# Licensed under a 3-clause BSD style license - see LICENSE.rst
"""
This module contains convenience functions for retrieving solar system
ephemerides from jplephem.
"""
from urllib.parse import urlparse
from collections import OrderedDict
import numpy as np
import os.path
from .sky_coordinate import SkyCoord
from astropy.utils.data import download_file
from astropy.utils.decorators import classproperty
from astropy.utils.state import ScienceState
from astropy.utils import indent
from astropy import units as u
from astropy import _erfa as erfa
from astropy.constants import c as speed_of_light
from .representation import CartesianRepresentation
from .orbital_elements import calc_moon
from .builtin_frames import GCRS, ICRS
from .builtin_frames.utils import get_jd12
__all__ = ["get_body", "get_moon", "get_body_barycentric",
"get_body_barycentric_posvel", "solar_system_ephemeris"]
DEFAULT_JPL_EPHEMERIS = 'de430'
"""List of kernel pairs needed to calculate positions of a given object."""
BODY_NAME_TO_KERNEL_SPEC = OrderedDict([
('sun', [(0, 10)]),
('mercury', [(0, 1), (1, 199)]),
('venus', [(0, 2), (2, 299)]),
('earth-moon-barycenter', [(0, 3)]),
('earth', [(0, 3), (3, 399)]),
('moon', [(0, 3), (3, 301)]),
('mars', [(0, 4)]),
('jupiter', [(0, 5)]),
('saturn', [(0, 6)]),
('uranus', [(0, 7)]),
('neptune', [(0, 8)]),
('pluto', [(0, 9)])
])
"""Indices to the plan94 routine for the given object."""
PLAN94_BODY_NAME_TO_PLANET_INDEX = OrderedDict(
(('mercury', 1),
('venus', 2),
('earth-moon-barycenter', 3),
('mars', 4),
('jupiter', 5),
('saturn', 6),
('uranus', 7),
('neptune', 8)))
_EPHEMERIS_NOTE = """
You can either give an explicit ephemeris or use a default, which is normally
a built-in ephemeris that does not require ephemeris files. To change
the default to be the JPL ephemeris::
>>> from astropy.coordinates import solar_system_ephemeris
>>> solar_system_ephemeris.set('jpl') # doctest: +SKIP
Use of any JPL ephemeris requires the jplephem package
(https://pypi.org/project/jplephem/).
If needed, the ephemeris file will be downloaded (and cached).
One can check which bodies are covered by a given ephemeris using::
>>> solar_system_ephemeris.bodies
('earth', 'sun', 'moon', 'mercury', 'venus', 'earth-moon-barycenter', 'mars', 'jupiter', 'saturn', 'uranus', 'neptune')
"""[1:-1]
[docs]class solar_system_ephemeris(ScienceState):
"""Default ephemerides for calculating positions of Solar-System bodies.
This can be one of the following::
- 'builtin': polynomial approximations to the orbital elements.
- 'de430' or 'de432s': short-cuts for recent JPL dynamical models.
- 'jpl': Alias for the default JPL ephemeris (currently, 'de430').
- URL: (str) The url to a SPK ephemeris in SPICE binary (.bsp) format.
- PATH: (str) File path to a SPK ephemeris in SPICE binary (.bsp) format.
- `None`: Ensure an Exception is raised without an explicit ephemeris.
The default is 'builtin', which uses the ``epv00`` and ``plan94``
routines from the ``erfa`` implementation of the Standards Of Fundamental
Astronomy library.
Notes
-----
Any file required will be downloaded (and cached) when the state is set.
The default Satellite Planet Kernel (SPK) file from NASA JPL (de430) is
~120MB, and covers years ~1550-2650 CE [1]_. The smaller de432s file is
~10MB, and covers years 1950-2050 [2]_. Older versions of the JPL
ephemerides (such as the widely used de200) can be used via their URL [3]_.
