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Celestial mechanics are, of course, the reason for seasonal variation. To
get a better idea of the interplay of planetary orbits and the Sun, try playing
with the Java applets at
http://alpha.fesg.tu-muenchen.de/dieter/java/methods.html.
Some are just spreadsheets but others are simulations
of the Solar System showing orbits in different epochs. For a more
straightforward tour around the Solar System see
http://www.magicnet. net/~jdk/ssframes.html.