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planets BCRS coordinates obtained from de440.bsp are far away from Horizons site

See original GitHub issue

I compare planet’s BCRS positions computed using obj.at(t) by de430.bsp and data queried from Horizons web site. The difference is small enough.

But when I replace de430.bsp with de440.bsp, the difference are VERY LARGE. I wonder what is going on?

Following is my code snippet

from skyfield.api import load
from astroquery.jplhorizons import Horizons
from astropy.time import Time
import astropy.units as u
import numpy as np



def compare_obj(planets, code, year, month, day, hour=0, minute=0, second=0):
    '''
    compare position between jpl query and skyfield computation

    parameters:
    ----------
    code : 1,2,3,4,5,6,7,8,9,10,199,299,399,301

    year: int,
    month: int,
    day: int,
    hour: int default 0,
    minute: int default 0,
    second float default 0,
    '''

    # print(code)
    ts = load.timescale()
    t = ts.utc(year, month, day, hour, minute, second)
    objname = planets.names()[code][0]
    # print(objname)
    obj = planets[objname]
    icrf_sf = obj.at(t)
    # print(icrf_sf.position)
    icrf_sf_np = icrf_sf.position.km

    time = Time(t.utc_iso())
    times = time.tt+np.linspace(0, 1, 2)*u.s
    objquery = Horizons(id=str(code), id_type='majorbody', location='@0',
                        epochs={'start': times.value[0], 'stop': times.value[1], 'step': '1'})
    icrf_jpl = objquery.vectors(refplane='earth')
    icrf_jpl['x'].convert_unit_to('km')
    icrf_jpl['y'].convert_unit_to('km')
    icrf_jpl['z'].convert_unit_to('km')
    icrf_jpl_np = np.array(
        [icrf_jpl['x'][0], icrf_jpl['y'][0], icrf_jpl['z'][0]])
    print(objname + ' icrf_sf_np in km', icrf_sf_np)
    print(objname + ' icrf_jpl_np in km', icrf_jpl_np)
    print(objname + ' icrf_sf_np-icrf_jpl_np in km', icrf_sf_np-icrf_jpl_np)
    print()

def testDE(defile):
    print(defile)
    planets = load(defile)
    codes = [code for code in planets.codes if code != 0]
    ts = load.timescale()
    for code in codes:
        compare_obj(planets, code, 2021, 1, 30, 3, 6, 0)


testDE('de440.bsp')
testDE('de430.bsp')

the output are

de440.bsp
MERCURY_BARYCENTER icrf_sf_np in km [ 3870649.02606443 41392819.19301444 21542810.35938832]
MERCURY_BARYCENTER icrf_jpl_np in km [ 3870755.23463293 41392865.94384275 21542816.10309695]
MERCURY_BARYCENTER icrf_sf_np-icrf_jpl_np in km [-106.2085685   -46.75082832   -5.74370863]

VENUS_BARYCENTER icrf_sf_np in km [ 14475475.97449725 -97060411.02453132 -44648132.07199989]
VENUS_BARYCENTER icrf_jpl_np in km [ 14475582.42719842 -97060364.29934852 -44648126.3223459 ]
VENUS_BARYCENTER icrf_sf_np-icrf_jpl_np in km [-106.45270116  -46.7251828    -5.749654  ]

EARTH_BARYCENTER icrf_sf_np in km [-9.60526964e+07  1.04132030e+08  4.51595476e+07]
EARTH_BARYCENTER icrf_jpl_np in km [-9.60525902e+07  1.04132076e+08  4.51595532e+07]
EARTH_BARYCENTER icrf_sf_np-icrf_jpl_np in km [-106.21787445  -46.6572369    -5.60996395]

MARS_BARYCENTER icrf_sf_np in km [3.59179214e+07 2.08828095e+08 9.47860347e+07]
MARS_BARYCENTER icrf_jpl_np in km [3.59180277e+07 2.08828142e+08 9.47860402e+07]
MARS_BARYCENTER icrf_sf_np-icrf_jpl_np in km [-106.25607501  -46.78110483   -5.5933331 ]

JUPITER_BARYCENTER icrf_sf_np in km [ 4.79548433e+08 -5.37038375e+08 -2.41865441e+08]
JUPITER_BARYCENTER icrf_jpl_np in km [ 4.79548552e+08 -5.37038322e+08 -2.41865417e+08]
JUPITER_BARYCENTER icrf_sf_np-icrf_jpl_np in km [-118.78297108  -52.91103995  -24.01297405]

SATURN_BARYCENTER icrf_sf_np in km [ 8.39201230e+08 -1.12705480e+09 -5.01674692e+08]
SATURN_BARYCENTER icrf_jpl_np in km [ 8.39201338e+08 -1.12705476e+09 -5.01674685e+08]
SATURN_BARYCENTER icrf_sf_np-icrf_jpl_np in km [-108.02585697  -48.04796076   -6.4681049 ]

