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      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

Calling Sequence

      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.

Optional Inputs

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

Optional Input Keywords

      /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

Revision History

      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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