Contents of the JMOONS.DOC file
Moons of Jupiter Orbital Clock
Version 0.00 - April 1988
Most of us are familiar with the path and phases of Earth's only
satellite - the moon. Orbiting the Earth every 27 days or so, the
moon controls the tides of our oceans, provides illumination at night,
and also gives us an excellent object to observe in our telescopes,
binoculars, or just the naked eye.
Imagine what it might be like if the Earth had four satellites of
different sizes, different orbital periods, different topographical
and geographical features, all having an effect on the tides and
eclipsing each other and the sun. Quite a scenario, wouldn't you
think? That's what it would be like if you lived on the planet
Jupiter. Although the moons of Jupiter are tiny in comparison to the
size of the planet itself, they are so varied in features and
properties that they are the most unique satellites in our solar
While Jupiter is on the average more than 2000 times farther away
from us than our moon is, we can still marvel at the four largest
satellites circling this vast gas giant of a planet. In a telescope
and even in a pair of strong binoculars, the movements of these moons
can be observed. Below are the names and some interesting data about
each of Jupiter's four largest moons.
Average Distance Time to Orbit Diameter
Name from planet (km) planet (km)
Io 422,000 1d 18h 28m 3632
Europa 671,000 3d 13h 14m 3126
Ganymede 1,070,000 7d 3h 43m 5276
Callisto 1,885,000 16d 16h 32m 4820
And just for comparison, here's the same data for our moon.
Moon 384,500 27d 7h 43m 3476
All this data for the moons of Jupiter add up to a never ending
panorama of circling objects. From our viewpoint here on Earth,
however, we can really see only one dimension of these movements,
motion that occurs from side to side. That is the purpose of this
program. To show the relative position of each of the moons in
relation to Jupiter itself at a given date and time.
To start the program, type the word "MOONS" in at your DOS prompt
and press the enter key. The first screen that appears is where you
respond to four inquiries so the program will know what you would
DATE, TIME, AND TIME ZONE
Here's what the program needs to know:
1) DATE - The moons will appear in their positions as they
were/are/will be on the date you type in. The format
required is MM-DD-YYYY. Since the program does not keep
time itself, but rather relies on your computer to do it,
there are some restrictions as to what dates you may enter.
Basically, the rules are the same for this date prompt as
they are for the DOS DATE command. No dates earlier than
January 1, 1980 (01-01-1980) will be accepted, nor will any
dates later than December 31, 2099 (12-31-2099). A quick
shortcut around having to type in the current date all the
time is to just enter the letter "D" (upper or lower case).
This will retrieve the DOS date from your computer. You
entered the DOS date when you turned on your computer or the
DOS date may have been set by your computer's internal
2) TIME - The moons will appear in their positions as they
were/are/will be at the time you enter. The required format
for this prompt is HH:MM:SS in military style. That's the
same as 24-hour format. For example, if the regular time is
08:36:15 PM, the corresponding 24-hour format would be
20:36:15. If you just type in the letter "T" (upper or
lower case), the time will be retrieved from DOS. Again,
either you set the time yourself or your computer's internal
clock did it for you. By the way, don't worry about
resetting the date and time before you exit the program, you
will be prompted before the program returns you to DOS.
3) ZONE - This is a value between -12 and +15 indicating the
number of hours before or after Universal Coordinated Time.
Universal Coordinated Time, or UTC for short, is the time at
the zero meridian which crosses the Eastern portion of
Europe and Africa. This imaginary line passes through
Greenwich, England, therefore UTC is also known as Greenwich
Mean Time (GMT). The continental United States covers four
different zones that are offset from UTC by -5,-6,-7, and -8
hours when in Standard time and -4,-5,-6, and -7 hours when
in Daylight Saving Time. These four zones are Eastern,
Central, Mountain, and Pacific respectively. A table is
shown at the bottom of the input screen that includes all
the time zones in North America, Alaska, Hawaii, Australia,
and a few other locations around the world.
4) NAME - This field is for descriptive purposes only. Whatever
3-letter abbreviation you enter here will be shown on the
Orbital Clock display. The zone table lists the time zone
abbreviations for the continental U.S. You may just press
enter at this prompt if you do not wish to have the
abbreviation appear or don't know what the abbreviation is.
