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1986-10-04
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Jupiter Satellite Simulator/Charter
(JSSC)
Ron Fowler
NightOwl Software, Inc.
JSSC provides both real-time and simulated displays of Jupiter's
Galilean satellites; it runs on IBM-compatible computers and
requires a Color Graphics Adapter (or compatible video card).
In addition to simulation, JSSC provides a charting function to
produce a graph of the Jovian system over a period of time (sim-
ilar to the one which appears each month in Sky & Telescope).
-------------------------
JSSC always comes up in the simulator mode, where Jupiter is
displayed in the center of the a telescopic viewfield, with each
of the four Gallilean satellites plotted as points in Jupiter's
equatorial plane (each labeled with the first letter of its
name). The simulator can be in one of three modes: Realtime,
Skip and Freeze. If you're using the JSSC.DAT file supplied,
JSSC will be in Freeze mode (described below). Skip and Realtime
modes can be selected via commands, which are entered in as
keyboard characters whenever the simulator is "idling" (i.e., not
prompting for other input). Whichever mode you select when you
exit JSSC will be stored in JSSC.DAT, so that the next time you
run JSSC, you'll come up in the same mode as when you last left.
Online help is available via the 'H' command (simply key an 'H'
at the simulation screen). Each of the available commands is
described as follows:
1) H (Help) prints a short help text. '?' also works.
2) R (Realtime mode): JSSC advances the display as your system
clock advances and uses the system's time and date as "current"
time and date.
3) S (Skip mode): JSSC adds an increment to the current time once
each second, then re-displays. The "current" time and date start
out the same as the system's, but may be changed with the D
command (below).
4) F (Freeze mode): JSSC does not automatically update the dis-
play. Two subcommands are available from Freeze mode: + (move
the time forward by the increment amount) and - (move backward by
the increment amount). This mode is handiest for manually moving
the Jovian system backward and/or forward. Use "F" to toggle in
and out of Freeze mode.
5) D (set Date/Time): this command is used to set the current
date and time; ("T" is a synonym for "D"). JSSC will prompt you
for a new date and time. The D command forces Skip mode, since D
does not change the system's date or time, and thus is not "real-
time".
6) I (set Increment): JSSC will ask you for a new Increment
value for the skip mode. You may enter any number of days, along
with an appropriate number of hours, minutes and seconds. The
Increment is saved between runs in JSSC.DAT.
7) Z (time zone): JSSC will print a list of time zones, from
which you may select a three-letter mnemonic (be sure and key in
all 3 letters). If your time zone is not listed, you can enter a
number instead. This number is the number of hours required to
add to Local time (LCT) to convert to Greenwich Mean Time (6
hours for Central Standard Time, for example). JSSC will save
your time zone in JSSC.DAT; thus, after initial setup, you should
require this command only twice per year.
8) V (Viewfield): toggles on and off the simulated telescopic
field through which Jupiter and its satellites appear.
7) Cursor control keys: Magnification can be increased using the
page-up key (coarse adjustment) or the uparrow key (fine ad-
justment). Similarly, page-down and downarrow decrease magni-
fication. Leftarrow moves the image in the "telescope" left
while rightarrow moves the image right. All of these keys do
intensive graphics manipulation, and thus are not extremely fast.
8) C (Chart)
This command plots a graph of the Jovian system over time; JSSC
will prompt you for a starting date/time, and a duration in days
and hours, minutes and seconds. The chart screen will display GMT
dates and times on the left, and local dates and times on the
right. Letters identifying the satellites are printed period-
ically down the graph, each letter near the curve for its sat-
ellite. The plot may be aborted at any time with the escape key.
I recommand a maximum duration of five to seven days ... much
more than that, and the graph tends to be squeezed too tightly.
After the plot has completed, JSSC will prompt for "X" or "A";
"X" will exit back to the simulation mode. "A", on the other
hand, will display an arrow on the screen, which may be moved
with the cursor keys. The arrow is used for querying date and
time at a particular point on the graph. Move the arrow to an
area, then use the "I" key to identify that point's time and
date.
Notes
-----
1) JSSC does not calculate latitude variations. The satellites
are all displayed in Jupiter's equatorial plane.
2) JSSC does not differentiate between transits and eclipses. In
simulation modes, the satellites always display in the fore-
ground, regardless of whether the disk of Jupiter is in front of
or behind the satellite (this is true of the chart function as
well, except that the satellites appear to pass behind the band
representing Jupiter).
3) In chart mode, dates and times are prefixed with "G" or "L",
to indicate GMT or LCT.
4) Whenever JSSC prompts for a date or time, if you respond with
only your enter key, JSSC will use the CURRENT date or time.
When you specify a time or date, be sure to use the following
forms:
DATE: MM-DD-YYYY (e.g., 09-12-1986)
TIME: HH:MM:SS (e.g., 14:21:17)
While JSSC is fussy about the format (it'll beep and re-prompt if
it doesn't like your entry), it does no error checking on the
actual numbers entered.
Responding with carriage-returns to both time and date during the
Increment prompts cause a 1-second default increment. Similarly,
you can select a default 5-day period for the Chart period by
entering carriage-return to both of the period prompts.
5) It's sometimes handy to align JSSC's Chart function with a
Sky & Telescope chart. When doing this, remember that S&T charts
are labeled for GMT; you should, therefore, start your chart at
midnight GMT on the desired date. This will require a little
forethought. For example, from my time zone (5 hours behind GMT
in the summer), to start a September 1 plot in alignment with
S&T's chart, the start date is 08-31-1986 and the start time is
19:00:00 (which is the time here on 31 August when Greenwich
Midnight occurs on 1 September)
Credits
-------
Few programs are entirely original works, and this is certainly
true of JSSC:
- JSSC would have been a lot sloppier without David Bulger's
orbital calculations (taken, along with a few other
goodies, from David's JUPZOOM.BAS).
- Local-time/date to Julian date calculation routines came
from David Bulger's JULIAN.BAS (David does write a *lot*
of good stuff!)
- Julian-to-local conversion came from JDCAL.BAS, a pro-
gram originally published in Sky & Telescope magazine.
Ron Fowler
NightOwl Software, Inc.
October 1, 1986
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