Britannica
Sources used in this answer
How does orbital eccentricity change the shape of an ellipse?
Created from Britannica articles
using AI.
AI can make mistakes, so verify using Britannica articles.
Celestial mechanics
Ptolemaic diagram of a geocentric system, from the star atlas Harmonia Macrocosmica by the cartographer Andreas Cellarius, 1660.
In celestial mechanics, the eccentricity of an ellipse is a measure of how much the shape deviates from a perfect circle.12
The relationship between eccentricity and the shape of an orbit is defined as follows:
- Eccentricity of 0: The orbit is a perfect circle, where the semimajor axis (a) and semiminor axis (b) are equal.12
- Increasing Eccentricity: As the eccentricity increases toward 1, the orbit becomes increasingly elongated or "flattened". Mathematically, this corresponds to a greater distance between the two foci of the ellipse relative to the length of the major axis.123
- Physical Interpretation: Eccentricity quantifies the elongation of the path; for example, Earth’s low eccentricity of 0.0167 indicates an orbit that is nearly circular, whereas objects with higher eccentricities, such as certain comets or the dwarf planet Pluto, follow much more elongated paths.12
In essence, eccentricity describes the degree of departure from circularity, with higher values indicating a more stretched or elliptical orbital path.2