PRECESS

      Precess coordinates from EQUINOX1 to EQUINOX2.

      For interactive display, one can use the procedure ASTRO which calls
      PRECESS or use the /PRINT keyword.   The default (RA,DEC) system is
      FK5 based on epoch J2000.0 but FK4 based on B1950.0 is available via
      the /FK4 keyword.
      Use BPRECESS and JPRECESS to convert between FK4 and FK5 systems

      PRECESS, ra, dec, [ equinox1, equinox2, /PRINT, /FK4, /RADIAN ]

      RA - Input right ascension (scalar or vector) in DEGREES, unless the
              /RADIAN keyword is set
      DEC - Input declination in DEGREES (scalar or vector), unless the
              /RADIAN keyword is set
      The input RA and DEC are modified by PRECESS to give the
      values after precession.

      EQUINOX1 - Original equinox of coordinates, numeric scalar.  If
               omitted, then PRECESS will query for EQUINOX1 and EQUINOX2.
      EQUINOX2 - Equinox of precessed coordinates.

      /PRINT - If this keyword is set and non-zero, then the precessed
               coordinates are displayed at the terminal.    Cannot be used
               with the /RADIAN keyword
      /FK4   - If this keyword is set and non-zero, the FK4 (B1950.0) system
               will be used otherwise FK5 (J2000.0) will be used instead.
      /RADIAN - If this keyword is set and non-zero, then the input and
               output RA and DEC vectors are in radians rather than degrees

       Accuracy of precession decreases for declination values near 90
       degrees.  PRECESS should not be used more than 2.5 centuries from
       2000 on the FK5 system (1950.0 on the FK4 system).

       (1) The Pole Star has J2000.0 coordinates (2h, 31m, 46.3s,
               89d 15' 50.6"); compute its coordinates at J1985.0
       IDL> precess, ten(2,31,46.3)*15, ten(89,15,50.6), 2000, 1985, /PRINT
               ====> 2h 16m 22.73s, 89d 11' 47.3"
       (2) Precess the B1950 coordinates of Eps Ind (RA = 21h 59m,33.053s,
       DEC = (-56d, 59', 33.053") to equinox B1975.
       IDL> ra = ten(21, 59, 33.053)*15
       IDL> dec = ten(-56, 59, 33.053)
       IDL> precess, ra, dec ,1950, 1975, /fk4

       Algorithm from Computational Spherical Astronomy by Taff (1983),
       p. 24. (FK4). FK5 constants from "Astronomical Almanac Explanatory
       Supplement 1992, page 104 Table 3.211.1.

       Function PREMAT - computes precession matrix

       Written, Wayne Landsman, STI Corporation  August 1986
       Correct negative output RA values   February 1989
       Added /PRINT keyword      W. Landsman   November, 1991
       Provided FK5 (J2000.0)  I. Freedman   January 1994
       Precession Matrix computation now in PREMAT   W. Landsman June 1994
       Added /RADIAN keyword                         W. Landsman June 1997
       Converted to IDL V5.0   W. Landsman   September 1997
       Correct negative output RA values when /RADIAN used    March 1999
       Work for arrays, not just vectors  W. Landsman    September 2003

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