MPHASE

       Return the illuminated fraction of the Moon at given Julian date(s)

       MPHASE, jd, k

       JD - Julian date, scalar or vector, double precision recommended

       k - illuminated fraction of Moon's disk (0.0 < k < 1.0), same number
           of elements as jd.   k = 0 indicates a new moon, while k = 1 for
           a full moon.

       Plot the illuminated fraction of the moon for every day in July
       1996 at 0 TD (~Greenwich noon).
       IDL> jdcnv, 1996, 7, 1, 0, jd         ;Get Julian date of July 1
       IDL> mphase, jd+dindgen(31), k        ;Moon phase for all 31 days
       IDL> plot, indgen(31),k               ;Plot phase vs. July day number

       Algorithm from Chapter 46 of "Astronomical Algorithms" by Jean Meeus
       (Willmann-Bell, Richmond) 1991.   SUNPOS and MOONPOS are used to get
       positions of the Sun and the Moon (and the Moon distance).   The
       selenocentric elongation of the Earth from the Sun (phase angle)
       is then computed, and used to determine the illuminated fraction.

       MOONPOS, SUNPOS

       Written W. Landsman     Hughes STX           June 1996
       Converted to IDL V5.0   W. Landsman   September 1997
       Use /RADIAN keywords to MOONPOS, SUNPOS internally  W. Landsman Aug 2000

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