Now on to the Orbital Clock!
THE ORBITAL CLOCK
The large rectangular box that appears at the top of your screen
is the window looking into the Jovian system. South is at the top of
the display, matching a view through a telescope. The large circular
object in the center is the planet Jupiter, and the four smaller
objects are the four Galilean satellites which are not shown to scale
in relation to Jupiter. Just what order the satellites appear in from
left to right depends on the date and time. Speaking of the date and
time, they appear in the lower center of the Orbital Clock. The line
immediately above the date and time, but below the planet and
satellites, shows the maximum orbits of each of the moons and looks
something like this:
The innermost brackets indicate the limits of Io's orbit, the
second set of brackets show the limits of Europa's orbit, the third,
Ganymede's and the fourth or outermost brackets mark the limits of
Callisto's orbit. For example, Io will always travel within the
innermost set of brackets. You may look in your telescope and see a
satellite that is close to Jupiter and a little above or below the
center of Jupiter. This means that all the satellites' orbits are not
in line with a 90 degree view from Earth, but inclined or declined a
bit. The Orbital Clock on your screen does not take that into
consideration. It shows a 90 degree view all the time. Also, the
moons may eclipse Jupiter and each other so one object may be hidden
by another. The Orbital Clock will show the satellites at all times
whether they are behind Jupiter or in front of it. (See the 15-Day
Orbital Graph below.) If, however, a satellite passes in front of
another satellite, the Orbital Clock may show the hidden one and not
the real one that is in front. (See the Bi-Dimensional display
The table of values in the left center of your screen are the
apparent distance of each moon from the center of Jupiter and the
percentage of each moon's maximum apparent distance. The first column
shows in kilometers how far away the satellite appears to be from the
center of Jupiter. Of course, each moon is really a fixed distance
away from Jupiter with only minor variations, but since the Orbital
Clock shows a head-on view, and the view through a telescope is
similar, it seems like the different satellites get nearer or farther
away from Jupiter. The second column of numbers represent the
percentage of the maximum apparent distance a particular moon can be
from Jupiter. This is handy if you wish to know exactly when a
satellite is passing directly in front of or behind Jupiter (the
percentage will be 0.0000), or when a satellite is at it's maximum
apparent distance from the planet (100.0000 %). The negative and
positive values just show which side of the display the object
appears, negative for the left side and positive for the right side.
Two keys on your computer keyboard will act on the Orbital Clock
and table of values. The Orbital Clock first appears in a frozen
state (your computer is still advancing the time internally,
however). The "S" key will toggle the Orbital Clock on, setting the
clock in motion, which means that every second the Date, Time, and
table of values will automatically be updated. When the minute
changes, the display of satellites is updated. Pressing the "S" key
again will toggle the Orbital Clock off into the frozen state.
Pressing the "U" key will update the Orbital Clock, Date, Time,
and table of values. This feature will work when the Orbital Clock is
in motion or in the frozen state.
When you press the "G" key on your keyboard, the 15-Day Orbital
Graph is created at the lower right hand portion of your screen. The
reason it isn't displayed for you automatically is that on some
computers it may take a couple of minutes to complete. So, it will
only appear if you request it.
On this graph are 15 days, seven before and seven after the date
you specified on the Date, Time, and Zone screen. A line will appear
within this date that corresponds to the time of day you specified.
In the middle of the graph, running from top to bottom is a thick line
that represents the diameter of Jupiter. Swirling around this central
line are four graphs or sine waves. These represent the apparent
distance of the four moons from Jupiter. Again, South is at the top,
matching the view in a telescope.
The one wave that completes a period (starting at one point on
the sine wave and continuing down to the same relative point) of a
little more than a day and a half and never gets too far away from
Jupiter belongs to Io. The next sine wave that completes a period in
about 3 and a half days is Europa's. The third is Ganymede's with a
period of about 7 days, and the last is Callisto's whose period is
approximately 16.75 days and therefore one full period cannot be shown
on this graph. If your computer displays color while running this
program, you may notice that on the graph the sine waves may disappear
while about to pass over the center Jupiter line. This indicates that
the moon, from our view point on Earth, is passing behind Jupiter.
