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CODING OF LUNAR OCCULTATION PREDICTIONS
GENERATED BY THE "OCCULT" SOFTWARE PACKAGE
DEVELOPED BY DAVID HERALD IN CANBERRA
 

OCCULT for DOS  OUTPUT  (Part of one of David Herald's support files)


1. Occultation Predictions.

This is the main computational routine. It computes occultation predictions for all selected sites for the range of specified dates.

On selection, the screen displays the selected star catalogue, and whether output will be sent to the printer or to a disk file. It also displays the sites for which predictions are to be computed.

The program asks for the year, the first month and day, and the last month and day. Pressing <enter> on blank fields enters the year, month, and day from the computer's clock, and for the end date copies the month or day from the start date.

If the selected year differs from the computers clock by more than 5 years, the program requires confirmation of the year (this reduces the chance of an incorrectly entered date being unnoticed, with the program unnecessarily computing a lunar ephemeris for that date.)
 

NOTE. If any part of the UT period 22h 30m to 1h 30m occurs with the moon above the horizon, predictions MUST be computed for the relevant two UT dates to obtain complete predictions. This is because the program functions by computing predictions for all stars in lunar geocentric conjunction on the relevant UT day; conjunctions on one day can lead to occultations in the other day in this period.
 

The predictions are displayed on screen (and sent to printer or file). All predictions are corrected for the lunar limb using data from the Watts charts. Further, the corrections to the Watts charts determined by Morrison & Appleby MNRAS (1981) 196, 1013-1020, are applied.

The predictions commence with a header identifying the site. The predicted information is as follows:

Day   The day of the event. A quirk is that on the first or last day of the month, the day may occasionally appear in the wrong month. This is because the program selects events by the time of the geocentric conjunction of the star and moon; however an occultation may occur up to 1.5 hours from this time. Thus for events falling within 1.5 hours of the end or start of a month may be listed in the adjacent month. Such occurrences are obvious when they occur.

Time  The universal time, in hours, mins and secs.

P     The type of event (phase):

      D  disappearance
      d  -  star less than 1 mag brighter than predicted visibility limit.

      R  reappearance
      r  -  star less than 1 mag brighter than predicted visibility limit.

      Gr grazing occultation at site. At mid-occultation, or closest
         approach, the star is less than 4" from the limb of the moon
         (either above or below).
      gr -  star less than 1 mag brighter than predicted visibility limit.
 

Star No  the star identification number, with the catalogue indicated as
         follows:

        nnnn     ZC catalogue no.

       nnnnn or
      nnnnnn    SAO catalogue number

      Xnnnnn    XZ94 catalogue no.
     Gnnnnnnnn  the Hubble Guide Star Catalogue number (note that this
                continues through the double star and spectral type
                fields.)
      ? nnnn    catalogue no. in another catalogue (With the initial
                letter of the catalogue file name leading)
 

D     the double star code. The following definitions apply:

      A  Listed by Aitken or Burnham
      B  Close double, with third star nearby with separate XZ entry
      C  Listed by Innes, Cousteau, or other visual observers
      D  Primary of double, secondary has separate XZ entry
      E  Secondary of double, primary has separate XZ entry
      F  Following component
      G  A or C with second star either M, J, U or V, with a third star
         referred to second star
      H  Triple: J or U or V, and M
      I  O, with secondary either J, U, or V (third star's data referred
         to secondary)
      J  One-line spectroscopic binary, separation probably <".01
      K  U or V, but duplicity doubtful
      L  Triple: J or U, and V; or all V; or all J
      M  Mean position of close pair
      N  North component
      O  Orbital elements available
      P  Preceding component
      Q  Triple; J or U or V, and O
      R  Triple; O and O
      S  South component
      T  Triple, V, and A or C; or all A and/or C
      U  Separation <".01 (usually a double-line spectroscopic binary)
      V  Separation >".01 but not visual (occultation, interferometric or
         speckle component)
      W  Triple; J or U, and A or C
      X  Probably a close double, but not certain
      Y  Triple; K or X, and A or C
      Z  Triple; O, and A or C or V or X or L
      $  G except M rather than A or C for 1-2 stars
      NOTE: Visual observers will usually not notice the duplicity of
      stars with codes J or U.

