Orbit

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In Physics, an Orbit is the gravitationally curved path of one object around a point or another body, for example the gravitational orbit of a Planet around a Star.

[edit] Planetary orbits

Within a Planetary system; Planets, Dwarf planets, Asteroids (a.k.a. minor planets), Comets, and Space debris orbit the central Star in elliptical orbits. A comet in a parabolic or hyperbolic orbit about a central star is not gravitationally bound to the star and therefore is not considered part of the star's planetary system. To date, no comet has been observed in our Solar System with a distinctly hyperbolic orbit. Bodies which are gravitationally bound to one of the planets in a planetary system, either natural or artificial satellites, follow orbits about that planet.

Owing to mutual gravitational perturbations, the eccentricities of the orbits of the planets in our Solar System vary over time. Mercury, the smallest planet in the Solar System, has the most eccentric orbit. At the present epoch, Mars has the next largest eccentricity while the smallest eccentricities are those of the orbits of 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 from each other. (More specific terms are used for specific bodies. For example, perigee and apogee are the lowest and highest parts of an Earth orbit, respectively.)

In the elliptical orbit, the center of mass of the orbiting-orbited system will sit at one focus of both Orbits, with nothing present at the other focus. As a Planet approaches periapsis, the planet will increase in speed, or velocity. As a planet approaches apoapsis, the Planet will decrease in velocity.