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orbital element
subject			fact

orbital element	is required to define the size, shape, and orientation of an orbit in space
	see also mean element
	see also osculating element
	has definition One of seven quantities that must be established from observations in order to define the size, shape, and orientation of an orbit in space.
	is a kind of orbital quantity
quantity	has unit
number	has value

Kinds of orbital element :

• argument of the perihelion (6 facts) (ω, longitude of the perihelion) - An orbital element representing the longitude of the ascending node plus the angle along the orbit from the ascending node to the perihelion point., Angular distance (measured in the plane of the object's orbit and in the direction of its motion) from the ascending node to the perihelion point.
• eccentricity (7 facts) (e) - An orbital element representing the eccentricity of the elliptical orbit, In astronomy, the extent to which an elliptical orbit departs from a circular one. It is usually expressed as a decimal fraction, regarding a circle as having an eccentricity of 0., A measure of how round or elliptical an orbit is. A perfect circle has an eccentricity of 0 percent, and an extremely elliptical orbit has an eccentricity of just under 100 percent. The Sun has an orbital eccentricity of 6 percent, which means that at perigalacticon the Sun is 6 percent closer to the Galactic center than its mean distance and at apogalacticon the Sun is 6 percent farther from the Galactic center than its mean distance., A parameter that specifies the shape of a conic section; one of the standard elements used to describe an elliptic orbit. (See elements, orbital.), The amount by which the elliptical orbit deviates from circularity: e = c/a, where c is the distance from the center to a focus and a is the semimajor axis.
• epoch (7 facts) (T) - An orbital element representing the time of perihelion passage., A point of time selected as a fixed reference., An arbitrary fixed instant of time or date used as a chronological reference datum for calendars (see calendar), celestial reference systems, star catalogs, or orbital motions (see orbit)., A date and time that specifies the reference system to which celestial coordinates are referred. Prior to 1984 coordinates of star catalogs were commonly referred to the mean equator and equinox of the beginning of a Besselian year. Beginning with 1984 the Julian year has been used, as denoted by the prefix J, e.g., J2000.0.
• inclination (6 facts) (i) - An orbital element representing the inclination of the object's orbital plane to the ecliptic, The angle between one plane and another. The (equatorial) inclination of a planet is the angle between the plane of its equator and that of its orbit. The inclination of the orbit of a planet in the Solar System other than Earth is the angle between the plane of that orbit and the ecliptic., The angle between two planes or their poles; usually the angle between an orbital plane and a reference plane.
• longitude of the ascending node (4 facts) (Ω) - An orbital element representing the point where the body crosses the ecliptic from south to north
• period (4 facts) (P) - An orbital element representing the time required to complete an orbit. This parameter is required when determining the orbit of a binary star system in which the mass is not known.
• semimajor axis (4 facts) (a) - An orbital element representing half the length of the major axis of an ellipse.