Function aper 

pub fn aper(theta: f64, astrom: &mut IauAstrom)

Insert ERA into context

In the star-independent astrometry parameters, update only the Earth rotation angle, supplied by the caller explicitly.

This function is part of the International Astronomical Union’s SOFA (Standards of Fundamental Astronomy) software collection.

Status: support function.

Given:

   theta   double      Earth rotation angle (radians, Note 2)
   astrom  iauASTROM*  star-independent astrometry parameters:
    pmt    double       not used
    eb     double[3]    not used
    eh     double[3]    not used
    em     double       not used
    v      double[3]    not used
    bm1    double       not used
    bpn    double[3][3] not used
    along  double       longitude + s' (radians)
    xpl    double       not used
    ypl    double       not used
    sphi   double       not used
    cphi   double       not used
    diurab double       not used
    eral   double       not used
    refa   double       not used
    refb   double       not used

Returned:

   astrom  iauASTROM*  star-independent astrometry parameters:
    pmt    double       unchanged
    eb     double[3]    unchanged
    eh     double[3]    unchanged
    em     double       unchanged
    v      double[3]    unchanged
    bm1    double       unchanged
    bpn    double[3][3] unchanged
    along  double       unchanged
    xpl    double       unchanged
    ypl    double       unchanged
    sphi   double       unchanged
    cphi   double       unchanged
    diurab double       unchanged
    eral   double       "local" Earth rotation angle (radians)
    refa   double       unchanged
    refb   double       unchanged

Notes:

1. This function exists to enable sidereal-tracking applications to avoid wasteful recomputation of the bulk of the astrometry parameters: only the Earth rotation is updated.

2. For targets expressed as equinox based positions, such as classical geocentric apparent (RA,Dec), the supplied theta can be Greenwich apparent sidereal time rather than Earth rotation angle.

3. The function iauAper13 can be used instead of the present function, and starts from UT1 rather than ERA itself.

4. 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

     iauApcg iauApcg13    geocentric      ICRS <-> GCRS
     iauApci iauApci13    terrestrial     ICRS <-> CIRS
     iauApco iauApco13    terrestrial     ICRS <-> observed
     iauApcs iauApcs13    space           ICRS <-> GCRS
     iauAper iauAper13    terrestrial     update Earth rotation
     iauApio iauApio13    terrestrial     CIRS <-> observed

Those with names ending in “13” use contemporary SOFA 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.