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Man Pages


Manual Reference Pages  -  PLANETS (1)

NAME

planets - Gravitational simulation of planetary bodies

CONTENTS

Description
Keybindings
     Universe definition
     Physics
     Display control
     Program control
Technical Details
Bugs
Author

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).

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--> PLANETS (1) April 20, 2003

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