.. [1] https://naif.jpl.nasa.gov/pub/naif/generic_kernels/spk/planets/aareadme_de430-de431.txt
.. [2] https://naif.jpl.nasa.gov/pub/naif/generic_kernels/spk/planets/aareadme_de432s.txt
.. [3] https://naif.jpl.nasa.gov/pub/naif/generic_kernels/spk/planets/a_old_versions/
"""
_value = 'builtin'
_kernel = None
[docs] @classmethod
def validate(cls, value):
# make no changes if value is None
if value is None:
return cls._value
# Set up Kernel; if the file is not in cache, this will download it.
cls.get_kernel(value)
return value
[docs] @classmethod
def get_kernel(cls, value):
# ScienceState only ensures the `_value` attribute is up to date,
# so we need to be sure any kernel returned is consistent.
if cls._kernel is None or cls._kernel.origin != value:
if cls._kernel is not None:
cls._kernel.daf.file.close()
cls._kernel = None
kernel = _get_kernel(value)
if kernel is not None:
kernel.origin = value
cls._kernel = kernel
return cls._kernel
@classproperty
def kernel(cls):
return cls.get_kernel(cls._value)
@classproperty
def bodies(cls):
if cls._value is None:
return None
if cls._value.lower() == 'builtin':
return (('earth', 'sun', 'moon') +
tuple(PLAN94_BODY_NAME_TO_PLANET_INDEX.keys()))
else:
return tuple(BODY_NAME_TO_KERNEL_SPEC.keys())
def _get_kernel(value):
"""
Try importing jplephem, download/retrieve from cache the Satellite Planet
Kernel corresponding to the given ephemeris.
"""
if value is None or value.lower() == 'builtin':
return None
try:
from jplephem.spk import SPK
except ImportError:
raise ImportError("Solar system JPL ephemeris calculations require "
"the jplephem package "
"(https://pypi.org/project/jplephem/)")
if value.lower() == 'jpl':
value = DEFAULT_JPL_EPHEMERIS
if value.lower() in ('de430', 'de432s'):
value = ('https://naif.jpl.nasa.gov/pub/naif/generic_kernels'
'/spk/planets/{:s}.bsp'.format(value.lower()))
elif os.path.isfile(value):
return SPK.open(value)
else:
try:
urlparse(value)
except Exception:
raise ValueError('{} was not one of the standard strings and '
'could not be parsed as a file path or URL'.format(value))
return SPK.open(download_file(value, cache=True))
def _get_body_barycentric_posvel(body, time, ephemeris=None,
get_velocity=True):
"""Calculate the barycentric position (and velocity) of a solar system body.
Parameters
----------
body : str or other
The solar system body for which to calculate positions. Can also be a
kernel specifier (list of 2-tuples) if the ``ephemeris`` is a JPL
kernel.
time : `~astropy.time.Time`
Time of observation.
ephemeris : str, optional
Ephemeris to use. By default, use the one set with
``astropy.coordinates.solar_system_ephemeris.set``
get_velocity : bool, optional
Whether or not to calculate the velocity as well as the position.
Returns
-------
position : `~astropy.coordinates.CartesianRepresentation` or tuple
Barycentric (ICRS) position or tuple of position and velocity.
Notes
-----
No velocity can be calculated with the built-in ephemeris for the Moon.
Whether or not velocities are calculated makes little difference for the
built-in ephemerides, but for most JPL ephemeris files, the execution time
roughly doubles.
"""
if ephemeris is None:
ephemeris = solar_system_ephemeris.get()
if ephemeris is None:
raise ValueError(_EPHEMERIS_NOTE)
kernel = solar_system_ephemeris.kernel
else:
kernel = _get_kernel(ephemeris)
jd1, jd2 = get_jd12(time, 'tdb')
if kernel is None:
body = body.lower()
earth_pv_helio, earth_pv_bary = erfa.epv00(jd1, jd2)
if body == 'earth':
body_pv_bary = earth_pv_bary
elif body == 'moon':
if get_velocity:
raise KeyError("the Moon's velocity cannot be calculated with "
"the '{}' ephemeris.".format(ephemeris))
return calc_moon(time).cartesian
else:
sun_pv_bary = erfa.pvmpv(earth_pv_bary, earth_pv_helio)
if body == 'sun':
body_pv_bary = sun_pv_bary
else:
try:
body_index = PLAN94_BODY_NAME_TO_PLANET_INDEX[body]
except KeyError:
raise KeyError("{}'s position and velocity cannot be "
"calculated with the '{}' ephemeris."
