Coordinate system; units

[lancelet]

Thanks for your work on this library.

I think it would be useful in the documentation to mention the coordinate system and the units.

AFAICT, units for body positions and velocities are km and km / s respectively.

The coordinate system and time reference are a little more difficult to figure out, but from the GUI interface to Horizons, the relevant chunks seem to be:

    Reference epoch: J2000.0
    X-Y plane: adopted Earth orbital plane at the reference epoch
               Note: IAU76 obliquity of 84381.448 arcseconds wrt ICRF X-Y plane
    X-axis   : ICRF
    Z-axis   : perpendicular to the X-Y plane in the directional (+ or -) sense
               of Earth's north pole at the reference epoch.

Wikipedia has a somewhat bewildering description of ICRF: https://en.wikipedia.org/wiki/International_Celestial_Reference_System_and_Frame The origin is clear, but the orientation not so much. 😸

For time; this may be the same as what is returned from the API:

TIME

  Barycentric Dynamical Time ("TDB" or T_eph) output was requested. This
continuous relativistic coordinate time is equivalent to the relativistic
proper time of a clock at rest in a reference frame comoving with the
solar system barycenter but outside the system's gravity well. It is the
independent variable in the solar system relativistic equations of motion.

  TDB runs at a uniform rate of one SI second per second and is independent
of irregularities in Earth's rotation.

  Calendar dates prior to 1582-Oct-15 are in the Julian calendar system.
Later calendar dates are in the Gregorian system.

[ocramz]

Thank you for raising these, all good points.

[ocramz]

I would also like a way to retrieve planet masses, for computing the momentum vector. Perhaps one could use wikipedia data for this

On Mon, 11 Apr 2022 at 08:23, Jonathan Merritt @.***> wrote:

Thanks for your work on this library.

I think it would be useful in the documentation to mention the coordinate system and the units.

AFAICT, units for body positions and velocities are km and km / s respectively.

The coordinate system and time reference are a little more difficult to figure out, but from the GUI interface to Horizons, the relevant chunks seem to be:

Reference epoch: J2000.0

X-Y plane: adopted Earth orbital plane at the reference epoch

           Note: IAU76 obliquity of 84381.448 arcseconds wrt ICRF X-Y plane

X-axis   : ICRF

Z-axis   : perpendicular to the X-Y plane in the directional (+ or -) sense

           of Earth's north pole at the reference epoch.

Wikipedia has a somewhat bewildering description of ICRF: https://en.wikipedia.org/wiki/International_Celestial_Reference_System_and_Frame The origin is clear, but the orientation not so much. 😸

For time; this may be the same as what is returned from the API:

TIME

Barycentric Dynamical Time ("TDB" or T_eph) output was requested. This

continuous relativistic coordinate time is equivalent to the relativistic

proper time of a clock at rest in a reference frame comoving with the

solar system barycenter but outside the system's gravity well. It is the

independent variable in the solar system relativistic equations of motion.

TDB runs at a uniform rate of one SI second per second and is independent

of irregularities in Earth's rotation.

Calendar dates prior to 1582-Oct-15 are in the Julian calendar system.

Later calendar dates are in the Gregorian system.

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