URANUS_BARYCENTER icrf_sf_np in km [2.28436557e+09 1.73166246e+09 7.26112140e+08]
URANUS_BARYCENTER icrf_jpl_np in km [2.28436563e+09 1.73166231e+09 7.26112205e+08]
URANUS_BARYCENTER icrf_sf_np-icrf_jpl_np in km [-63.51256084 147.313416   -65.57285929]

NEPTUNE_BARYCENTER icrf_sf_np in km [ 4.40851526e+09 -6.70704912e+08 -3.84279840e+08]
NEPTUNE_BARYCENTER icrf_jpl_np in km [ 4.40851483e+09 -6.70704784e+08 -3.84279619e+08]
NEPTUNE_BARYCENTER icrf_sf_np-icrf_jpl_np in km [ 431.83857346 -127.86859035 -221.2449652 ]

PLUTO_BARYCENTER icrf_sf_np in km [ 2.11522736e+09 -4.22814862e+09 -1.95678724e+09]
PLUTO_BARYCENTER icrf_jpl_np in km [ 2.11522725e+09 -4.22814934e+09 -1.95678776e+09]
PLUTO_BARYCENTER icrf_sf_np-icrf_jpl_np in km [114.71878219 719.76608706 514.04651737]

SUN icrf_sf_np in km [-1024502.89778636   793741.74069273   362391.18422153]
SUN icrf_jpl_np in km [-1024396.56180323   793788.47208616   362396.88179922]
SUN icrf_sf_np-icrf_jpl_np in km [-106.33598314  -46.73139343   -5.69757769]

MERCURY icrf_sf_np in km [ 3870649.02606443 41392819.19301444 21542810.35938832]
MERCURY icrf_jpl_np in km [ 3870755.23463293 41392865.94384275 21542816.10309695]
MERCURY icrf_sf_np-icrf_jpl_np in km [-106.2085685   -46.75082832   -5.74370863]

VENUS icrf_sf_np in km [ 14475475.97449725 -97060411.02453132 -44648132.07199989]
VENUS icrf_jpl_np in km [ 14475582.42719842 -97060364.29934852 -44648126.3223459 ]
VENUS icrf_sf_np-icrf_jpl_np in km [-106.45270116  -46.7251828    -5.749654  ]

MOON icrf_sf_np in km [-9.63633733e+07  1.04305841e+08  4.52684662e+07]
MOON icrf_jpl_np in km [-9.63632671e+07  1.04305887e+08  4.52684719e+07]
MOON icrf_sf_np-icrf_jpl_np in km [-106.21872723  -46.65832654   -5.61086642]

EARTH icrf_sf_np in km [-9.60488751e+07  1.04129892e+08  4.51582079e+07]
EARTH icrf_jpl_np in km [-9.60487689e+07  1.04129938e+08  4.51582135e+07]
EARTH icrf_sf_np-icrf_jpl_np in km [-106.21782386  -46.65724593   -5.6099669 ]

de430.bsp
MERCURY_BARYCENTER icrf_sf_np in km [ 3870755.19165879 41392865.94682641 21542816.10914535]
MERCURY_BARYCENTER icrf_jpl_np in km [ 3870755.23463293 41392865.94384275 21542816.10309695]
MERCURY_BARYCENTER icrf_sf_np-icrf_jpl_np in km [-0.04297413  0.00298365  0.0060484 ]

VENUS_BARYCENTER icrf_sf_np in km [ 14475582.45257788 -97060364.29550089 -44648126.32222056]
VENUS_BARYCENTER icrf_jpl_np in km [ 14475582.42719842 -97060364.29934852 -44648126.3223459 ]
VENUS_BARYCENTER icrf_sf_np-icrf_jpl_np in km [0.02537947 0.00384763 0.00012533]

EARTH_BARYCENTER icrf_sf_np in km [-9.60525902e+07  1.04132076e+08  4.51595532e+07]
EARTH_BARYCENTER icrf_jpl_np in km [-9.60525902e+07  1.04132076e+08  4.51595532e+07]
EARTH_BARYCENTER icrf_sf_np-icrf_jpl_np in km [-0.01715797 -0.01308142 -0.00567057]

MARS_BARYCENTER icrf_sf_np in km [3.59180277e+07 2.08828142e+08 9.47860402e+07]
MARS_BARYCENTER icrf_jpl_np in km [3.59180277e+07 2.08828142e+08 9.47860402e+07]
MARS_BARYCENTER icrf_sf_np-icrf_jpl_np in km [-0.0169936   0.00380459  0.00220387]