.format(body, ephemeris))
body_pv_helio = erfa.plan94(jd1, jd2, body_index)
body_pv_bary = erfa.pvppv(body_pv_helio, sun_pv_bary)
body_pos_bary = CartesianRepresentation(
body_pv_bary['p'], unit=u.au, xyz_axis=-1, copy=False)
if get_velocity:
body_vel_bary = CartesianRepresentation(
body_pv_bary['v'], unit=u.au/u.day, xyz_axis=-1,
copy=False)
else:
if isinstance(body, str):
# Look up kernel chain for JPL ephemeris, based on name
try:
kernel_spec = BODY_NAME_TO_KERNEL_SPEC[body.lower()]
except KeyError:
raise KeyError("{}'s position cannot be calculated with "
"the {} ephemeris.".format(body, ephemeris))
else:
# otherwise, assume the user knows what their doing and intentionally
# passed in a kernel chain
kernel_spec = body
# jplephem cannot handle multi-D arrays, so convert to 1D here.
jd1_shape = getattr(jd1, 'shape', ())
if len(jd1_shape) > 1:
jd1, jd2 = jd1.ravel(), jd2.ravel()
# Note that we use the new jd1.shape here to create a 1D result array.
# It is reshaped below.
body_posvel_bary = np.zeros((2 if get_velocity else 1, 3) +
getattr(jd1, 'shape', ()))
for pair in kernel_spec:
spk = kernel[pair]
if spk.data_type == 3:
# Type 3 kernels contain both position and velocity.
posvel = spk.compute(jd1, jd2)
if get_velocity:
body_posvel_bary += posvel.reshape(body_posvel_bary.shape)
else:
body_posvel_bary[0] += posvel[:4]
else:
# spk.generate first yields the position and then the
# derivative. If no velocities are desired, body_posvel_bary
# has only one element and thus the loop ends after a single
# iteration, avoiding the velocity calculation.
for body_p_or_v, p_or_v in zip(body_posvel_bary,
spk.generate(jd1, jd2)):
body_p_or_v += p_or_v
body_posvel_bary.shape = body_posvel_bary.shape[:2] + jd1_shape
body_pos_bary = CartesianRepresentation(body_posvel_bary[0],
unit=u.km, copy=False)
if get_velocity:
body_vel_bary = CartesianRepresentation(body_posvel_bary[1],
unit=u.km/u.day, copy=False)
return (body_pos_bary, body_vel_bary) if get_velocity else body_pos_bary
[docs]def get_body_barycentric_posvel(body, time, ephemeris=None):
"""Calculate the barycentric position and velocity of a solar system body.
Parameters
----------
body : str or other
The solar system body for which to calculate positions. Can also be a
kernel specifier (list of 2-tuples) if the ``ephemeris`` is a JPL
kernel.
time : `~astropy.time.Time`
Time of observation.
ephemeris : str, optional
Ephemeris to use. By default, use the one set with
``astropy.coordinates.solar_system_ephemeris.set``
Returns
-------
position, velocity : tuple of `~astropy.coordinates.CartesianRepresentation`
Tuple of barycentric (ICRS) position and velocity.
See also
--------
get_body_barycentric : to calculate position only.
This is faster by about a factor two for JPL kernels, but has no
speed advantage for the built-in ephemeris.
Notes
-----
The velocity cannot be calculated for the Moon. To just get the position,
use :func:`~astropy.coordinates.get_body_barycentric`.
"""
return _get_body_barycentric_posvel(body, time, ephemeris)
get_body_barycentric_posvel.__doc__ += indent(_EPHEMERIS_NOTE)[4:]
[docs]def get_body_barycentric(body, time, ephemeris=None):
"""Calculate the barycentric position of a solar system body.
Parameters
----------
body : str or other
The solar system body for which to calculate positions. Can also be a
kernel specifier (list of 2-tuples) if the ``ephemeris`` is a JPL
kernel.
time : `~astropy.time.Time`
Time of observation.
ephemeris : str, optional
Ephemeris to use. By default, use the one set with
``astropy.coordinates.solar_system_ephemeris.set``
Returns
-------
position : `~astropy.coordinates.CartesianRepresentation`
Barycentric (ICRS) position of the body in cartesian coordinates
See also
--------
get_body_barycentric_posvel : to calculate both position and velocity.
Notes
-----
"""
return _get_body_barycentric_posvel(body, time, ephemeris,
get_velocity=False)
get_body_barycentric.__doc__ += indent(_EPHEMERIS_NOTE)[4:]
def _get_apparent_body_position(body, time, ephemeris, obsgeoloc=None):
"""Calculate the apparent position of body ``body`` relative to Earth.
This corrects for the light-travel time to the object.