Sp    the star's spectral type

mag   the star's magnitude, usually visual.

%illum the percent illumination of the moon. If followed by a +, values
      are for a waxing moon; - for a waning moon and E for illumination
      during a lunar eclipse.

elon  the elongation of the moon from the sun, in degrees.

sunalt    the altitude of the sun, but only if it is greater than -12 deg.

moon alt  the altitude of the moon

moon az   the azimuth of the moon

CA    Cusp Angle - the angle of the event around the limb of the moon,
      measured from the nearest cusp. -'ve values indicate a bright limb
      event. The cusps are usually N (north) or S (south), but near full
      moon can be E (East) or W (west).

      If a lunar eclipse is in progress, CA gives is the % distance from
      the centre of the umbra, and is followed by a 'U'. Values up to 103%
      are possible.

PA    Position Angle - the angle of the event around the limb of the moon,
      measured from true north

WA    Watts Angle - the angle of the event around the limb of the moon,
      measured eastward from the moon's north pole. Essential for
      reappearance, as it locates the event with reference to lunar
      features. To use, mark a map of the moon around the circumference at
      10 deg intervals, starting at the north pole. Mare Crisium is at
      about 300 deg. This provides the Watts angle scale.

Long Libn  the libration of the moon in longitude, as seen from the site
           at the time of the event

Lat Libn   the libration of the moon in latitude;
 

A     coefficient for correcting the prediction for changes in site
      location. The units are seconds of time per minutes of arc. The
      correction to the prediction for a change in site, in seconds of
      time, is found by multiplying A by the change in site longitude (in
      minutes of arc, +'ve to the East) from the prediction site.

B     same as for A, but for changes in latitude (+'ve to the north).
 

If output is sent to Disk, with the disk option set as 'Extra', the following
is given at the end of the line:

VA    Vertex Angle - the angle of the event around the limb of the moon
      measured anticlockwise from the vertex of the lunar limb - ie the
      point on the limb highest from the horizon.

RA    The apparent Right Ascension of the star. (For planets, this
      position is not the position at the time of the event, but at
      an integral hour near the time of the event.)

Dec   The apparent Declination of the star.
 
 

The predictions also give special messages, in the following circumstances:
 

- if a dark-limb occultation occurs less than 20" from the terminator, the
  distance to the terminator, and to a theoretical 3" high mountain beyond
  the terminator which could be sunlit, is given.

- if there is a grazing occultation of the star nearby. The distances used
  to indicate whether a graze-nearby message should be generated are set
  in the "Edit Configuration Data // Edit Default Values" option from the
  main menu of OCCULT.

  The message gives the latitude of the graze path at the longitude of the
  prediction site, the change of that latitude with a change in longitude
  (+'ve to the East), and the cusp angle of the graze.

- magnitude, separation and position angle for the components of double
  stars;

- for variable stars, the variable name, max and min magnitudes, type of
  variable, and (if the elements are known) the phase in its cycle. Note
  that for some variables, the magnitude variation is given in place of
  the minimum magnitude; and

- for occultations of planets and asteroids, the duration of the partial
  occultation.
 

Predictions are also given for a small number of open clusters (objects with M (Messier), NGC, IC, Rup (Ruprecht) &c numbers - and Spectral type set to ' C'. The clusters have been selected on the basis of the magnitude of the brightest cluster member, and the magnitude of that star is listed in the predictions (not the Total Magnitude of the cluster). These predictions also give the time taken for the whole cluster to be occulted (in mins).

The predictions on the screen are not sorted by time. However, predictions sent to the printer or disk are sorted by time. Note that OCCULT computes predictions on the basis of conjunctions occurring in 24 hr periods. Events arising from conjunctions in different UT days will not be sorted together.
 
 

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