord(ra=11*u.degree, dec=10*u.degree, distance=11.5*u.pc, frame='icrs')
>>> c1.separation_3d(c2)
<Distance 1.52286024 pc>


### Convenience methods¶

SkyCoord defines a number of convenience methods that support, for example, computing on-sky (i.e. angular) and 3D separations between two coordinates:

>>> c1 = SkyCoord(ra=10*u.degree, dec=9*u.degree, frame='icrs')
>>> c2 = SkyCoord(ra=11*u.degree, dec=10*u.degree, frame='fk5')
>>> c1.separation(c2)  # Differing frames handled correctly
<Angle 1.40453359 deg>


cross-matching catalog coordinates (detailed in Matching Catalogs):

>>> target_c = SkyCoord(ra=10*u.degree, dec=9*u.degree, frame='icrs')
>>> # read in coordinates from a catalog...
>>> catalog_c = ...
>>> idx, sep, _ = target_c.match_to_catalog_sky(catalog_c)


The astropy.coordinates subpackage also provides a quick way to get coordinates for named objects assuming you have an active internet connection. The from_name method of SkyCoord uses Sesame to retrieve coordinates for a particular named object:

>>> SkyCoord.from_name("PSR J1012+5307")
<SkyCoord (ICRS): (ra, dec) in deg
(153.1393271, 53.117343)>


For sites (primarily observatories) on the Earth, astropy.coordinates provides a quick way to get an EarthLocation:

>>> from astropy.coordinates import EarthLocation
>>> EarthLocation.of_site('Apache Point Observatory')
<EarthLocation (-1463969.30185172, -5166673.34223433,  3434985.71204565) m>


To see the list of site names available, use astropy.coordinates.EarthLocation.get_site_names().

For arbitrary Earth addresses (e.g., not observatory sites), use the of_address classmethod. Any address passed to this function uses Google maps to retrieve the latitude and longitude and can also (optionally) query Google maps to get the height of the location. As with Google maps, this works with fully specified addresses, location names, city names, and etc.:

>>> EarthLocation.of_address('1002 Holy Grail Court, St. Louis, MO')
<EarthLocation (-26726.98216371, -4997009.8604809, 3950271.16507911) m>
>>> EarthLocation.of_address('1002 Holy Grail Court, St. Louis, MO',
...                          get_height=True)
<EarthLocation (-26727.6272786, -4997130.47437768, 3950367.15622108) m>
<EarthLocation ( 1364606.64511651, -4593292.9428273,  4195415.93695139) m>


Note

from_name, of_site, and of_address are for convenience, and hence are by design rather simple. If you need precise coordinates for an object you should find the appropriate reference and input the coordinates manually, or use more specialized functionality like that in the astroquery or astroplan affiliated packages.

Also note that these methods retrieve data from the internet to determine the celestial or Earth coordinates. The online data may be updated, so if you need to guarantee that your scripts are reproducible in the long term, see the Usage tips/suggestions for methods that access remote resources section.

This functionality can be combined to do more complicated tasks like computing barycentric corrections to radial velocity observations (also a supported high-level SkyCoord method - see Radial Velocity Corrections):

>>> from astropy.time import Time
>>> obstime = Time('2017-2-14')
>>> target = SkyCoord.from_name('M31')
>>> keck = EarthLocation.of_site('Keck')
<Quantity -22.359784554780255 km / s>


### Velocities (Proper Motions and Radial Velocities)¶

New in Astropy v2.0, the coordinate frame classes can now store and transform velocities along with positional coordinate information. This is not available from the SkyCoord class as this new functionality is experimental, but is accessible for more information see the Working with velocities in Astropy coordinates page.

## Overview of astropy.coordinates concepts¶

Note

The coordinates package from v0.4 onward builds from previous versions of the package, and more detailed information and justification of the design is available in APE (Astropy Proposal for Enhancement) 5.

