planets

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).