CODING OF LUNAR OCCULTATION
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 - star less than 1 mag brighter than predicted visibility limit.
r - star less than 1 mag brighter than predicted visibility limit.
Gr grazing occultation at site. At mid-occultation,
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
nnnn ZC catalogue no.
nnnnnn SAO catalogue number
Xnnnnn XZ94 catalogue
Gnnnnnnnn the Hubble Guide Star Catalogue number (note that this
continues through the double star and spectral type
? 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
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
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%
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
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
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
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,
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
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
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
- for variable stars, the variable name, max and min magnitudes, type
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
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.