Note

This is an old version of the documentation. See http://docs.astropy.org/en/stable for the latest version.

Solar System Ephemerides

astropy.coordinates can calculate the SkyCoord of some of the major solar system objects. By default, it uses approximate orbital elements calculated using built-in ERFA routines, but it can also use more precise ones using the JPL ephemerides (which are derived from dynamical models). The default JPL ephemerides (DE430) provide predictions valid roughly for years between 1550 and 2650. The file is 115 MB and will need to be downloaded the first time you use this functionality, but will be cached after that.

Note

Using JPL ephemerides requires that the jplephem package be installed. This is most easily achieved via pip install jplephem, although whatever package management system you use might have it as well.

Three functions are provided; get_body(), get_moon() and get_body_barycentric(). The first two functions return SkyCoord objects in the GCRS frame, whilst the latter returns a CartesianRepresentation of the barycentric position of a body (i.e in the ICRS frame).

Here is an example of using these functions with built-in ephemerides, i.e., without the need to download a large ephemerides file:

>>> from astropy.time import Time
>>> from astropy.coordinates import solar_system_ephemeris, EarthLocation
>>> from astropy.coordinates import get_body_barycentric, get_body, get_moon
>>> t = Time("2014-09-22 23:22")
>>> loc = EarthLocation.of_site('greenwich') 
>>> with solar_system_ephemeris.set('builtin'):
...     jup = get_body('jupiter', t, loc) 
>>> jup  
<SkyCoord (GCRS: obstime=2014-09-22 23:22:00.000, obsgeoloc=(3949481.68990863, -550931.91188162, 4961151.73733451) m, obsgeovel=(40.15954083, 287.47876693, -0.04597867) m / s): (ra, dec, distance) in (deg, deg, AU)
    (136.91116209, 17.02935409, 5.94386022)>

Above, we used solar_system_ephemeris as a context, which sets the default ephemeris while in the with clause, and resets it at the end.

To get more precise positions, one could use the de430 ephemeris mentioned above, but between 1950 and 2050 one could also opt for the de432s ephemeris, which is stored in a smaller, ~10 MB, file (which will be downloaded and cached when the ephemeris is set):

>>> solar_system_ephemeris.set('de432s') 
<ScienceState solar_system_ephemeris: 'de432s'>
>>> get_body('jupiter', t, loc) 
<SkyCoord (GCRS: obstime=2014-09-22 23:22:00.000, obsgeoloc=( 3949481.68990897, -550931.9118838,  4961151.73733447) m, obsgeovel=( 40.1745933,  288.00078051,  0.) m / s): (ra, dec, distance) in (deg, deg, km)
    ( 136.90234781,  17.03160686,   8.89196019e+08)>
>>> get_moon(t, loc) 
<SkyCoord (GCRS: obstime=2014-09-22 23:22:00.000, obsgeoloc=( 3949481.6899252, -550931.91194065,  4961151.73733445) m, obsgeovel=( 40.1745933,  288.00078051,  0.) m / s): (ra, dec, distance) in (deg, deg, km)
        ( 165.51849193,  2.32863887,  407229.65033585)>
>>> get_body_barycentric('moon', t) 
<CartesianRepresentation (x, y, z) in km
    (  1.50107535e+08, -866789.11996916, -418963.55218495)>

For one-off calculations with a given ephemeris, one can also pass it directly to the various functions:

>>> get_body_barycentric('moon', t, ephemeris='de432s')
... 
<CartesianRepresentation (x, y, z) in km
    (  1.50107535e+08, -866789.11996916, -418963.55218495)>
>>> get_body_barycentric('moon', t, ephemeris='builtin')
... 
<CartesianRepresentation (x, y, z) in km
    (  1.50107513e+08, -866838.51786769, -418988.57509287)>

For a list of the bodies for which positions can be calculated, do:

>>> solar_system_ephemeris.bodies 
('sun',
 'mercury',
 'venus',
 'earth-moon-barycenter',
 'earth',
 'moon',
 'mars',
 'jupiter',
 'saturn',
 'uranus',
 'neptune',
 'pluto')
>>> solar_system_ephemeris.set('builtin')
<ScienceState solar_system_ephemeris: 'builtin'>
>>> solar_system_ephemeris.bodies
('earth',
 'sun',
 'moon',
 'mercury',
 'venus',
 'earth-moon-barycenter',
 'mars',
 'jupiter',
 'saturn',
 'uranus',
 'neptune')

Note

While the sun is included in the these ephemerides, it is important to recognize that get_sun always uses the built-in, polynomial model (as this requires no special download). So it is not safe to assume that get_body(time, 'sun') and get_sun(time) will give the same result.