JUPITER_BARYCENTER icrf_sf_np in km [ 4.79548552e+08 -5.37038322e+08 -2.41865417e+08]
JUPITER_BARYCENTER icrf_jpl_np in km [ 4.79548552e+08 -5.37038322e+08 -2.41865417e+08]
JUPITER_BARYCENTER icrf_sf_np-icrf_jpl_np in km [0.00735855 0.0060811  0.00242743]

SATURN_BARYCENTER icrf_sf_np in km [ 8.39201338e+08 -1.12705476e+09 -5.01674685e+08]
SATURN_BARYCENTER icrf_jpl_np in km [ 8.39201338e+08 -1.12705476e+09 -5.01674685e+08]
SATURN_BARYCENTER icrf_sf_np-icrf_jpl_np in km [0.00549626 0.0037775  0.0013234 ]

URANUS_BARYCENTER icrf_sf_np in km [2.28436563e+09 1.73166231e+09 7.26112205e+08]
URANUS_BARYCENTER icrf_jpl_np in km [2.28436563e+09 1.73166231e+09 7.26112205e+08]
URANUS_BARYCENTER icrf_sf_np-icrf_jpl_np in km [-0.00323009  0.00332499  0.00150168]

NEPTUNE_BARYCENTER icrf_sf_np in km [ 4.40851483e+09 -6.70704784e+08 -3.84279619e+08]
NEPTUNE_BARYCENTER icrf_jpl_np in km [ 4.40851483e+09 -6.70704784e+08 -3.84279619e+08]
NEPTUNE_BARYCENTER icrf_sf_np-icrf_jpl_np in km [0.0006609  0.00368536 0.00149208]

PLUTO_BARYCENTER icrf_sf_np in km [ 2.11522726e+09 -4.22814965e+09 -1.95678772e+09]
PLUTO_BARYCENTER icrf_jpl_np in km [ 2.11522725e+09 -4.22814934e+09 -1.95678776e+09]
PLUTO_BARYCENTER icrf_sf_np-icrf_jpl_np in km [  13.38765478 -306.40300035   38.77710915]

SUN icrf_sf_np in km [-1024396.56181172   793788.47207897   362396.8817964 ]
SUN icrf_jpl_np in km [-1024396.56180323   793788.47208616   362396.88179922]
SUN icrf_sf_np-icrf_jpl_np in km [-8.48737545e-06 -7.19248783e-06 -2.82318797e-06]

MERCURY icrf_sf_np in km [ 3870755.19165879 41392865.94682641 21542816.10914535]
MERCURY icrf_jpl_np in km [ 3870755.23463293 41392865.94384275 21542816.10309695]
MERCURY icrf_sf_np-icrf_jpl_np in km [-0.04297413  0.00298365  0.0060484 ]

VENUS icrf_sf_np in km [ 14475582.45257788 -97060364.29550089 -44648126.32222056]
VENUS icrf_jpl_np in km [ 14475582.42719842 -97060364.29934852 -44648126.3223459 ]
VENUS icrf_sf_np-icrf_jpl_np in km [0.02537947 0.00384763 0.00012533]

MOON icrf_sf_np in km [-9.63632671e+07  1.04305887e+08  4.52684719e+07]
MOON icrf_jpl_np in km [-9.63632671e+07  1.04305887e+08  4.52684719e+07]
MOON icrf_sf_np-icrf_jpl_np in km [-0.01673733 -0.01252432 -0.00531527]

EARTH icrf_sf_np in km [-9.60487689e+07  1.04129938e+08  4.51582135e+07]
EARTH icrf_jpl_np in km [-9.60487689e+07  1.04129938e+08  4.51582135e+07]
EARTH icrf_sf_np-icrf_jpl_np in km [-0.01716313 -0.01308827 -0.00567494]

Issue Analytics

  • State:closed
  • Created 3 years ago
  • Comments:5 (4 by maintainers)

github_iconTop GitHub Comments

1reaction
brandon-rhodescommented, Apr 9, 2021

An email today from HORIZONS, as they move to adopt DE440 and DE441, states:

“Inclusion of 30 new Kuiper-belt masses, and the Kuiper Belt ring mass, results in a time-varying shift of ~100 km in DE441’s barycenter relative to DE431.”

So it’s the Kuiper Belt’s fault.

0reactions
brandon-rhodescommented, Feb 15, 2021

As it has now been 2 weeks without further comment on the issue, I’m going to assume that there’s no action for Skyfield to take here, and that the new ephemeris has simply refined the position of the Solar System barycenter by 100–200 km. Anyone should feel free to comment further here, though, if they have more questions about Skyfield and the new ephemeris.

Read more comments on GitHub >

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