<?xml version="1.0" encoding="ISO-8859-1"?>

<screensaver name="epicycle" _label="Epicycle">

  <command arg="--root"/>

  <video href="https://www.youtube.com/watch?v=rpk3zxQxaR8"/>

  <number id="delay" type="slider" arg="--delay %"
          _label="Frame rate" _low-label="Low" _high-label="High"
          low="0" high="100000" default="20000"
          convert="invert"/>

  <number id="ncolors" type="slider" arg="--colors %"
            _label="Number of colors" _low-label="Two" _high-label="Many"
            low="1" high="255" default="100"/>

  <number id="holdtime" type="slider" arg="--holdtime %"
          _label="Linger" _low-label="1 second" _high-label="30 seconds"
          low="1" high="30" default="2"/>

  <hgroup>
  <number id="linewidth" type="spinbutton" arg="--linewidth %"
          _label="Line thickness" low="1" high="50" default="4"/>

  <number id="harmonics" type="spinbutton" arg="--harmonics %"
          _label="Harmonics" low="1" high="20" default="8"/>
  </hgroup>

  <boolean id="showfps" _label="Show frame rate" arg-set="--fps"/>

  <xscreensaver-updater />

  <_description>
A pre-heliocentric model of planetary motion.

This draws the path traced out by a point on the edge of a
circle.  That circle rotates around a point on the rim of another
circle, and so on, several times.

The geometry of epicycles was perfected by Hipparchus of Rhodes at some time
around 125 B.C., 185 years after the birth of Aristarchus of Samos, the
inventor of the heliocentric universe model.  Hipparchus applied epicycles to
the Sun and the Moon.  Ptolemy of Alexandria went on to apply them to what was
then the known universe, at around 150 A.D.  Copernicus went on to apply them
to the heliocentric model at the beginning of the sixteenth century.  Johannes
Kepler discovered that the planets actually move in elliptical orbits in about
1602.  The inverse-square law of gravity was suggested by Boulliau in 1645.
Isaac Newton's Principia Mathematica was published in 1687, and proved that
Kepler's laws derived from Newtonian gravitation.

https://en.wikipedia.org/wiki/Deferent_and_epicycle

Written by James Youngman; 1998.
  </_description>
</screensaver>
