J-Moons! Documentation


J-Moons! Icon Computes the locations of the Galilean satellites of Jupiter for a user specified date and time.


  • Graphical output area that J-Moons! uses to display Jupiter and the Galilean satellites. This can be inverted for white on black display using the preferences menu.
  • Jupiter is represented by the largest round object in the graphical display.
  • A "floating" moon legend below the graphical output area. The C represents Callisto, the moon like object to the left of Jupiter in this example. The capital C indicates that the moon is between Earth and the rotational axis of Jupiter. Lowercase letters mean that the rotational axis of Jupiter is between us and the moon.
  • Tap a moon to identify it when the legend has multiple moons near to each other. When moons are tap identified a fact sheet is displayed containing facts about that particular moon.
  • Select dates using the PalmOS Date Selector.
  • Use the left triangle button for going back in time 1 day.
  • Use the right triangle button for going forward in time 1 day.
  • The display can be inverted on the "y-axis" so that left of the display is "East" or telescope view, or "West" which is binocular view.
  • The display can be inverted on the "x-axis" so that North is up or South is up. Using this and the previous feature, the view can be oriented to match that of your observing equipment.
  • Set or view the time being used for the graphical output.
  • Use Star Pilot's Location Manager to enter your location, Time Zone, and DST rules.
  • Set a user defined "step time" in decimal hours and watch how the moons change their location. Use the - (minus) button to step backwards in time, use the + (plus) button to go forward in time by the step value. You can also press the "down arrow" to select from a menu of predefined minutes of step values. Press the R trigger to get the program to automatically step forward or backwards for the selected step interval.
  • Press the Go button whenever you update the date or time using Grafitti and need to recalculate the position of the moons.
  • Use Now to display the current positions of the moons.
  • If Star Pilot is installed, the diamond button launches it when pressed. Likewise, in the lower right hand corner of the display, S will launch Sol! II, M will launch Moon Info!, and T will launch Sidereal Time.
  • Using the up and down hardware buttons, you can go backwards or forwards by date, or by the step value for time depending on your preference set in the preferences screen.

The Main Screen

J-Moons! Main screen

Other Features

  • Depending on the magnification power of your telescope, you may not want to see the "y-axis" calculations of Jupiter's Galilean satellites. Check Y Adj to turn "y-axis" calculations on or off. This is set on the preferences screen.
  • User settings (preferences) are saved between uses of the program.

Possible Future Plans

  • Develop a native version of this program on iOS devices and Android.
  • Color coding of the moons, and colorized application.


This program requires PalmOS version 2 or later.

J-Moons! requires MathLib.prc and Star Pilot's Location Manager to be installed before it can be used.

MathLib is a free shared library that can be used by any OS 2.0 Pilot program that needs IEEE 754 double precision math functions. It's distributed under the terms of the GNU General Public License, and is freely available with full source code and documentation at the MathLib Information web page at http://www.probe.net/~rhuebner/mathlib.html (deprecated). It's not a part of the CASL runtime programs, and you're not paying anything for its use; a copy is simply included in this archive for your convenience.
After installation, the following Icons will be installed on your Palm: J-Moons! Icon


When you run J-Moons!, the current date and time and the default location selected in Location Manager will be used to show the positions of the moons around Jupiter.

You can change the date by tapping the displayed date and selecting the month, day and year that you want your calculations to be made for. When you select a date, the moons positions are recalculated for the selected date, and whatever the time was set for. Enter a time using Graffiti in the field between the Now and Go buttons. Press the Go button to force a recalculation. The time should be entered in HH.MM 24 hour format. (For example 9:45pm is 21.45.)

Hours are assumed when you enter a time. For example, if you enter the time 16, J-Moons! converts that to 16.00. Likewise 4 is 4.00. To enter 42 minutes after midnight as a time, enter 0.42. Times can be only be entered as HH.MM format.

To get the position of the Galilean moons at the current time and date (using your current location) you can press the Now button.

Press the - (minus) button to decrement the selected time by the number of hours (in decimal) displayed in the Step field and redraw a new representation of the moons. You can enter fractional hours in the field, for example, 1.5 represents 1 hour 30 minutes. The default value, initially assigned when you first run the program is 0.5, or 1/2 hour. Press the + (plus) button to increment the time by the Step amount.

The program defaults to displaying the satellites with the Left of the display representing West and North is up. To flip the display so that Left is East, press the E button. To switch back to West, press the W button.

