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Text File | 1992-02-15 | 28.5 KB | 357 lines

Moons of Jupiter Orbital Clock ── Version 1.00 ── 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 system. 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 3630 Europa 671,000 3d 13h 14m 3138 Ganymede 1,070,000 7d 3h 43m 5262 Callisto 1,883,000 16d 16h 32m 4800 And just for comparison, here's the same data for our moon. Moon 384,400 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 "JMOONS" in at your DOS prompt and press the enter key. After a brief blurb, the first screen to appear is where you respond to four inquiries so the program will know what you would liked displayed. 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. No dates earlier than January 1, 1801 (01-01-1801) will be accepted, nor will any dates later than December 31, 2200 (12-31-2200). These aren't just arbitrary dates, but a range where the program operates fairly efficiently. Dates outside this range produced unpredictable results. A quick shortcut around having to type in the current date all the time is to just press the "ENTER" key. 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 clock. 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 press the "ENTER" key, 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, the program does not affect the computer's internal date and time. (A few seconds may be lost internally while the program is run - a result of doing intensive graphics.) 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. (These 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 at the upper right above the Orbital Clock. The satellites are color-coded throughout the program for easy identification. Io will appear in red, Europa in blue, Ganymede in green, and Callisto in magenta (purple). 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 head-on view from Earth, but inclined up or declined down a bit. The Orbital Clock on your screen does not take that into consideration. It shows a direct head-on 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 3- or 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 below.) 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. Both the Orbital Clock and table of values will be automatically updated at the beginning of every minute as shown in the time display. ORBITAL GRAPHS When you press the "5" 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 several seconds 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. (Appearing in the color red.) The next sine wave that completes a period in about 3 and a half days is Europa's. (In blue.) The third is Ganymede's with a period of about 7 days (green), and the last is Callisto's (magenta) whose period is approximately 16.75 days and therefore one full period cannot be shown on this graph. 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 "3" 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 Jupiter. 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. This display shows a view of Jupiter and it's moons from an angle slightly above the head-on view 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. The moons in this window 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. Again, the moons are color-coded in the same scheme as above. This display is also updated at the beginning of every minute as shown in the time display in the upper right corner of the screen. Quick Reference to active keys The following keys may be pressed while the Orbital Clock is displayed: "3" = 3 Day Orbital Graph. "4" = Allows changes in Date, Time, and Time Zone. "5" = 15 Day Orbital Graph. "9" = Exits the program and returns you to DOS. 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. An EGA card and monitor is required for this program. 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 telescope! Moons of Jupiter Orbital Clock by: Andrew Jones JMOONS is written in BASIC and compiled using Microsoft's QuickBASIC 2.0 JMOONS.EXE and JMOONS.DOC are hereby declared public domain and may be freely copied and distributed. Comments and inquiries may be sent to me through the Starport Information Service or to Compuserve ID number 73767,557 or to: Andrew Jones Eagle Rock Village 4-6B Budd Lake, NJ 07828 Bibliography 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. U. S. Naval Observatory. 1987. Astronomical Almanac.