Here we provide an overview of the package and associated framework. This background information is not necessary for simply using coordinates, particularly if you use the SkyCoord high- level class, but it is helpful for more advanced usage, particularly creating your own frame, transformations, or representations. Another useful piece of background information are some Important Definitions as they are used in coordinates.

coordinates is built on a three-tiered system of objects: representations, frames, and a high-level class. Representations classes are a particular way of storing a three-dimensional data point (or points), such as Cartesian coordinates or spherical polar coordinates. Frames are particular reference frames like FK5 or ICRS, which may store their data in different representations, but have well- defined transformations between each other. These transformations are all stored in the astropy.coordinates.frame_transform_graph, and new transformations can be created by users. Finally, the high-level class (SkyCoord) uses the frame classes, but provides a more accessible interface to these objects as well as various convenience methods and more string-parsing capabilities.

Separating these concepts makes it easier to extend the functionality of coordinates. It allows representations, frames, and transformations to be defined or extended separately, while still preserving the high-level capabilities and simplicity of the SkyCoord class.

Examples:

See Determining and plotting the altitude/azimuth of a celestial object for an example of using the coordinates functionality to prepare for an observing run.

## Using astropy.coordinates¶

More detailed information on using the package is provided on separate pages, listed below.

In addition, another resource for the capabilities of this package is the astropy.coordinates.tests.test_api_ape5 testing file. It showcases most of the major capabilities of the package, and hence is a useful supplement to this document. You can see it by either looking at it directly if you downloaded a copy of the astropy source code, or typing the following in an IPython session:

In [1]: from astropy.coordinates.tests import test_api_ape5
In [2]: test_api_ape5??


Some references particularly useful in understanding subtleties of the coordinate systems implemented here include:

• USNO Circular 179
A useful guide to the IAU 2000/2003 work surrounding ICRS/IERS/CIRS and related problems in precision coordinate system work.
• Standards Of Fundamental Astronomy
The definitive implementation of IAU-defined algorithms. The “SOFA Tools for Earth Attitude” document is particularly valuable for understanding the latest IAU standards in detail.
• IERS Conventions (2010)
An exhaustive reference covering the ITRS, the IAU2000 celestial coordinates framework, and other related details of modern coordinate conventions.
• Meeus, J. “Astronomical Algorithms”
A valuable text describing details of a wide range of coordinate-related problems and concepts.

## Reference/API¶

### astropy.coordinates Package¶

This subpackage contains classes and functions for celestial coordinates of astronomical objects. It also contains a framework for conversions between coordinate systems.

The diagram below shows all of the coordinate systems built into the coordinates package, their aliases (useful for converting other coordinates to them using attribute-style access) and the pre-defined transformations between them. The user is free to override any of these transformations by defining new transformations between these systems, but the pre-defined transformations should be sufficient for typical usage.

The color of an edge in the graph (i.e. the transformations between two frames) is set by the type of transformation; the legend box defines the mapping from transform class name to color.

• AffineTransform:

• FunctionTransform:

• FunctionTransformWithFiniteDifference:

• StaticMatrixTransform:

• DynamicMatrixTransform:

Note

The ecliptic coordinate systems (added in Astropy v1.1) have not been extensively tested for accuracy or consistency with other implementations of ecliptic coordinates. We welcome contributions to add such testing, but in the meantime, users who depend on consistency with other implementations may wish to check test inputs against good datasets before using Astropy’s ecliptic coordinates.

#### Functions¶

 cartesian_to_spherical(x, y, z) Converts 3D rectangular cartesian coordinates to spherical polar coordinates. concatenate(coords) Combine multiple coordinate objects into a single SkyCoord. get_body(body, time[, location, ephemeris]) Get a SkyCoord for a solar system body as observed from a location on Earth in the GCRS reference system. get_body_barycentric(body, time[, ephemeris]) Calculate the barycentric position of a solar system body. get_body_barycentric_posvel(body, time[, …]) Calculate the barycentric position and velocity of a solar system body. get_constellation(coord[, short_name, …]) Determines the constellation(s) a given coordinate object contains. get_icrs_coordinates(name) Retrieve an ICRS object by using an online name resolving service to retrieve coordinates for the specified name. get_moon(time[, location, ephemeris]) Get a SkyCoord for the Earth’s Moon as observed from a location on Earth in the GCRS reference system. get_sun(time) Determines the location of the sun at a given time (or times, if the input is an array Time object), in geocentric coordinates. match_coordinates_3d(matchcoord, catalogcoord) Finds the nearest 3-dimensional matches of a coordinate or coordinates in a set of catalog coordinates. match_coordinates_sky(matchcoord, catalogcoord) Finds the nearest on-sky matches of a coordinate or coordinates in a set of catalog coordinates. search_around_3d(coords1, coords2, distlimit) Searches for pairs of points that are at least as close as a specified distance in 3D space. search_around_sky(coords1, coords2, seplimit) Searches for pairs of points that have an angular separation at least as close as a specified angle. spherical_to_cartesian(r, lat, lon) Converts spherical polar coordinates to rectangular cartesian coordinates.

