Kepler's Laws of Planetary Motion

1. Each planet moves in an elliptical orbit with the Sun at one focus.

That's fine. But what is an "ellipse" -- and what is a "focus" of an ellipse?

You can draw an ellipse by placing two thumb tacks into a pad of paper and tying a string to them. Then use a pen to draw a "curve" by keeping the string taute or tight. This means the sum of the distances from the two thumb tacks is constant. This "curve" you will draw is an ellipse and the two thumb tacks are at the two focuses (or foci) of the ellipse!

Kepler's Laws

Kepler's Laws

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(C) 2003, Doug Davis; all rights reserved