Kepler’s First Two Laws

• PATH: A planet travels in an orbit which is an ellipse with the Sun at one focus:

• SPEED: A planet travels at such a rate that the radius vector (sun to planet) sweeps out equal areas in equal times.

F

F

a = semi-major axis

b = semi-minor axis

a

b

The sum of the distances from two “foci” F to a point on an ellipse is a constant.

t1

t2

t3

t4

A12

A34

A12 = A34

if t2-t1 = t4-t3

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