Kepler’s Laws, #3
•1619  Publishes third law, showing that there is a relationship orbital period and semi-major axis:
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•Exact relationship is P2 µ a3 .
–Outer planets orbit more slowly than inner ones
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•Example:  Earth  P = 365 days, a = 1.00  AU.
 Mars   p = 687 days, a = 1.524  AU  
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