Usage - Browser notes - Copyright notice - Source - History - Glossary of astronomical terms
Important note for Internet Explorer 6 users:
If you don't see any satellites around Jupiter when loading Jupiter Tool please try the following:
Select a place from the Places selection list. This should restore the display and make all controls working.
Don't forget to store the location by clicking 'Store'. Then IE6 should start properly next time.
The problem seems to be related to a bug in the IE6 javascript interpreter.
The error messages in IE6 are next to useless and the apparent position
of the javascript error according to IE6 makes no sense in relation to the symptoms.
Mozilla and Firefox do not show any problems, so consider switching to one of
these browsers if you can't make IE6 working.
Jupiter Tool is an application for calculating Jovian events and simulating the movements of the Galilean satellites and the Great Red Spot (GRS) on Jupiter. The longitudes of the central meridian of Jupiter's system I and II are also displayed (useful for observation of features on Jupiter).
Jupiter Tool is part of the Javascript Astrotools suite. The application is written entirely in javascript and executed by the Javascript interpreter of your web browser.
Jupiter is the largest planet of our solar system and a favourite target for observation by amateur astronomers. Jupiter is a gas giant without a solid surface and different parts of the planet rotate at slightly different speeds. Three rotational 'systems' are recognised. System I comprises the equatorial zone between the two equatorial belts and has a rotation period of 9 hours 50 minutes and 30 seconds. System II comprises the regions between the equatorial belts and the poles and rotates once in 9 hours 56 minutes and 41 seconds. There is also a third system rooted deeply inside Jupiter. This system is only detectable by its radio emissions and has no visual effects on Jupiter. The most famous feature of Jupiter is the Great Red Spot (GRS) which is a gigantic storm that has persisted for centuries. It is generally attached to System II but slowly drifts in longitude, see further below. Other well-known but less persistent features are white ovals and dark 'barges' in the equatorial belts. Observers of such features need to know the longitude of the Central Meridian of each system at the time of observation in order to track these features between different rotations.
The Galilean Moons
The four large satellites, Io, Europa, Ganymede and Callisto, were discovered by Galileo and therefore
often called the Galilean Moons. In Jupiter Tool they are labelled 1 (Io), 2 (Europa), 3 (Ganymede) and
4 (Callisto). The satellites and their movements from hour to hour is easily observed with standard
handheld binoculars.
When a satellite passes between the Sun and Jupiter it will cast a shadow on the surface
of Jupiter.
Such transits of satellite shadows on the disk of Jupiter are easy to
observe in most amateur telescopes and observing such events is a favoured
pastime for amateur astronomers. On the other hand observing the transits of the
satellites themselves is much more difficult due to the low contrast of
particularly Io and Europa against the surface of Jupiter. Ganymede and
Callisto are considably darker than Jupiter itself and transits of these moons
may be confused with shadow transits. The primary aim of Juptool is to
predict and simulate such events.
The user interface is an adapted version of the interface of AstroTools Ephemeris. Only parts specific to Jupiter Tool are discussed here.
At the top you see a window displaying Jupiter and visible satellites. North is up and east to
the left. At the lower left the current azimuth and altitude of Jupiter is shown together with the Sun's
altitude. At the lower right are shown the longitudes of the central meridian (CM) of system I and II.
Below you find the controls for setting the time, controlling zoom and animation etc.
Further down you find controls for generating a list of Jupiter events.
The bottom section of the interface comprises the controls for setting the
observer location and time zone (see Ephemeris Tool for information about changing your location or time zone).
Note that Jupiter Tool shares the observatory cookie with other components of Astrotools thus it will default
to the observatory location defined in Ephemeris Tool.
Time
Enter the desired date and time directly or use the time step buttons to step forwards or backwards in
time.
Zoom
You can choose between
a full-width view showing the complete orbits of all satellites or a zoomed-in view showing Jupiter
in greater details including the Great Red Spot. Press 'zoom in' and 'zoom out' buttons to switch between
these two views.
Animation
To animate the motion of the satellites and the GRS press either the forward or the backward 'Animate'
button. You can control the speed of the animation from the selection list to the right of the
animation buttons. Stop the animation by pressing STOP or any of the time step buttons.
Longitude of the GRS
The Great Red Spot is in general attached to Jupiter's System II but drifts slowly in longitude.
To show the GRS correctly enter the current longitude in the 'Long. of GRS' input field.
The default value is 94°. You can get
the latest known longitude of the GRS from 'Sky and Telescope' magazine or their
web site.
List events
By clicking this button you will get a list of satellite events within the near future. Select the
desired period from the selection list. You can also include crossings of the Great Red Spot by checking
the GRS checkbox. Events visible from the observer location are shown in bold
('Visibility' is, somewhat arbitrarily, chosen
to be when Jupiter is at least 3° above the horizon and the Sun at least 3° below the horizon).
Please notice that the event calculations are based on interpolation techniques
(for speed reasons) and the times indicated can be 3-4 minutes off those listed in accurate tables.
The simulated display is more precise than the calculated contact times because it is based on instantaneous calculated positions.
The times refer to the 'touching' of the center of the satellite to the limb of Jupiter. As an event really
starts when the limb of the satellite and Jupiter make contact the real starting or ending time may be
a few minutes earlier/later than the center contact time. The extreme situation is Callisto that may skim
the polar regions of Jupiter by less than the radius of the satellite
without ever being detected as a transit/occultation by Juptool.
In order to get a decent animation speed Jupiter Tool trades precision for speed. The calculations of the physical features of Jupiter (longitudes of System I and II etc) are based on the 'lower accuracy' method in chapter 43 of "Astronomical Algorithms", 2nd edition by Jean Meeus. The positions of the satellites and their shadows are based on the 'high accuracy' method in chapter 44 of 'Meeus' but with several 'short cuts' (truncations of terms etc) to speed up calculations. I believe that relative positions of satellites are accurate to within half an arcsecond or so. Longitudes of system I and II should be accurate to about 0.2°
Some aspects are not simulated. The 'planetocentric declination of Earth' (the tilt of the equator of Jupiter) is not shown (difficult to do that with Javascript graphics). Neither is the phase of Jupiter. The position of the center of a satellite (shadow) determines if it is shown as eclipsed or a shadow is visible. Thus the script may miss partial shadows at the limb and partly eclipsed satellites.
Jupiter Tool is demanding on the browser. It requires a current browser supporting at least javascript v1.3 and CSS2. It will fail on older browsers like Netscape 4 and IE5. It has been tested on Mozilla 1.7.3, Firefox 1.0PR and Internet Explorer 6.
You need a fast computer to get a smooth animation. A two GHz computer or better is recommended. On a sub-gigahertz machine the animation is quite jerky.
Astrotools and all its subcomponents like Jupiter Tool have been released under the GNU General Public Licence (GNU GPL).
Copyright (C) 2004-2005 Ole Nielsen
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
Please see gpl.txt for the full text of the GNU GPL.
Please also visit the Javascript Ephemeris web site of Peter Hayes. His site and javascript code has been the inspiration for my application. Although the majority of my code is new or completely rewritten some code fragments can still be traced back to his original code. Any code provided by Peter Hayes is copyright © Peter Hayes 1999-2001.
You can run Astrotools off-line from your own hard disk. Download astrotools.zip to your computer and unzip the archive to a folder of your choice. Open "astrotools2.html" in your browser to start AstroTools. Don't forget to bookmark this file in your browser for later use.
You may redistribute Astrotools under the terms of the GNU GPL (see above). You must include all files including the text of the GNU GPL (gpl.txt).
Home page: Ole's Astronomy Site
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