*(This page refers to eccitricity in astrodynamics. For other uses, see the disambiguation page eccentricity.)* In astrodynamics, under standard assumptions any orbit must be of conic section shape. The eccentricity of this conic section, the **orbit's eccentricity**, is an important parameter of the orbit that defines its absolute shape. Eccentricity may be interpreted as a measure of how much this shape deviates from a circle. Under standard assumptions **eccentricity** () is strictly defined for all circular, elliptic, parabolic and hyperbolic orbits and may take following values: ## Calculation
**Eccentricity** of an orbit can be calculated from orbital state vectors as a magnitude of eccentricity vector: where: For elliptic orbits it can also be calculated from distance at periapsis and apoapsis: where: ## Examples For example, the eccentricity of the Earth's orbit today is 0.0167. Through time, the eccentricity of the Earth's orbit slowly changes from nearly 0 to almost 0.05 as a result of gravitational attractions between the planets (see graph [1] (*http://www.museum.state.il.us/exhibits/ice_ages/eccentricity_graph.html*)). Other values: Pluto 0.2488 (largest value among the planets of the Solar System), Mercury 0.2056, Moon 0.0554. For the values for all planets in one table, see de:Planet (Tabelle).
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