astropy.coordinates supports a rich system for transforming coordinates from one system to another. While common astronomy frames are built into Astropy, the transformation infrastructure is dynamic. This means it allows users to define new coordinate frames and their transformations. The topic of writing your own coordinate frame or transforms is detailed in Defining a New Frame, and this section is focused on how to use transformations.
The simplest method of transformation is shown below:
>>> import astropy.units as u
>>> from astropy.coordinates import SkyCoord
>>> gc = SkyCoord(l=0*u.degree, b=45*u.degree, frame='galactic')
>>> gc.fk5
<SkyCoord (FK5): equinox=J2000.000, ra=229.27250215 deg, dec=-1.12841764184 deg>
While this appears to be simple attribute-style access, it is actually syntactic sugar for the more general transform_to() method, which can accept either a frame name, class or instance:
>>> from astropy.coordinates import FK5
>>> gc.transform_to('fk5')
<SkyCoord (FK5): equinox=J2000.000, ra=229.27250215 deg, dec=-1.12841764184 deg>
>>> gc.transform_to(FK5)
<SkyCoord (FK5): equinox=J2000.000, ra=229.27250215 deg, dec=-1.12841764184 deg>
>>> gc.transform_to(FK5(equinox='J1980.0'))
<SkyCoord (FK5): equinox=J1980.000, ra=229.014681064 deg, dec=-1.05557823687 deg>
As a convenience it is also possible to use a SkyCoord object as the frame in transform_to(). This allows easily putting one coordinate object into the frame of another:
>>> sc = SkyCoord(ra=1.0, dec=2.0, unit='deg', frame=FK5, equinox='J1980.0')
>>> gc.transform_to(sc)
<SkyCoord (FK5): equinox=J1980.000, ra=229.014681064 deg, dec=-1.05557823687 deg>
The table below summarizes the built-in coordinate frames. For details of these frames and the transformations between them see the astropy.coordinates API documentation and the BaseCoordinateFrame class which forms the basis for all astropy.coordinates coordinate frames.
| Frame class | Frame name |
|---|---|
| ICRS | icrs |
| FK5 | fk5 |
| FK4 | fk4 |
| FK4NoETerms | fk4noeterms |
| Galactic | galactic |
Additionally, some coordinate frames (including FK5, FK4, and FK4NoETerms) support “self transformations”, meaning the type of frame doesn’t change, but the frame attributes do. Any example is precessing a coordinate from one equinox to another in an equatorial system. This is done by passing transform_to a frame class with the relevant attributes, as shown below. Note that these systems use a default equinox if you don’t specify one:
>>> fk5c = FK5('02h31m49.09s', '+89d15m50.8s')
>>> fk5c.equinox
<Time object: scale='utc' format='jyear_str' value=J2000.000>
>>> fk5c
<SkyCoord (FK5): equinox=J2000.000, ra=37.9545416667 deg, dec=89.2641111111 deg>
>>> fk5_2005 = FK5(equinox='J2005') # String initializes an astropy.time.Time object
>>> fk5c.transform_to(fk5_2005)
<SkyCoord (FK5): equinox=J2005.000, ra=39.3931763878 deg, dec=89.2858442155 deg>
You can also specify the equinox when you create a coordinate using an Time object:
>>> from astropy.time import Time
>>> fk5c = FK5('02h31m49.09s', '+89d15m50.8s',
... equinox=Time('J1970', scale='utc'))
>>> fk5_2000 = FK5(equinox=Time(2000, format='jyear', scale='utc'))
>>> fk5c.transform_to(fk5_2000)
<SkyCoord (FK5): equinox=2000.0, ra=48.0231710002 deg, dec=89.386724854 deg>
The same lower-level frame classes also have a transform_to() method that works the same as above, but they do not support attribute-style access. They are also subtly different in that they only use frame attributes present in the initial or final frame, while SkyCoord objects use any frame attributes they have for all transformation steps. So SkyCoord can always transform from one frame to another and back again without change, while low-level classes may lose information and hence often do not round-trip.