Procedure

     HALFPI ( Half the value of pi )

     DOUBLE PRECISION FUNCTION HALFPI ( )

Abstract

     Return half the value of pi (the ratio of the circumference of
     a circle to its diameter).

Required_Reading

     None.

Keywords

     CONSTANTS

Declarations

    None.

Brief_I/O

     The function returns half the value of pi.

Detailed_Input

     None.

Detailed_Output

     The function returns half the value of pi (the ratio of
     a circle's circumference to its diameter), determined by
     the ACOS function. That is,

           HALFPI = ACOS ( -1.D0 ) * 0.5D0

Parameters

     None.

Exceptions

     Error free.

Files

     None.

Particulars

     The first time the function is referenced, the value is computed
     as shown above. The value is saved, and returned directly upon
     subsequent reference.

Examples

     The subroutine shown below illustrates the use of HALFPI.

                 SUBROUTINE BFTRAN ( RA, DEC, W, TIPM )

           C
           C     Compute the transformation from inertial to body
           C     fixed coordinates, given the directions of the north
           C     pole and prime meridian of the body.
           C
                 DOUBLE PRECISION    RA
                 DOUBLE PRECISION    DEC
                 DOUBLE PRECISION    W
                 DOUBLE PRECISION    TIPM ( 3,3 )

           C
           C     SPICELIB functions
           C
                 DOUBLE PRECISION    HALFPI

           C
           C     The transformation is defined by the compound
           C     rotation
           C
           C        [W] [pi/2 - Dec] [RA + pi/2]
           C           3            1           3
           C
                 CALL ROTATE (       RA + HALFPI(),  3, TIPM)
                 CALL ROTMAT (TIPM,  HALFPI() - DEC, 1, TIPM)
                 CALL ROTMAT (TIPM,  W,              3, TIPM)

                 RETURN
                 END

Restrictions

     None.

Literature_References

     None.

Author_and_Institution

     J. Diaz del Rio    (ODC Space)
     W.L. Taber         (JPL)
     I.M. Underwood     (JPL)

Version

    SPICELIB Version 1.0.3, 09-JUL-2020 (JDR)

        Edited the header to comply with NAIF standard.

    SPICELIB Version 1.0.2, 08-APR-2015 (JDR)

        Minor edit to example comments eliminating typos.

    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 (WLT) (IMU)