Parameters
----------
body : str or other
The solar system body for which to calculate positions. Can also be a
kernel specifier (list of 2-tuples) if the ``ephemeris`` is a JPL
kernel.
time : `~astropy.time.Time`
Time of observation.
ephemeris : str, optional
Ephemeris to use. By default, use the one set with
``~astropy.coordinates.solar_system_ephemeris.set``
obsgeoloc : `~astropy.coordinates.CartesianRepresentation`, optional
The GCRS position of the observer
Returns
-------
cartesian_position : `~astropy.coordinates.CartesianRepresentation`
Barycentric (ICRS) apparent position of the body in cartesian coordinates
"""
if ephemeris is None:
ephemeris = solar_system_ephemeris.get()
# builtin ephemeris and moon is a special case, with no need to account for
# light travel time, since this is already included in the Meeus algorithm
# used.
if ephemeris == 'builtin' and body.lower() == 'moon':
return get_body_barycentric(body, time, ephemeris)
# Calculate position given approximate light travel time.
delta_light_travel_time = 20. * u.s
emitted_time = time
light_travel_time = 0. * u.s
earth_loc = get_body_barycentric('earth', time, ephemeris)
if obsgeoloc is not None:
earth_loc += obsgeoloc
while np.any(np.fabs(delta_light_travel_time) > 1.0e-8*u.s):
body_loc = get_body_barycentric(body, emitted_time, ephemeris)
earth_distance = (body_loc - earth_loc).norm()
delta_light_travel_time = (light_travel_time -
earth_distance/speed_of_light)
light_travel_time = earth_distance/speed_of_light
emitted_time = time - light_travel_time
return get_body_barycentric(body, emitted_time, ephemeris)
_get_apparent_body_position.__doc__ += indent(_EPHEMERIS_NOTE)[4:]
[docs]def get_body(body, time, location=None, ephemeris=None):
"""
Get a `~astropy.coordinates.SkyCoord` for a solar system body as observed
from a location on Earth in the `~astropy.coordinates.GCRS` reference
system.
Parameters
----------
body : str or other
The solar system body for which to calculate positions. Can also be a
kernel specifier (list of 2-tuples) if the ``ephemeris`` is a JPL
kernel.
time : `~astropy.time.Time`
Time of observation.
location : `~astropy.coordinates.EarthLocation`, optional
Location of observer on the Earth. If not given, will be taken from
``time`` (if not present, a geocentric observer will be assumed).
ephemeris : str, optional
Ephemeris to use. If not given, use the one set with
``astropy.coordinates.solar_system_ephemeris.set`` (which is
set to 'builtin' by default).
Returns
-------
skycoord : `~astropy.coordinates.SkyCoord`
GCRS Coordinate for the body
Notes
-----
"""
if location is None:
location = time.location
if location is not None:
obsgeoloc, obsgeovel = location.get_gcrs_posvel(time)
else:
obsgeoloc, obsgeovel = None, None
cartrep = _get_apparent_body_position(body, time, ephemeris, obsgeoloc)
icrs = ICRS(cartrep)
gcrs = icrs.transform_to(GCRS(obstime=time,
obsgeoloc=obsgeoloc,
obsgeovel=obsgeovel))
return SkyCoord(gcrs)
get_body.__doc__ += indent(_EPHEMERIS_NOTE)[4:]
[docs]def get_moon(time, location=None, ephemeris=None):
"""
Get a `~astropy.coordinates.SkyCoord` for the Earth's Moon as observed
from a location on Earth in the `~astropy.coordinates.GCRS` reference
system.
Parameters
----------
time : `~astropy.time.Time`
Time of observation
location : `~astropy.coordinates.EarthLocation`
Location of observer on the Earth. If none is supplied, taken from
``time`` (if not present, a geocentric observer will be assumed).
ephemeris : str, optional
Ephemeris to use. If not given, use the one set with
``astropy.coordinates.solar_system_ephemeris.set`` (which is
set to 'builtin' by default).
Returns
-------
skycoord : `~astropy.coordinates.SkyCoord`
GCRS Coordinate for the Moon
Notes
-----
"""
return get_body('moon', time, location=location, ephemeris=ephemeris)
get_moon.__doc__ += indent(_EPHEMERIS_NOTE)[4:]
def _apparent_position_in_true_coordinates(skycoord):
"""
Convert Skycoord in GCRS frame into one in which RA and Dec
are defined w.r.t to the true equinox and poles of the Earth
"""
jd1, jd2 = get_jd12(skycoord.obstime, 'tt')
_, _, _, _, _, _, _, rbpn = erfa.pn00a(jd1, jd2)
return SkyCoord(skycoord.frame.realize_frame(
skycoord.cartesian.transform(rbpn)))