Changing the display so that North or South is up is as simple as pressing N to have North up, or S to have South up.

Just below the graphic display of the satellites, letters representing each moon will be display directly below each satellite. G of g is Ganymede, I or i is Io, E or e is Europa, and C or c is Callisto. The larger circular object near the center of the graph is Jupiter and is not labeled.

Sometimes the moons will overlap each other as seen from Earth. When this happens, the letters that represent the moons names will overlap. The program displays the moons and the letters in smaller orbit to larger orbit format, so when this happens you'll see the letter of the moon with largest overlapping orbit. If Callisto and Io are in the same position, you'll see the C but not the I. When a moon is behind or in front of Jupiter, you'll see it's letter below the Jupiter graphic.

Other Screens

Calculations Screen

The Calculations Screen shows the various values of different variables computed to display the current positions of the moons. These values are named identically to the ones used by Jean Meeus in his book Astronomical Formulae for Calculators, 4th Edition.

  • d - Number of days (including fractional days) since 2000 January 1 at 12h ET.
  • V - Long period term in the motion of Jupiter.
  • M - Mean anomaly of Earth.
  • N - Mean anomaly of Jupiter.
  • J - Difference between the mean heliocentric longitudes of Earth and Jupiter.
  • A - Equation of Center of Earth.
  • B - Equation of Center of Jupiter.
  • K - J + A - B.
  • u1 - u4 - Angle measured from the inferior conjunction with Jupiter.
  • R - Radius vector of the Earth.
  • r - Radius vector of Jupiter.
  • x1,y1 - x4,y4 - Location of each moon with respect to Jupiter at the center (0,0) coordinates and measured in radii of Jupiter.
All angle values are in degrees.

