Skip to content

Latest commit

 

History

History
85 lines (65 loc) · 2.6 KB

era.fk5hz.md

File metadata and controls

85 lines (65 loc) · 2.6 KB
Raw

eraFk5hz

[rh, dh] = ERFA.fk5hz(r5, d5, date1, date2)

Transform an FK5 (J2000.0) star position into the system of the Hipparcos catalogue, assuming zero Hipparcos proper motion.

Given:

   r5           double   FK5 RA (radians), equinox J2000.0, at date
   d5           double   FK5 Dec (radians), equinox J2000.0, at date
   date1,date2  double   TDB date (Notes 1,2)

Returned:

   rh           double   Hipparcos RA (radians)
   dh           double   Hipparcos Dec (radians)

Notes:

  1. This function converts a star position from the FK5 system to the Hipparcos system, in such a way that the Hipparcos proper motion is zero. Because such a star has, in general, a non-zero proper motion in the FK5 system, the function requires the date at which the position in the FK5 system was determined.

  2. The TT date date1+date2 is a Julian Date, apportioned in any convenient way between the two arguments. For example, JD(TT)=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.

  1. The FK5 to Hipparcos transformation is modeled as a pure rotation and spin; zonal errors in the FK5 catalogue are not taken into account.

  2. The position returned by this function is in the Hipparcos reference system but at date date1+date2.

  3. See also eraFk52h, eraH2fk5, eraHfk5z.

Called:

   eraS2c       spherical coordinates to unit vector
   eraFk5hip    FK5 to Hipparcos rotation and spin
   eraSxp       multiply p-vector by scalar
   eraRv2m      r-vector to r-matrix
   eraTrxp      product of transpose of r-matrix and p-vector
   eraPxp       vector product of two p-vectors
   eraC2s       p-vector to spherical
   eraAnp       normalize angle into range 0 to 2pi

Reference:

F.Mignard & M.Froeschle, 2000, Astron.Astrophys. 354, 732-739.

This revision: 2021 May 11

Copyright (C) 2013-2021, NumFOCUS Foundation. Derived, with permission, from the SOFA library.