CO_NUTATE

     Calculate changes in RA and Dec due to nutation of the Earth's rotation

     Calculates necessary changes to ra and dec due to
     the nutation of the Earth's rotation axis, as described in Meeus, Chap 23.
     Uses formulae from Astronomical Almanac, 1984, and does the calculations
     in equatorial rectangular coordinates to avoid singularities at the
     celestial poles.

     CO_NUTATE, jd, ra, dec, d_ra, d_dec, [EPS=, D_PSI =, D_EPS = ]

    JD: Julian Date [scalar or vector]
    RA, DEC : Arrays (or scalars) of the ra and dec's of interest
   Note: if jd is a vector, ra and dec MUST be vectors of the same length.

    d_ra, d_dec: the corrections to ra and dec due to nutation (must then
                                be added to ra and dec to get corrected values).

    EPS: set this to a named variable that will contain the obliquity of the
             ecliptic.
    D_PSI: set this to a named variable that will contain the nutation in the
           longitude of the ecliptic
    D_EPS: set this to a named variable that will contain the nutation in the
                       obliquity of the ecliptic

    (1) Example 23a in Meeus: On 2028 Nov 13.19 TD the mean position of Theta
        Persei is 2h 46m 11.331s 49d 20' 54.54".    Determine the shift in
        position due to the Earth's nutation.
        IDL> jd = JULDAY(11,13,2028,.19*24)       ;Get Julian date
        IDL> CO_NUTATE, jd,ten(2,46,11.331)*15.,ten(49,20,54.54),d_ra,d_dec
              ====> d_ra = 15.843"   d_dec = 6.217"

    NUTATE

    Written  Chris O'Dell, 2002
    Vector call to NUTATE   W. Landsman   June 2002

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