Name

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    CO_NUTATE

Purpose

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

Explanation

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    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.

Calling Sequence

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

Inputs

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    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.

Outputs

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    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).

Optional Output Keywords

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    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

Example

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    (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"

Procedures Used

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    NUTATE

Revision History

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