Eccentricity
Eccentricity measures how stretched an orbit is, from 0 for a perfect circle towards 1 for a long, thin ellipse.
Eccentricity describes the shape of an orbit. A value of 0 means a circle. Values between 0 and 1 mean an ellipse, growing more stretched as the number rises. Exactly 1 is a parabola, and values above 1 describe a hyperbola, an open path that never returns.
Earth's orbit has an eccentricity of about 0.017, almost circular. Mercury's is about 0.21. Many near-Earth asteroids have eccentricities between 0.3 and 0.7, which is why they cross the orbits of several planets. Halley's Comet has an eccentricity of about 0.97, so it swings from inside Venus's orbit to beyond Neptune's.
Eccentricity is how astronomers recognised interstellar objects. 1I/ʻOumuamua had an eccentricity of about 1.2, and 2I/Borisov about 3.4, both too large for any orbit bound to the Sun. They were passing through on their way between the stars.
Related terms
Level: Enthusiast Teens and adults