pvstar¶
- erfa.pvstar(pv)[source]¶
Convert star position+velocity vector to catalog coordinates.
- Parameters:
- pvdouble array
- Returns:
- radouble array
- decdouble array
- pmrdouble array
- pmddouble array
- pxdouble array
- rvdouble array
Notes
Wraps ERFA function
eraPvstar
. The ERFA documentation is:- - - - - - - - - - e r a P v s t a r - - - - - - - - - - Convert star position+velocity vector to catalog coordinates. Given (Note 1): pv double[2][3] pv-vector (au, au/day) Returned (Note 2): ra double right ascension (radians) dec double declination (radians) pmr double RA proper motion (radians/year) pmd double Dec proper motion (radians/year) px double parallax (arcsec) rv double radial velocity (km/s, positive = receding) Returned (function value): int status: 0 = OK -1 = superluminal speed (Note 5) -2 = null position vector Notes: 1) The specified pv-vector is the coordinate direction (and its rate of change) for the date at which the light leaving the star reached the solar-system barycenter. 2) The star data returned by this function are "observables" for an imaginary observer at the solar-system barycenter. Proper motion and radial velocity are, strictly, in terms of barycentric coordinate time, TCB. For most practical applications, it is permissible to neglect the distinction between TCB and ordinary "proper" time on Earth (TT/TAI). The result will, as a rule, be limited by the intrinsic accuracy of the proper-motion and radial-velocity data; moreover, the supplied pv-vector is likely to be merely an intermediate result (for example generated by the function eraStarpv), so that a change of time unit will cancel out overall. In accordance with normal star-catalog conventions, the object's right ascension and declination are freed from the effects of secular aberration. The frame, which is aligned to the catalog equator and equinox, is Lorentzian and centered on the SSB. Summarizing, the specified pv-vector is for most stars almost identical to the result of applying the standard geometrical "space motion" transformation to the catalog data. The differences, which are the subject of the Stumpff paper cited below, are: (i) In stars with significant radial velocity and proper motion, the constantly changing light-time distorts the apparent proper motion. Note that this is a classical, not a relativistic, effect. (ii) The transformation complies with special relativity. 3) Care is needed with units. The star coordinates are in radians and the proper motions in radians per Julian year, but the parallax is in arcseconds; the radial velocity is in km/s, but the pv-vector result is in au and au/day. 4) The proper motions are the rate of change of the right ascension and declination at the catalog epoch and are in radians per Julian year. The RA proper motion is in terms of coordinate angle, not true angle, and will thus be numerically larger at high declinations. 5) Straight-line motion at constant speed in the inertial frame is assumed. If the speed is greater than or equal to the speed of light, the function aborts with an error status. 6) The inverse transformation is performed by the function eraStarpv. Called: eraPn decompose p-vector into modulus and direction eraPdp scalar product of two p-vectors eraSxp multiply p-vector by scalar eraPmp p-vector minus p-vector eraPm modulus of p-vector eraPpp p-vector plus p-vector eraPv2s pv-vector to spherical eraAnp normalize angle into range 0 to 2pi Reference: Stumpff, P., 1985, Astron.Astrophys. 144, 232-240. This revision: 2023 May 4 Copyright (C) 2013-2023, NumFOCUS Foundation. Derived, with permission, from the SOFA library. See notes at end of file.