Name

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      MPHASE

Purpose

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

Calling Sequence

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      MPHASE, jd, k

Input

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

Output

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      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.

Example

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      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

Method

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      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.

Procedures Called

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      MOONPOS, SUNPOS

Revision History

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      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