apcg¶
- erfa.apcg(date1, date2, ebpv, ehp)[source]¶
For a geocentric observer, prepare star-independent astrometry parameters for transformations between ICRS and GCRS coordinates.
- Parameters:
- date1double array
- date2double array
- ebpvdouble array
- ehpdouble array
- Returns:
- astromeraASTROM array
Notes
Wraps ERFA function
eraApcg
. The ERFA documentation is:- - - - - - - - e r a A p c g - - - - - - - - For a geocentric observer, prepare star-independent astrometry parameters for transformations between ICRS and GCRS coordinates. The Earth ephemeris is supplied by the caller. The parameters produced by this function are required in the parallax, light deflection and aberration parts of the astrometric transformation chain. Given: date1 double TDB as a 2-part... date2 double ...Julian Date (Note 1) ebpv double[2][3] Earth barycentric pos/vel (au, au/day) ehp double[3] Earth heliocentric position (au) Returned: astrom eraASTROM star-independent astrometry parameters: pmt double PM time interval (SSB, Julian years) eb double[3] SSB to observer (vector, au) eh double[3] Sun to observer (unit vector) em double distance from Sun to observer (au) v double[3] barycentric observer velocity (vector, c) bm1 double sqrt(1-|v|^2): reciprocal of Lorenz factor bpn double[3][3] bias-precession-nutation matrix along double unchanged xpl double unchanged ypl double unchanged sphi double unchanged cphi double unchanged diurab double unchanged eral double unchanged refa double unchanged refb double unchanged Notes: 1) The TDB date date1+date2 is a Julian Date, apportioned in any convenient way between the two arguments. For example, JD(TDB)=2450123.7 could be expressed in any of these ways, among others: date1 date2 2450123.7 0.0 (JD method) 2451545.0 -1421.3 (J2000 method) 2400000.5 50123.2 (MJD method) 2450123.5 0.2 (date & time method) The JD method is the most natural and convenient to use in cases where the loss of several decimal digits of resolution is acceptable. The J2000 method is best matched to the way the argument is handled internally and will deliver the optimum resolution. The MJD method and the date & time methods are both good compromises between resolution and convenience. For most applications of this function the choice will not be at all critical. TT can be used instead of TDB without any significant impact on accuracy. 2) All the vectors are with respect to BCRS axes. 3) This is one of several functions that inserts into the astrom structure star-independent parameters needed for the chain of astrometric transformations ICRS <-> GCRS <-> CIRS <-> observed. The various functions support different classes of observer and portions of the transformation chain: functions observer transformation eraApcg eraApcg13 geocentric ICRS <-> GCRS eraApci eraApci13 terrestrial ICRS <-> CIRS eraApco eraApco13 terrestrial ICRS <-> observed eraApcs eraApcs13 space ICRS <-> GCRS eraAper eraAper13 terrestrial update Earth rotation eraApio eraApio13 terrestrial CIRS <-> observed Those with names ending in "13" use contemporary ERFA models to compute the various ephemerides. The others accept ephemerides supplied by the caller. The transformation from ICRS to GCRS covers space motion, parallax, light deflection, and aberration. From GCRS to CIRS comprises frame bias and precession-nutation. From CIRS to observed takes account of Earth rotation, polar motion, diurnal aberration and parallax (unless subsumed into the ICRS <-> GCRS transformation), and atmospheric refraction. 4) The context structure astrom produced by this function is used by eraAtciq* and eraAticq*. Called: eraApcs astrometry parameters, ICRS-GCRS, space observer This revision: 2013 October 9 Copyright (C) 2013-2023, NumFOCUS Foundation. Derived, with permission, from the SOFA library. See notes at end of file.