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=(3949481.69230491, -550931.90674055, 4961151.73597586) m, obsgeovel=(40.15954083, 287.47863521, -0.0459789) m / s): (ra, dec, distance) in (deg, deg, km)
(136.90234802, 17.03160667, 8.89196021e+08)>
>>> get_moon(t, loc)
<SkyCoord (GCRS: obstime=2014-09-22 23:22:00.000, obsgeoloc=(3949481.69230491, -550931.90674055, 4961151.73597586) m, obsgeovel=(40.15954083, 287.47863521, -0.0459789) m / s): (ra, dec, distance) in (deg, deg, km)
(165.51849203, 2.32863886, 407229.6503193)>
>>> 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.