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Orbits

IN SHORT - Most orbits, this path a celestial body is following around another, are of the elliptical type. The planets of the solar system are orbiting counterclockwise around the Sun as seen from the north pole of the ecliptic. Various data

allow to characterize an orbit, like its axis or its elongation relative to a circle -its eccentricity. Some points, like the nodes, or the apsides, are of interest too, as further technical notions are applied to the orbits

Orbit is the path a celestial body describes about another one. As far as solar system is concerned, it is Kepler who first studied scientifically orbits of solar system planets in the 17th century, and it is Newton which through gravity gave the fundamental reason to planets' motion. These theoretical concepts are always valid and may be generally applied to most of celestial objects

Most of celestial bodies' orbits have the form of an ellipse, some of a parabola or of an hyperbola. Most of solar system planets have elliptical orbits. An orbit may be counterclockwise (orbit is termed direct) or clockwise (retrograde) as seen from the north pole of the ecliptic. All planets of the solar system have a direct motion. An elliptical orbit is a section of cone and has two focii (instead of the one a circle has). An ellipse has two axis: the major axis which intersects the two focii; the minor axis, which is the perpendicular to the previous at the ellipse's center. The semimajor axis of an orbit is the average distance from the orbiting body to the central body. As name tells it is the half of the major axis. Ellipses are diversely elongated relative to a circle shape: elongation of an orbit is described as its eccentricity; it ranges from 0 (the orbit is circular) to > 1 (orbit is an hyperbola). 1 is for a parabola. An elliptical orbit has an eccentricity 0 > e < 1. Technically, eccentricity is the ratio of distance between the foci (d) of the ellipse and the length of the major axis (a): e = d / a. A parabolic orbit is performed when a body has reached what is called its "escape velocity" -the speed which is needed for an orbiting body to leave its orbit around the main body. An hyperbolic orbit occurs whent the orbiting body is moving faster than the escape velocity. An hyperbolic orbit is a more flattened curve than a parabolic one. Period is the amount of time it takes to the body to complete a revolution (revolution is the full length of the orbit)

orbit as ellipse, elongation, parabola, hyperbola

Orbits of celestial bodies are defined relatively to a plane of reference. In the solar system, this plane is Earth's orbit's. It is called the ecliptic. Inclination is the technical name for the orbit's plane tilt relative to the reference plane; it is the angle between the plane of the orbit of the body and the reference plane (more precisely between the body's axis and a perpendicular drawn from the reference plane). Nodes are points of the orbit where the body, along its course, crosses, up or down the plane of reference (ascending node is the point where the body crosses plane upwards; descending node is the point where the body crosses downwards). Line intersecting both nodes is called the line of nodes. As most orbits are ellipses, two other points are of importance: apsides, which are the points where the orbiting body is either closer to the central body (this points is said periapsis) or farther of it (apoapsis); for bodies orbiting the Sun, these points are said perihelion and aphelion (from the Greek "helios", Sun); for a body orbiting Earth, perigee and apogee are used. Due to Kepler's second law (equal areas are sweeped in equal amounts of time) a body moves swifter at its peripasis and slower at its apoapsis. Due to the general movement of orbits, lines of nodes and apsides are moving with time (for nodes it is said progression if in the same direction than the orbit, regression otherwise)

More technical data may be known about orbits:

orbit's semimajor axis is said "a"; eccentricity is said "e"; orbit's inclination relative to the reference plane is said "i"
for solar system bodies, "x" (which may be said too vernal point, "first point of the Aries", vernal equinox) is the line starting at the main body's center (Sun in case of the solar system) and pointing to the vernal equinox. For the solar system and any other frame of reference, x is part of a three-axis coordinate system applying to the orbit (y being the polar axis of the main body, z the third axis)
W, in the solar system, is the angle between x and the ascending node (or, the same, it is the right ascension, R.A. of the ascending node)
w is the argument of the perigee. The argument of the perigee is the angle from the ascending node to the perigee
n is the true anomaly. The true anomaly is the angle from the perigee to such or such position of the orbiting body