Sky science
Orbital Eccentricity
Direct answer
Orbital Eccentricity is a dimensionless measure of how much a conic orbit departs from a circle.
It quantifies orbital shape from circular to open trajectories.
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At a glance
- Found in
- Astronomy & Skywatching
- Related
- Orbital Elements, Semimajor Axis, Orbital Inclination, Longitude of Ascending Node
It quantifies orbital shape from circular to open trajectories.
A scientific orientation to scale, light, or motion; the article supplies the precise definition.
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The story behind Orbital Eccentricity
Ellipse geometry supplied eccentricity before Newton explained orbital gravitation.
A one-minute story
The Wandering Stars
How the moving planets were watched, named and found, and what a transit is.
Why Orbital Eccentricity matters
Orbital Eccentricity distinguishes nearly circular planetary orbits from elongated cometary paths.
How to read Orbital Eccentricity
Interpret eccentricity with the conic type and current element epoch.
Orbital Eccentricity in practice: an example
An ellipse has eccentricity between zero and one; a parabola has one.
Where readings of Orbital Eccentricity go wrong
Eccentricity alone does not give orbit size, orientation, or current position.
Sources and editorial method
Research references used for this entry include:
- NASA Science: Orbits and Kepler’s Laws - NASA explanation of orbital ellipses, orbital periods, and Kepler’s laws of planetary motion.
- U.S. Naval Observatory: Astronomical Almanac Glossary - Definitions for astronomical coordinates, time, reference systems, and observable phenomena.
- Source
- The Great Almanac Dictionary
- Category
- Astronomy & Skywatching
- Last updated
- 2026-09-26
Compare circular and elongated orbits
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Common questions
What is Orbital Eccentricity?
Orbital Eccentricity is a dimensionless measure of how much a conic orbit departs from a circle.
What do people get wrong about Orbital Eccentricity?
Eccentricity alone does not give orbit size, orientation, or current position.
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