Planets For Linux

Planets is a simple interactive program for playing with simulations of planetary systems, released under the GPL. The project runs on Linux and Windows, and could doubtless be ported to your favorite flavor of Unix.

Planets was originally designed for kids, in particular, for my then 4-year old nephew who is fascinated by astronomy. The user interface is aimed at being simple enough that a fairly young kid can get some joy out of it. But the adults who have used it have found it to be pretty fun as well.

The code is not bug-free, and Planets is missing some significant features. But it's pretty stable and is a fun toy to play with. If you do download it, please drop me an email and tell me about your experience with it.

Here are some key features of "Planets":

� Saving and loading of universes � Infinite undo (erase last action) and goback (return to point in time just after last action). This allows for undoing mistakes and replaying interesting configurations. � Traces of planet trajectories � Two ways of dealing with planet collisions: - merges, where the colliding planets are merged into one planet, and - bounces, where the colliding planets are bounced off each other elastically. This itself comes in two varieties: - force bouncing, where the force between planets is made repulsive at close quarters. - true bouncing, where simple pool-table physics calculations are made to determine when planets collide, and compute the appropriate bounce from said collision. � kidmode, a mode where the focus is (mostly) locked on the application, and interesting changes are initiated by merely banging on the keyboard. This mode is aimed at 1-5 year olds. � Center-of-mass following: it is possible to follow the center of mass of a subset of the planets. Thus, if you have a sun-moon-planet system, you can have the view automatically track the moon-planet pair. � Can display kinetic, potential and total energy of the system. � Both the gravitational constant and the gravitational exponent can be changed. � There is a simple control panel that makes it possible to see and change the simulation options. � Zooming, panning, and centering on the center of mass.

Requirements:

� A working installation of ocaml 3.08 or later. � Tcl/Tk version 8.3

Limitations:

� The simulation technique is a simple and inaccurate first order approximation. It shouldn't be too hard to incorporate a higher-order approximation, and eventually it might be nice to have an adaptive simulation that increased the number of simulation rounds when accuracy is detectable at stake. This should enable both better accuracy, and increase the scalability of the simulation. � Force-bouncing does not work for all values of the gravitational constant or for different gravitational exponents. � Energy is not particularly well conserved. This will likely be improved by improving the simulation technique � When using true bouncing, random planets may be placed so that they overlap, which allows the planets to get arbitrarily close, and leads to planetary motions that expose the poorness of the underlying simulation techniques.

What's New in This Release:

� Traces were fixed so that they no longer fail with recent versions of Tcl/Tk.