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GALILEAN.DOC
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1993-05-10
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GALILEAN SIMULATOR PROGRAM
This program will calculate the position of the Galilean satellites and
it will also display the position of the great Red spot. The calculations
involved are quite complex and in animation run's the update time can
be quite long on slow XT machines and low spec 286's.
The program will run on machines which have VGA or EGA graphics
cards installed. The program also displays messages referring to the
satellite's orbit, namely Superior conjunction, Eastern elongation, Western
elongation and inferior conjunction. The program will also indicate if a
transit of any satellite is occurring and will actually display the shadow
of the satellite on the surface of Jupiter in another colour. It will also
indicate when an occultation of a satellite by Jupiter is occurring. Owing to the
planetocentric angular distance of the Earth from Jupiter, the shadows may actually
miss their intended objects altogether.
The formulae used to calculate the positions of the satellites are from
the excellent astronomy book "Astronomical Formulae For Calculators" by
Jean Meeus, and the techniques used to predict eclipse, transits, occultations,
and shadow transits were developed by the author. All of the values that the
program calculates such as positions, times of Shadow Ingress (beginning),
Shadow Egress (ending), Eclipse disappearance (beginning), Eclipse reappearance,
(ending), Occultation disappearance, Occultation reappearance, Transit Ingress
and Transit Egress have all been compared with values published in The
Handbook of the BAA and all the values have been within 3 minutes of the
published values except for satellite IV whose Shadow and Eclipse times were
out by up to 20 minutes which is not so surprising as it is so far away from
Jupiter.
The position of the great red spot is determined by assuming that it
has a constant longitude and latitude. If the program is run very far from 1988
then the values will be in error. Nothing can be done about this and
it really cannot be predicted in advance. In fact over the previous century,
it has retrograded many times from it's predicted longitude position in
system II. The program assumes that it is located at a position of latitude
23 degrees south, longitude 6 degrees. Hopefully in the next release of the
program I will improve this arrangement.
The program requires the following parameters in order to run.
A)
The date to begin simulation for. The values should each be separated
by at least one space or alternatively one can be typed in at a time
followed by hitting the return key. The program uses the following
standard format for dates HOUR MINUTE DAY MONTH YEAR. All these values
should be in integer format without any decimal points.
B)
The ending date for the simulation. The format for these values is
exactly the same as for A) above. If you just one to see the position
for just one instant, then this value should be the same as A).
C)
The stepping period. This is the value by which the time increments
by during each update of the screen during the animation. As a rough
guide 0.04 of a day is equal to one hour.
D)
The delay between each screen update. This is the time in milliseconds that
the image stays on the screen before it is overwritten by the next screen.
If the program is being run on a slow computer then this value should be
set to a small value to update the screen quicker. It is handy to slow down
the update time if an interesting event is being investigated as the slower
update time allows the user to observe exactly when the event occurred.
E)
Display the trails. If you want to observe the path of the satellite as it
moves around then this option allows the user this option. If you want to
leave on the tracks then type in a small y else anything else.
F)
Error band before info is displayed about the red spot
This value in degrees is used so that when the red spot is within this
limit of the central meridian then it will be indicated.
A value of about 20 degrees is o.k. for normal use but if high precision
work is required such as determining when exactly the red spot is on the
central meridian then this can be achieved by putting this value to say
2 or 3 degrees and increasing the update time so that the moment can be
found exactly.
G)
Error band before info is displayed about the satellites
This value is used in exactly the same way as F) above except that if
refers to when the satellites are going through the orbit cycle. The
values indicated are Inferior Conjunction, Superior Conjunction, Western
Elongation and Eastern Elongation.
H)
Normal or Inverted view
This allows the user to switch between the view that a naked eye or
binocular would see and the inverted view that an astronomical telescope
gives.
I)
Longitude and Latitude of a user defined location on Jupiter. This option
may come in handy when a particular location is being observed and you
want to find when it will be visible.
When the program is run the following information appears on the screen
along with the graphic of the Moons.
1) The date in Gregorian form.
2) A compass to indicate directions.
3) system 1 longitude: this is the central meridians longitude.
4) system 2 longitude: this is the central meridians longitude.
5) d_s: this is the planetcentric angular distance of the sun from the
equator of Jupiter.
6) d_e: this is the planetcentric angular distance of the earth from the
equator of Jupiter.
7) phase angle: this is the angle between Sun - Jupiter - Earth
8) The angle to inferior conjunction of each satellite measured in a
clockwise sense when viewed from above the north pole of Jupiter.
9) The distance from (center to center) each satellite to Jupiter,
measured in units of Jupiter's equatorial radius.
10) The x-value of each satellite from Jupiter (again in terms of the
equatorial radius.
11) The y-value of each satellite from Jupiter (again in terms of the
equatorial radius.
The program also displays info when a special event such as an eclipse or
shadow is present. It also displays info about the phases of the orbits of
the moons such as elongation's (eastern and western) and conjunctions
(inferior and superior) occur. These messages are colour coded corresponding
to the colours displayed on the graphic.
NOTE:
The program won't work unless it has access to the .bgi files which
accompanies it.
This program is presented as is and users are free to give copies to friends
If you find this program useful I would be grateful if you could send me
some donation to help with producing further improvements or if you have any
suggestions contact the author at:
P.J. Naughter,
Cahore,
Ballygarrett,
Gorey,
Co. Wexford,
Ireland.