Clever guy that I am, I am writing a small paper on Celestial Mechanics...simplified...really really simplified. However, if I bore a tunnel through the Earth and drop a watermelon through it, the watermelon returns to my hand in 76 minutes. This is an example of an elliptical orbit of e=1 But if I use the same figures and shoot the watermelon around the Earth in a grazing orbit, with eccentricity of zero, the orbital period is 6 minutes longer. Why? BTW: I suspect any minimum energy subway tunnel of hypocycloidal shape has 1/2 the transit time of the watermelon, or 38 minutes, or 76 minutes round trip. A hypocycloid where the generating circle diameter is the same as the base circle radius generates a straight line through the center. Comments?