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Lunar and Planetary Institute: Sky Tellers: Myths, Magic, Mysteries of Universe
Very nice website with excellent graphics depicting the change of the seasons and how the seasons are different between the northern and southern hemisphere. Also has a nice informational table of the spring and fall equinox and summer...
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Montana State University: Spring and Fall Equinox
This Montana State University site provides a simple chart on the spring and fall equinox and a description of the process.