.TH planets 1 "April 20, 2003" 
.SH NAME
planets \- Gravitational simulation of planetary bodies
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.SH DESCRIPTION
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.B 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.
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.SH KEYBINDINGS
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.SS Universe definition
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.IP a
Add Planet
.IP j
Place random orbital planet
.IP r
Place random planet
.IP u
Undo (undoes last planet insertion)
.IP e
Reset to empty universe
.IP g
Go Back (goes back to just after last planet insertion)
.IP Mouse
Click on a planet to delete it
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.SS Physics
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.IP b
Toggle bounce (experimental)
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.SS Display control
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.TP
Cursor keys
Panning
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.TP 
c, Space
Move display to center of mass
.IP x
Initiate center of mass tracking
.IP =
Zoom in
.IP -
Zoom out
.IP p
Toggle Pause
.IP o
Change all colors randomly
.IP t
Toggle Trace
.IP d
Double Trace Length
.IP h
Halve Trace Length
.IP Mouse
Drag a box around a set of planets to follow the center of mass of
those planets
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.SS Program control
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.IP H
Display help dialog
.IP k
Display option dialog
.IP 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.
.IP l
Load Universe
After pressing l, press any other character to load the universe with
that name. Universes are stored in ~/.planets/ .
.IP s
Save Universe
.\"
After pressing s, press any other character to save the universe with
that name. Universes are saved in ~/.planets/ .
.TP 
q, Esc
Quit
.\"
.SH TECHNICAL DETAILS
.B 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.  
.B 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.
.SH 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.
.SH AUTHOR
.B 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).