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MOONPOS

MOONPOS

Name


      MOONPOS

Purpose


      To compute the RA and Dec of the Moon at specified Julian date(s).

Calling Sequence


      MOONPOS, jd, ra, dec, dis, geolong, geolat, [/RADIAN ]

Inputs


      JD - Julian ephemeris date, scalar or vector, double precision suggested

Outputs


      Ra - Apparent right ascension of the moon in DEGREES, referred to the
              true equator of the specified date(s)
      Dec - The declination of the moon in DEGREES
      Dis - The Earth-moon distance in kilometers (between the center of the
            Earth and the center of the Moon).
      Geolong - Apparent longitude of the moon in DEGREES, referred to the
              ecliptic of the specified date(s)
      Geolat - Apparent longitude of the moon in DEGREES, referred to the
              ecliptic of the specified date(s)
      The output variables will all have the same number of elements as the
      input Julian date vector, JD. If JD is a scalar then the output
      variables will be also.

Optional Input Keyword


      /RADIAN - If this keyword is set and non-zero, then all output variables
              are given in Radians rather than Degrees

Examples


      (1) Find the position of the moon on April 12, 1992
      IDL> jdcnv,1992,4,12,0,jd ;Get Julian date
      IDL> moonpos, jd, ra ,dec ;Get RA and Dec of moon
      IDL> print,adstring(ra,dec,1)
              ==> 08 58 45.23 +13 46 6.1
      This is within 1" from the position given in the Astronomical Almanac
     
      (2) Plot the Earth-moon distance for every day at 0 TD in July, 1996
      IDL> jdcnv,1996,7,1,0,jd ;Get Julian date of July 1
      IDL> moonpos,jd+dindgen(31), ra, dec, dis ;Position at all 31 days
      IDL> plot,indgen(31),dis, /YNOZ

Method


      Derived from the Chapront ELP2000/82 Lunar Theory (Chapront-Touze' and
      Chapront, 1983, 124, 50), as described by Jean Meeus in Chapter 47 of
      ``Astronomical Algorithms'' (Willmann-Bell, Richmond), 2nd edition,
      1998. Meeus quotes an approximate accuracy of 10" in longitude and
      4" in latitude, but he does not give the time range for this accuracy.
      Comparison of this IDL procedure with the example in ``Astronomical
      Algorithms'' reveals a very small discrepancy (~1 km) in the distance
      computation, but no difference in the position calculation.
      This procedure underwent a major rewrite in June 1996, and the new
      calling sequence is *incompatible with the old* (e.g. angles now
      returned in degrees instead of radians).

Procedures Called


      CIRRANGE, ISARRAY(), NUTATE, TEN() - from IDL Astronomy Library
      POLY() - from IDL User's Library

Modification History


      Written by Michael R. Greason, STX, 31 October 1988.
      Major rewrite, new (incompatible) calling sequence, much improved
              accuracy, W. Landsman Hughes STX June 1996
      Added /RADIAN keyword W. Landsman August 1997
      Converted to IDL V5.0 W. Landsman September 1997
      Use improved expressions for L',D,M,M', and F given in 2nd edition of
            Meeus (very slight change), W. Landsman November 2000
      Avoid 32767 overflow W. Landsman January 2005
     



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