Usage Tips/Suggestions for Methods That Access Remote Resources

There are currently two methods that rely on getting remote data to work.

The first is the SkyCoord from_name() method, which uses Sesame to retrieve coordinates for a particular named object:

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

The second is the EarthLocation of_site() method, which provides a similar quick way to get an EarthLocation from an observatory name:

>>> from astropy.coordinates import EarthLocation
>>> apo = EarthLocation.of_site('Apache Point Observatory')  
>>> apo  
<EarthLocation (-1463969.3018517173, -5166673.342234327, 3434985.7120456537) m>
The full list of available observatory names can be obtained with

astropy.coordinates.EarthLocation.get_site_names().

While these methods are convenient, there are several considerations to take into account:

  • Since these methods access online data, the data may evolve over time (for example, the accuracy of coordinates might improve, and new observatories may be added). Therefore, this means that a script using these and currently running may give a different answer in five years. Therefore, users concerned with reproducibility should not use these methods in their final scripts, but can instead use them to get the values required, and then hard-code them into the scripts. For example, we can check the coordinates of the Kitt Peak Observatories using:

    >>> loc = EarthLocation.of_site('Kitt Peak')

    Note that this command requires an internet connection.

    We can then view the actual Cartesian coordinates for the observatory:

    >>> loc  
<EarthLocation (-1994502.6043061386, -5037538.54232911, 3358104.9969029757) m>

    This can then be converted into code:

    >>> loc = EarthLocation(-1994502.6043061386, -5037538.54232911, 3358104.9969029757, unit='m')

    This latter line can then be included in a script and will ensure that the results stay the same over time.

  • The online data may not be accurate enough for your purposes. If maximum accuracy is paramount, we recommend that you determine the celestial or Earth coordinates yourself and hard-code these, rather than using the convenience methods.

  • These methods will not function if an internet connection is not available. Therefore, if you need to work on a script while offline, follow the instructions in the first bullet point above to hard-code the coordinates before going offline.