What does the term eccentricity refer to in relation to orbits?

Eccentricity is a key parameter in the study of orbital mechanics, specifically related to the shape of orbits. When discussing orbits, particularly those that are elliptical—such as the orbits of planets around the Sun—eccentricity quantifies how much an orbit deviates from being circular.

An elliptical orbit can range from being nearly circular (with an eccentricity close to 0) to highly elongated (with an eccentricity approaching 1). The precise value of eccentricity captures the degree of this elongation. For example, a perfect circle has an eccentricity of 0, while the more elongated the ellipse, the higher the value of eccentricity, indicating that the planet's distance from the Sun changes significantly over the course of its orbit.

Understanding eccentricity helps astronomers predict various orbital characteristics, such as how the speed of a planet changes as it moves closer to or further away from the Sun during its orbit, as well as the potential impact of gravitational interactions with other celestial bodies.