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 PyERFA 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 the 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. Other JPL ephemerides can be requested by name for specific
use cases you may have (see the examples below).
Note
Using JPL ephemerides requires that the jplephem package be installed. This is
most conveniently achieved via python -m pip install jplephem
, although
whatever package management system you use might have it as well.
Two functions are provided; get_body()
and
get_body_barycentric()
.
The first returns SkyCoord
objects in the GCRS
frame,
while the latter returns a CartesianRepresentation
of the
barycentric position of a body (i.e., in the ICRS
frame).
Examples#
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
>>> 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=(3949481.69182405, -550931.91022387, 4961151.73597633) m, obsgeovel=(40.159527, 287.47873161, -0.04597922) m / s): (ra, dec, distance) in (deg, deg, AU)
(136.91116253, 17.02935396, 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 than is possible with the built-in ephemeris
(see Precision of the Built-In Ephemeris), you
could use the de430
ephemeris mentioned above, or, if you only care about
times between 1950 and 2050, 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=(3949481.69182405, -550931.91022387, 4961151.73597633) m, obsgeovel=(40.159527, 287.47873161, -0.04597922) m / s): (ra, dec, distance) in (deg, deg, km)
(136.90234846, 17.03160654, 8.89196021e+08)>
>>> get_body('moon', t, loc)
<SkyCoord (GCRS: obstime=2014-09-22 23:22:00.000, obsgeoloc=(3949481.69182405, -550931.91022387, 4961151.73597633) m, obsgeovel=(40.159527, 287.47873161, -0.04597922) m / s): (ra, dec, distance) in (deg, deg, km)
(165.51854528, 2.32861794, 407229.55638763)>
>>> 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, you 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 AU
(1.00340683, -0.00579417, -0.00280064)>
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 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.
Note
When using JPL ephemerides, be aware that answers may change at levels that can be surprising if you are not careful about understanding the frame you are in. See for example the case of the DE440s ephemerides, which is described in more detail in astropy PR #11608. So it is usually best to stay within the same ephemerides for consistency.
Precision of the Built-In Ephemeris#
The algorithm for calculating positions and velocities for planets other than
Earth used by ERFA is due to J.L. Simon, P. Bretagnon, J. Chapront,
M. Chapront-Touze, G. Francou and J. Laskar (Bureau des Longitudes, Paris,
France). From comparisons with JPL ephemeris DE102, they quote the maximum
errors over the interval 1800-2050 below. For more details, see the PyERFA routine, erfa.plan94
.
For the Earth, the rms errors in position and velocity are about 4.6 km and
1.4 mm/s, respectively (see erfa.epv00
).
Planet |
L (arcsec) |
B (arcsec) |
R (km) |
Mercury |
4 |
1 |
300 |
Venus |
5 |
1 |
800 |
EMB |
6 |
1 |
1000 |
Mars |
17 |
1 |
7700 |
Jupiter |
71 |
5 |
76000 |
Saturn |
81 |
13 |
267000 |
Uranus |
86 |
7 |
712000 |
Neptune |
11 |
1 |
253000 |