The very next time that same wave crosses Jupiter, it will be visible
and indicates that the moon is passing in front of Jupiter.
An alternative to the 15-Day Graph is the 3-Day graph. This
option follows the same description above, but is displayed by
pressing the "H" key on your computer keyboard.
These graphs are useful for determining when all four moons will
be to the right or left of Jupiter, or when a particular moon is
eclipsing Jupiter or another moon, or the rare (maybe impossible) time
when all four satellites are either directly in front of or behind
BI-DIMENSIONAL ORBITAL DISPLAY
Another feature that helps to determine whether a satellite is in
front of or behind Jupiter is the Bi-Dimensional Orbital Display.
Activated by pressing the "P" key, this display shows a view of
Jupiter and it's moons from an angle slightly above the 90-degree
plane that the Orbital Clock displays. As with the Orbital Clock and
Graphs, South is at the top. Jupiter is in the middle of this
window. Satellites that are passing in front of Jupiter appear in the
lower half of this window, and those passing behind Jupiter appear in
the upper half. Once again, this display may take several minutes to
complete, so be prepared to wait if your computer is not a speed
demon. The moons are not shown to scale in relation to the size of
Jupiter. If they were shown to scale, you probably wouldn't be able
to see them on this small display. The four rings around Jupiter are
there just to show the paths of the four satellites. Use the Orbital
Clock display to determine which round spot on the Bi-Dimensional
display represents which satellite.
Quick Reference to active keys
The following keys may be pressed while the Orbital Clock is
displayed: (Upper or lower case, it doesn't matter.)
"G" = 15 Day Orbital Graph.
"H" = 3 Day Orbital Graph.
"P" = Bi-Dimensional Display.
"S" = Sets the Orbital Clock in motion or freezes it.
"U" = Updates the Orbital Clock.
"X" = Exits the program and returns you to DOS.
"Z" = Allows change in Date, Time, and Time Zone.
Computer Requirements and Notes
The Moons of Jupiter Orbital Clock is designed to run on MS-DOS
compatible machines. Memory should not be a problem, but 256K or more
seems like a good idea. A CPU that runs at 8 MHz or faster (80286 or
80386) is a good idea also, otherwise some of the displays may take
considerable amount of time to complete.
The inner workings of this program try to determine the best
graphics video mode that your computer and monitor can display. It is
recommended that you have an EGA card and monitor. This way the
program displays in glorious 640 x 350 resolution and the objects are
all color coded. A Color Graphics Adapter and monitor will reduce the
resolution to 640 x 200, which still is pretty good, but will only be
in one color. The sine waves in the Graphs and the satellites in the
Bi-Dimensional Display are a little difficult to separate.
The Moons of Jupiter Orbital Clock program is great for rainy
days, or cloudy nights. Sometimes your favorite observing time
doesn't coincide with the time Jupiter is in the night sky. Below is
a general list of when Jupiter is in the sky through 1989.
Key: r = Jupiter rises in the East during twilight.
R = Jupiter rises in the East after twilight.
t = Jupiter transits during twilight.
T = Jupiter transits after twilight.
s = Jupiter sets in the West during twilight.
S = Jupiter sets in the West after twilight.
1988 - JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
S S S s r R R R Rt RT rTs TS
Opposition: November 22, 1988
1989 - JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
tS S S S s r r R R Rt RT rT
Opposition: December 27, 1989
A monthly astronomy magazine will be more specific about the
dates Jupiter rises, transits, and sets, and usually has a chart
indicating the positions of Jupiter's satellites at a certain time
every day. These types of publications greatly enhance your knowledge
of astronomy, and this program can keep you up to date on the moons of
Jupiter, but don't forget to check out the real thing in your
Menzel, Donald H. and Pasachoff, Jay M. 1983. A Field Guide to
the Stars and Planets. Second Edition, Revised. Boston:
Houghton Mifflin Company.
Sennitt, Andrew G. 1988. World Radio TV Handbook. 1988 Edition.
Amsterdam, The Netherlands: Billboard A. G.
Rugg, Tom and Feldman, Phil. 1981. TRS-80 Color Computer
Programs. Radio Shack Book # 62-2313.