From a teacher:
We are looking at student develop F=GMm/r^2 and T^2 proportional to r^3 based on data and hence the latter can be derived from the former. I do know one method where students can use a generic approach to find the relationship between 2 variables (see attached). I guess an applet where students develop F=GMm/r^2 is useful.

Possible lesson flow:

   * Divide group of 4 students to
         o interact with applet and develop procedure to find out the attractive force between 2 masses (1/2 hr)
         o manipulate data to obtain T^2 proportional r^3 (1/2)
   * Students to find out the way to relate the two formula through circular motion (20 mins)

Changes:
1 added t and realized t/2 is the actual time in years http://www.compadre.org/osp/items/detail.cfm?ID=9757&S=2
2 added r the instantaneous distance of planet center to Sun center to allow students to determine the mean distance.
3 fixed a bug with the Mars select radio because after UserDef and try to select mars again, it can't. to fix this bug, just add
[code]selectMars ();
mars=true;[/code]
to the Action Off.
4 made the radiobuttons planet no deselect = "true" so that the radiobutton looks selected even if you select it twice.
5 added pictures of real planet courtesy of Nasa to all planets,
6 fix/disable a code "_view.orbitDrawingPanel3D.setScaleFactor"  that does not work on Ejs4.3
[code]    // error in Ejs4.3
// _view.orbitDrawingPanel3D.setScaleFactor(scale,scale,scale);
[/code]
7 redraw the trace on 3D panel
8 add extra planets: uranus, neptune, pluto on both view
9 combine the a, e, i, \Omega, \omega into the bottom view
10 fixed the bug in evolution page  to [b][color=red]2*[/color][/b]Math.PI*(1+e*Math.cos(phi))*(1+e*Math.cos(phi))/Math.pow(a*(1-e*e),1.5) so that time is correct is years.


useful data
http://en.wikipedia.org/wiki/Planet