Concepts
Orbital Eccentricity
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Orbital eccentricity is a dimensionless parameter describing how much an orbit's shape deviates from a perfect circle. A value of 0 is a circular orbit, values between 0 and 1 describe an ellipse, exactly 1 describes a parabolic escape or capture trajectory, and values greater than 1 describe a hyperbolic trajectory that will never return. It is one of the six classical orbital elements, alongside inclination, and follows directly from Johannes Kepler's description of planetary orbits as ellipses.
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Concept Domain Proposed ByKepler's first law, published in Astronomia Nova (1609), established that planetary orbits are ellipses rather than circles, which is what eccentricity quantifies. Cross-Tradition Connections
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