#### Classes¶

 AffineTransform(transform_func, fromsys, tosys) A coordinate transformation specified as a function that yields a 3 x 3 cartesian transformation matrix and a tuple of displacement vectors. AltAz(*args, **kwargs) A coordinate or frame in the Altitude-Azimuth system (Horizontal coordinates). Angle One or more angular value(s) with units equivalent to radians or degrees. Attribute([default, secondary_attribute]) A non-mutable data descriptor to hold a frame attribute. BarycentricTrueEcliptic(*args, **kwargs) Barycentric ecliptic coordinates. BaseAffineTransform(fromsys, tosys[, …]) Base class for common functionality between the AffineTransform-type subclasses. BaseCoordinateFrame(*args, **kwargs) The base class for coordinate frames. BaseDifferential(*args, **kwargs) A base class representing differentials of representations. BaseEclipticFrame(*args, **kwargs) A base class for frames that have names and conventions like that of ecliptic frames. BaseRADecFrame(*args, **kwargs) A base class that defines default representation info for frames that represent longitude and latitude as Right Ascension and Declination following typical “equatorial” conventions. BaseRepresentation(*args, **kwargs) Base for representing a point in a 3D coordinate system. BaseRepresentationOrDifferential(*args, **kwargs) 3D coordinate representations and differentials. BaseSphericalCosLatDifferential(*args, **kwargs) Differtials from points on a spherical base representation. BaseSphericalDifferential(*args, **kwargs) BoundsError Raised when an angle is outside of its user-specified bounds. CIRS(*args, **kwargs) A coordinate or frame in the Celestial Intermediate Reference System (CIRS). CartesianDifferential(d_x[, d_y, d_z, unit, …]) Differentials in of points in 3D cartesian coordinates. CartesianRepresentation(x[, y, z, unit, …]) Representation of points in 3D cartesian coordinates. CartesianRepresentationAttribute([default, …]) A frame attribute that is a CartesianRepresentation with specified units. CartesianRepresentationFrameAttribute(*args, …) CompositeTransform(transforms, fromsys, tosys) A transformation constructed by combining together a series of single-step transformations. ConvertError Raised if a coordinate system cannot be converted to another CoordinateAttribute(frame[, default, …]) A frame attribute which is a coordinate object. CoordinateTransform(fromsys, tosys[, …]) An object that transforms a coordinate from one system to another. CylindricalDifferential(d_rho, d_phi, d_z[, …]) Differential(s) of points in cylindrical coordinates. CylindricalRepresentation(rho, phi, z[, …]) Representation of points in 3D cylindrical coordinates. DifferentialAttribute([default, …]) A frame attribute which is a differential instance. Distance A one-dimensional distance. DynamicMatrixTransform(matrix_func, fromsys, …) A coordinate transformation specified as a function that yields a 3 x 3 cartesian transformation matrix. EarthLocation Location on the Earth. EarthLocationAttribute([default, …]) A frame attribute that can act as a EarthLocation. FK4(*args, **kwargs) A coordinate or frame in the FK4 system. FK4NoETerms(*args, **kwargs) A coordinate or frame in the FK4 system, but with the E-terms of aberration removed. FK5(*args, **kwargs) A coordinate or frame in the FK5 system. FunctionTransform(func, fromsys, tosys[, …]) A coordinate transformation defined by a function that accepts a coordinate object and returns the transformed coordinate object. FunctionTransformWithFiniteDifference(func, …) A coordinate transformation that works like a FunctionTransform, but computes velocity shifts based on the finite-difference relative to one of the frame attributes. GCRS(*args, **kwargs) A coordinate or frame in the Geocentric Celestial Reference System (GCRS). Galactic(*args, **kwargs) A coordinate or frame in the Galactic coordinate system. GalacticLSR(*args, **kwargs) A coordinate