J-Moons! Computations screen

Release History

  • 2013 June 29
    • Documentation and website updated.
    • Program operation running on StyleTap confirmed.
  • 2004 March 7 Version 4.2.0 released.
    • Menu options to run applications, and quick launch button for launching Saturn! added.
    • New Launcher Icon for PalmOS 5+ devices.
    • Star Pilot Edition (available through http://www.star-pilot.com/
  • 2003 May 11 Version 4.1.1 released.
    • Fixed a minor internal issue.
  • 2003 April 14
    • Palm OS 5 Compatibility added.
    • The date is now displayed according to the user set preferences.
  • 2000 Oct 1 Version 4.0.0 released.
    • Astro Lib 1 and MEW3 Lib are no longer required.
  • 2000 May 29 Version 3.6.0 released.
    • Black and white, or Color Bitmaps (if you're using a Palm IIIc) are added to the main display at the top of the screen to allow quickly jumping to Moon Info! or Sol! II if they are installed.
    • For Palm OS 3.1 and higher devices, a new button T to the right of the date selector allows you to use the Time Picker to select a time in addition to being able to enter a time directly in the time field.
    • Contact information updated.
  • 2000 April 18 Version 3.5.1 released.
    • A problem on OS 3.0 to OS 3.3 when the application manager was configured to display applications as a "list" instead of as an "icon" would cause the Palm device to "Fatal Reset" when J-Moons! was loaded. This has been corrected.
    • The shareware release ZIP file didn't have the correct versions of MEW3_lib.prc and astrolib.prc contained in the archive.
  • 2000 April 13 Version 3.5.0 released.
    • For all palm devices with Palm OS 2.0 or higher, includes color icon for OS 3.5 devices.
  • 2000 March 30 Version 3.4.1c released.
    • For the Palm IIIc only, this version had a color icon.
  • 2000 March 14 Version 3.4.1 released.
    • A small Icon resource was added for Palm OS 3.x and later.
  • 2000 March 13 Version 3.4.0 released.
    • Animation was added so that you can have the program automatically step forwards or backwards through time showing the moon orientation.
    • Tapping a moon now takes you directly to it's fact sheet.
    • A bug was corrected that would cause user entered times to sometimes step backwards one minute when the "Go" button was pressed.
    • Bugs related to program startup were corrected. When components required for J-Moons! to work were not installed, the program would cause a fatal reset. Now the program offers you information about the component that is missing and how to get it.
  • 2000 March 8 Version 3.3.0 released.
    • Two new preferences were added.
      • Fast Calculation turns off Z axis calculation and cuts computation time in half. This means no moon will show a lower case letter in the floating moon legend.
      • Show Julian Date shows the calculated julian date on the main display.
    • 4 Fast find buttons were added to the upper left hand corner of the registered version of the product. Press the button to flash cross hairs on the moon of the same name.
    • Pen Up Detection bug was corrected so that the "find moon" dialog wouldn't pop up when first starting the application.
  • 2000 March 4 Version 3.2.0 released.
    • A new preferences menu option was added.
    • Y Adjustment Preference was moved off the main screen to the new preferences screen.
    • Ability to "invert" the graphical representation of the moons was added and is optional on the preferences screen.
    • The preferences screen now lets you decide how the hardware up and down buttons should be used, either as step control for adjusting time by the Step Value, or by going backwards or forwards by 1 day.
    • A pop up list of predefined minutes of Step Value was added so you can quickly select different step values.
    • Coding enhancements result in better animation when using the hardware up and down buttons to step the time or date values.
    • A bug was fixed that would cause a fatal reset when viewing Calculations if you used a hardware application or silkscreen application button to exit J-Moons!
  • 2000 March 2 Version 3.1.0 released.
    • When the legend is obscured because moons overlap or are close to each other, you can now tap the moon to identify it.
    • When a moon is identified by tapping, you can click on the "i" information button for detailed facts about the moon.
  • 28 February 2000 Version 3.0.0 released.
    • Program has been rewritten in C, and the CASL components are no longer necessary.
    • Program requires astrolib.prc, MEW3_lib.prc, MathLib.prc
    • Program requires Star Pilot's Location Manager
    • New View Orientation push buttons to accomodate displaying Jupiter's moons just like observed through any optical instrument. This design is totally new, but was inspired by the unreleased version 2.40 of J-Moons!
    • User Interface redesigned for use with Star Pilot's Location Manager.
  • Never released Version 2.40
    • Experimental "view orientation" button to accomodate displaying Jupiter's moons just like observed through any optical instrument.
  • 16 January 1999 Version 2.32 released.
    • Quick Launch button for Glen Aspeslagh's Sidereal Time application.
    • Reconfigured time entry so that 4 entered alone is 4 am, and not 4 minutes after midnight.
  • 7 November 1998 Version 2.2 released.
    • Optional modules for pop up date selection have been removed.
    • Latest CASL runtime modules are included in this release.
  • 4 September 1998 Version 2.1 released.
    • Fixed a bug in the North direction indicator which caused it to be reversed.
  • 7 August 1998 Version 2.0a released.
    • Fixed date selection problem when using MEW3date instead of CASL Hack which basically prevented the pop up date selector from working.
  • 5 August 1998 Version 2.0 released.
    • Support for CASL Hack added.
    • True Time Zone support added with optional full featured TimeZone Database module.
    • Y Adj feature added.
    • Added relative position of the moon, between Earth and Jupiter, or Jupiter between Earth and moon using upper and lowercase moon legends.
    • Added remove databases screen.
    • Incorporated Star Pilot interface for location information
    • Added Star Pilot button and Menu Option
    • Bug in DST calculation fixed.
    • Program now defaults to current date and time when started instead of last time updated.
    • Made several GUI changes to improve the appearance of the program.
  • 19 February 1998 Version 1.4 released.
    • Pop up date selector is now an optional "plug-in", using the same module as Sun!. This saves approximately 4k for the new version of J-Moons!
  • 8 February 1998 Version 1.3 released.
    • A Pop up date selector is now available for easily selecting dates, and you can still manually enter them as well.
    • Automatic DST adjustment has been added as an option.
  • 2 January 1998 Version 1.2, Shareware release.
    • A shareware version of the enhanced 1.2 release is now available!
  • 21 December 1997 Version 1.2.
    • You can flip the display so that Left is East or West by pressing the West/East button.
    • The program now saves the date and time between uses.
    • The program correctly defaults to NOW with a step value of 0.5 hours the first time it is used.
    • The date and time are now entered into two fields, instead of 5 making input of dates and time easier.
    • The CASLrt_pro.prc file included is updated and offers new features that improve the graphics performance of the program.
    • The ability to remove the database file was added so that it doesn't get left lying around in memory if you decide to remove J-Moons!
  • 14 December 1997 Version 1.1.
    • Jupiter is displayed closer to scale.
    • The program now saves UTC and Step between uses.
    • The moons were made larger so they're easier to see on the display.
  • 28 November 1997 Initial release.