**Kepler's First Law: ***The orbit of a planet is an ellipse with the san at one of the two foci*

**Kepler's Second Law: ***A line segment joining a planet and the sun sweeps out equal areas during the same time interval*

**Kepler's Third Law:** *The square of the orbital period, T, of a planet is directly proportional to the cube of its average distance r from the sun *

T² ∝ r³

**Modeling Planetary orbits as circles**

centripetal force = gravitational force on planet

mv² / r = GMm / r²

v² = GM / r

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