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===Astronomical significance=== {{See also|Ellipse#Planetary orbits}} In the [[gravitation]]al [[two-body problem]], the orbits of the two bodies about each other are described by two overlapping conic sections with one of the foci of one being coincident with one of the foci of the other at the [[center of mass]] ([[Barycentric coordinates (astronomy)|barycenter]]) of the two bodies. Thus, for instance, the [[minor planet]] [[Pluto]]'s largest [[natural satellite|moon]] [[Charon (moon)|Charon]] has an elliptical orbit which has one focus at the Pluto-Charon system's barycenter, which is a point that is in space between the two bodies; and Pluto also moves in an ellipse with one of its foci at that same barycenter between the bodies. Pluto's ellipse is entirely inside Charon's ellipse, as shown in [[Charon (moon)#Orbit|this animation]] of the system. By comparison, the Earth's [[Moon]] moves in an ellipse with one of its foci at the barycenter of the Moon and the [[Earth]], this barycenter being within the Earth itself, while the Earth (more precisely, its center) moves in an ellipse with one focus at that same barycenter within the Earth. The barycenter is about three-quarters of the distance from Earth's center to its surface. Moreover, the Pluto-Charon system moves in an ellipse around its barycenter with the [[Sun]], as does the Earth-Moon system (and every other planet-moon system or moonless planet in the [[Solar System]]). In both cases the barycenter is well within the body of the Sun. Two [[binary star]]s also move in ellipses sharing a focus at their barycenter; for an animation, see [[Binary star#Astrophysics|here]].
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