or frame in the Local Standard of Rest (LSR), axis-aligned to the Galactic frame. Galactocentric(*args, **kwargs) A coordinate or frame in the Galactocentric system. GenericFrame(frame_attrs) A frame object that can’t store data but can hold any arbitrary frame attributes. GeocentricTrueEcliptic(*args, **kwargs) Geocentric ecliptic coordinates. HCRS(*args, **kwargs) A coordinate or frame in a Heliocentric system, with axes aligned to ICRS. HeliocentricTrueEcliptic(*args, **kwargs) Heliocentric ecliptic coordinates. ICRS(*args, **kwargs) A coordinate or frame in the ICRS system. ITRS(*args, **kwargs) A coordinate or frame in the International Terrestrial Reference System (ITRS). IllegalHourError(hour) Raised when an hour value is not in the range [0,24). IllegalHourWarning(hour[, alternativeactionstr]) Raised when an hour value is 24. IllegalMinuteError(minute) Raised when an minute value is not in the range [0,60]. IllegalMinuteWarning(minute[, …]) Raised when a minute value is 60. IllegalSecondError(second) Raised when an second value (time) is not in the range [0,60]. IllegalSecondWarning(second[, …]) Raised when a second value is 60. LSR(*args, **kwargs) A coordinate or frame in the Local Standard of Rest (LSR). Latitude Latitude-like angle(s) which must be in the range -90 to +90 deg. Longitude Longitude-like angle(s) which are wrapped within a contiguous 360 degree range. PhysicsSphericalDifferential(d_phi, d_theta, d_r) Differential(s) of 3D spherical coordinates using physics convention. PhysicsSphericalRepresentation(phi, theta, r) Representation of points in 3D spherical coordinates (using the physics convention of using phi and theta for azimuth and inclination from the pole). PrecessedGeocentric(*args, **kwargs) A coordinate frame defined in a similar manner as GCRS, but precessed to a requested (mean) equinox. QuantityAttribute([default, …]) A frame attribute that is a quantity with specified units and shape (optionally). QuantityFrameAttribute(*args, **kwargs) RadialDifferential(*args, **kwargs) Differential(s) of radial distances. RadialRepresentation(distance[, …]) Representation of the distance of points from the origin. RangeError Raised when some part of an angle is out of its valid range. RepresentationMapping This namedtuple is used with the frame_specific_representation_info attribute to tell frames what attribute names (and default units) to use for a particular representation. SkyCoord(*args, **kwargs) High-level object providing a flexible interface for celestial coordinate representation, manipulation, and transformation between systems. SkyCoordInfo([bound]) Container for meta information like name, description, format. SkyOffsetFrame(*args, **kwargs) A frame which is relative to some specific position and oriented to match its frame. SphericalCosLatDifferential(d_lon_coslat, …) Differential(s) of points in 3D spherical coordinates. SphericalDifferential(d_lon, d_lat, d_distance) Differential(s) of points in 3D spherical coordinates. SphericalRepresentation(lon, lat, distance) Representation of points in 3D spherical coordinates. StaticMatrixTransform(matrix, fromsys, tosys) A coordinate transformation defined as a 3 x 3 cartesian transformation matrix. Supergalactic(*args, **kwargs) Supergalactic Coordinates (see Lahav et al. TimeAttribute([default, secondary_attribute]) Frame attribute descriptor for quantities that are Time objects. TimeFrameAttribute(*args, **kwargs) TransformGraph() A graph representing the paths between coordinate frames. UnitSphericalCosLatDifferential(…[, copy]) Differential(s) of points on a unit sphere. UnitSphericalDifferential(d_lon, d_lat[, copy]) Differential(s) of points on a unit sphere. UnitSphericalRepresentation(lon, lat[, …]) Representation of points on a unit sphere. UnknownSiteException(site, attribute[, …]) solar_system_ephemeris() Default ephemerides for calculating positions of Solar-System bodies.