planets(1)

NAME
planets - Gravitational simulation of planetary bodies

DESCRIPTION
Planets is a simple interactive program for playing with simulations of planetary systems. It is great teaching tool for understanding how gravitation works on a planetary level. The user interface is aimed at being simple enough for a fairly young kid can get some joy of it. There's also a special kid-mode aimed at very young children which grabs the focus and converts key banging into lots of random planets.

KEYBINDINGS

  Universe definition

a   Add Planet

j   Place random orbital planet

r   Place random planet

u   Undo (undoes last planet insertion)

e   Reset to empty universe

g   Go Back (goes back to just after last planet insertion)

Mouse
    Click on a planet to delete it

  Physics

b   Toggle bounce (experimental)

  Display control

Cursor keys
    Panning

c, Space
    Move display to center of mass

x   Initiate center of mass tracking

=   Zoom in

-   Zoom out

p   Toggle Pause

o   Change all colors randomly

t   Toggle Trace

d   Double Trace Length

h   Halve Trace Length

Mouse
    Drag a box around a set of planets to follow the center of mass of those planets

  Program control

H   Display help dialog

k   Display option dialog

Ctrl-Shift-k
    Toggle kid-mode. Kid mode locks the keyboard and mouse, so the only way to get out is to toggle kid-mode again to get out.

l   Load Universe After pressing l, press any other character to load the universe with that name. Universes are stored in ~/.planets/ .

s   Save Universe After pressing s, press any other character to save the universe with that name. Universes are saved in ~/.planets/ .

q, Esc
    Quit

TECHNICAL DETAILS
Planets uses a fourth-order runge-kutta approximation for the simulation itself. Planet bouncing is achieved by adding a repulsive force to planets at close quarters. Planets is fairly flexible: you can change the gravitational constant, the time-slice of the simulation, and even the exponent used in the gravitational law. Universes are saved in the ~/.planets directory, and are simple human readable and editable files.

BUGS
Currently bouncing doesn't work very well unless you make the time-slice quite small. Ideally, it would be nice to have a billiard-style bounce system, but it's not clear how to do this accurately in the presence of a strong gravitational field.

AUTHOR
Planets was written by Yaron M. Minsky <yminsky@cs.cornell.edu> as a gift for his nephew, Eyal Minsky-Fenick. This manpage was contributed originally by Martin Pitt <martin@piware.de> for the Debian GNU/Linux system (but may be used by others).