Credit: National Air and Space Museum, Smithsonian Institution
Kepler’s First Law describes the shape of an orbit. The orbit of a planet around the Sun (or of a satellite around a planet) is not a perfect circle. It is an ellipse—a “flattened” circle. The Sun (or the center of the planet) occupies one focus of the ellipse. (A focus is one of the two internal points that help determine the shape of an ellipse. The distance from one focus to any point on the ellipse and then back to the second focus is always the same.)
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Flight Dynamics