astropy.coordinates supports a rich system for transforming coordinates from one system to another. The key concept is that a registry of all the transformations is used to determine which coordinates can convert to others. When you ask for a transformation, the registry (or “transformation graph”) is searched for the shortest path from your starting coordinate to your target, and it applies all of the transformations in that path in series. This allows only the simplest transformations to be defined, and the package will automatically determine how to combine those transformations to get from one system to another.
As described above, there are two ways of transforming coordinates. Coordinates that have an alias (created with coordinate_alias) can be converted by simply using attribute style access to any other coordinate system:
>>> gc = GalacticCoordinates(l=0, b=45, unit=(u.degree, u.degree))
>>> gc.fk5
<FK5Coordinates RA=229.27250 deg, Dec=-1.12842 deg>
>>> ic = ICRSCoordinates(ra=0, dec=45, unit=(u.degree, u.degree)))
>>> ic.fk5
<FK5Coordinates RA=0.00001 deg, Dec=45.00000 deg>
While this appears to be simple attribute-style access, it is actually just syntactic sugar for the transform_to method:
>>> from astropy.coordinates import FK5Coordinates
>>> gc.transform_to(FK5Coordinates)
<FK5Coordinates RA=229.27250 deg, Dec=-1.12842 deg>
>>> ic.transform_to(FK5Coordinates)
<FK5Coordinates RA=0.00001 deg, Dec=45.00000 deg>
The full list of supported coordinate systems and transformations is in the astropy.coordinates API documentation below.
Additionally, some coordinate systems support precessing the coordinate to produce a new coordinate in the same system but at a different equinox. Note that these systems have a default equinox they start with if you don’t specify one:
>>> fk5c = FK5Coordinates('02h31m49.09s +89d15m50.8s')
>>> fk5c.equinox
<Time object: scale='utc' format='jyear_str' vals=J2000.000>
>>> fk5c
<FK5Coordinates RA=37.95454 deg, Dec=89.26411 deg>
>>> fk5c.precess_to(Time(2100, format='jyear', scale='utc'))
<FK5Coordinates RA=88.32396 deg, Dec=89.54057 deg>
You can also specify the equinox when you create a coordinate using an astropy.time.Time object:
>>> from astropy.time import Time
>>> fk5c = FK5Coordinates('02h31m49.09s +89d15m50.8s', equinox=Time('J1970', scale='utc'))
<FK5Coordinates RA=37.95454 deg, Dec=89.26411 deg>
>>> fk5c.precess_to(Time(2000, format='jyear', scale='utc'))
<FK5Coordinates RA=48.02317 deg, Dec=89.38672 deg>
Coordinate systems do not necessarily all support an equinox nor precession, as it is a meaningless action for coordinate systems that do not depend on a particular equinox.
Furthermore, coordinates typically have an obstime attribute, intended to record the time of the observation. Some systems (especially FK4) require this information due to being non-inertial frames (i.e., they rotate over time due to motions of the defining stars).