Procedure

     RECRAD ( Rectangular coordinates to RA and DEC )

     SUBROUTINE RECRAD ( RECTAN, RANGE, RA, DEC )

Abstract

     Convert rectangular coordinates to range, right ascension,
     and declination.

Required_Reading

     None.

Keywords

     CONVERSION
     COORDINATES

Declarations

     IMPLICIT NONE

     DOUBLE PRECISION   RECTAN ( 3 )
     DOUBLE PRECISION   RANGE
     DOUBLE PRECISION   RA
     DOUBLE PRECISION   DEC

Brief_I/O

     VARIABLE  I/O  DESCRIPTION
     --------  ---  --------------------------------------------------
     RECTAN     I   Rectangular coordinates of a point.
     RANGE      O   Distance of the point from the origin.
     RA         O   Right ascension in radians.
     DEC        O   Declination in radians.

Detailed_Input

     RECTAN   are the rectangular coordinates of a point.

Detailed_Output

     RANGE    is the distance of the point from the origin.

              The units associated with RANGE are those
              associated with the input RECTAN.


     RA       is the right ascension of RECTAN. This is the angular
              distance measured toward the east from the prime
              meridian to the meridian containing the input point.
              The direction of increasing right ascension is from
              the +X axis towards the +Y axis.

              RA is output in radians. The range of RA is [0, 2*pi].


     DEC      is the declination of RECTAN. This is the angle from
              the XY plane of the ray from the origin through the
              point.

              DEC is output in radians. The range of DEC is
              [-pi/2, pi/2].

Parameters

     None.

Exceptions

     Error free.

     1)  If the X and Y components of RECTAN are both zero, the
         right ascension is set to zero.

     2)  If RECTAN is the zero vector, right ascension and declination
         are both set to zero.

Files

     None.

Particulars

     This routine returns the range, right ascension, and declination
     of a point specified in rectangular coordinates.

     The output is defined by a distance from a central reference
     point, an angle from a reference meridian, and an angle above
     the equator of a sphere centered at the central reference
     point.

Examples

     The following code fragment converts right ascension and
     declination from the B1950 reference frame to the J2000 frame.

        C
        C     Convert RA and DEC to a 3-vector expressed in
        C     the B1950 frame.
        C
              CALL RADREC ( 1.D0, RA, DEC, V1950 )
        C
        C     We use the SPICELIB routine PXFORM to obtain the
        C     transformation  matrix for converting vectors between
        C     the B1950 and J2000 reference frames. Since
        C     both frames are inertial, the input time value we
        C     supply to PXFORM is arbitrary. We choose zero
        C     seconds past the J2000 epoch.
        C
              CALL PXFORM ( 'B1950', 'J2000', 0.D0, MTRANS )
        C
        C     Transform the vector to the J2000 frame.
        C
              CALL MXV ( MTRANS, V1950, V2000 )
        C
        C     Find the RA and DEC of the J2000-relative vector.
        C
              CALL RECRAD ( V2000, R, RA, DEC )

Restrictions

     None.

Literature_References

     None.

Author_and_Institution

     C.H. Acton         (JPL)
     N.J. Bachman       (JPL)
     J. Diaz del Rio    (ODC Space)
     H.A. Neilan        (JPL)
     W.L. Taber         (JPL)

Version

    SPICELIB Version 1.1.0, 12-AUG-2021 (JDR)

        Added IMPLICIT NONE statement.

        Edited the header to comply with NAIF standard.

    SPICELIB Version 1.0.2, 30-JUL-2003 (NJB) (CHA)

        Various header changes were made to improve clarity. Some
        minor header corrections were made.

    SPICELIB Version 1.0.1, 10-MAR-1992 (WLT)

        Comment section for permuted index source lines was added
        following the header.

    SPICELIB Version 1.0.0, 31-JAN-1990 (HAN)