Editing Orbit (section)

==Planetary orbits==
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Within a [[planetary system]], planets, [[dwarf planet]]s, [[asteroid]]s and other [[minor planet]]s, [[comet]]s, and [[space debris]] orbit the system's [[Barycentric coordinates (astronomy)|barycenter]] in [[elliptical orbit]]s. A comet in a [[Parabolic trajectory|parabolic]] or [[Hyperbolic trajectory|hyperbolic]] orbit about a barycenter is not gravitationally bound to the star and therefore is not considered part of the star's planetary system. Bodies that are gravitationally bound to one of the planets in a planetary system, either [[natural satellite|natural]] or [[satellite|artificial satellites]], follow orbits about a barycenter near or within that planet.

Owing to mutual [[Perturbation (astronomy)|gravitational perturbations]], the [[eccentricity (orbit)|eccentricities]] of the planetary orbits vary over time. [[Mercury (planet)|Mercury]], the smallest planet in the Solar System, has the most eccentric orbit. At the present [[Epoch (astronomy)|epoch]], [[Mars]] has the next largest eccentricity while the smallest orbital eccentricities are seen with [[Venus]] and [[Neptune]].

As two objects orbit each other, the [[periapsis]] is that point at which the two objects are closest to each other and the [[apoapsis]] is that point at which they are the farthest. (More specific terms are used for specific bodies. For example, ''perigee'' and ''apogee'' are the lowest and highest parts of an orbit around Earth, while ''perihelion'' and ''aphelion'' are the closest and farthest points of an orbit around the Sun.)

In the case of planets orbiting a star, the mass of the star and all its satellites are calculated to be at a single point called the barycenter. The paths of all the star's satellites are elliptical orbits about that barycenter. Each satellite in that system will have its own elliptical orbit with the barycenter at one focal point of that ellipse. At any point along its orbit, any satellite will have a certain value of kinetic and potential energy with respect to the barycenter, and the sum of those two energies is a constant value at every point along its orbit. As a result, as a planet approaches [[periapsis]], the planet will increase in speed as its potential energy decreases; as a planet approaches [[apoapsis]], its velocity will decrease as its potential energy increases.