Orbital Eccentricity
Orbital eccentricity measures how much an orbit departs from a circle, shaping distances, speeds, and sunlight patterns along a path.
What orbital eccentricity is
Orbital eccentricity is a number that describes the shape of an orbit. A value of 0 means the path is a perfect circle. Values greater than 0 and less than 1 describe ellipses, with larger values meaning a more stretched path. Astronomers use eccentricity because it captures an important part of an orbit without needing a drawing: how strongly the distance between two bodies changes during each revolution.
Circles, ellipses, and focus
Most bound orbits are ellipses rather than exact circles. In an ellipse, the central body sits at one focus, not at the geometric center. This is why a planet, moon, comet, or spacecraft can be closer to the body it orbits at one part of its path and farther away at another. Eccentricity connects the focus position to the size of the ellipse, so it is part of the standard language of orbital elements.
Periapsis and apoapsis
The nearest point in an orbit is called periapsis, with special names such as perihelion around the Sun and perigee around Earth. The farthest point is called apoapsis, including aphelion and apogee. Higher eccentricity usually widens the gap between these points. That difference matters for spacecraft planning, satellite coverage, thermal design, and the brightness or visibility of objects in the sky.
Speed changes along an orbit
Objects in elliptical orbits do not move at one constant speed. Kepler's second law says an orbiting body sweeps out equal areas in equal times, which means it moves faster when it is close to the central mass and slower when it is farther away. Eccentricity therefore affects timing as well as shape, because a high-eccentricity orbit can spend a long time near apoapsis and a short, fast interval near periapsis.
Planets and satellites
Planetary orbits in the Solar System are elliptical, but many are close to circular. Artificial satellites can be placed into low-eccentricity or high-eccentricity orbits depending on their mission. Circular and near-circular orbits are useful for steady altitude and repeated coverage, while elongated orbits can dwell over particular regions or reach high altitudes without circularizing the whole path.
Comets and high eccentricity
Comets often have much higher eccentricities than planets. A comet in a very elongated orbit may travel from the cold outer Solar System to a close solar approach and back again. Some paths are not closed ellipses at all: parabolic and hyperbolic trajectories describe objects moving at escape speed or faster, so they pass through rather than remaining permanently bound.
Climate and Milankovitch cycles
Earth's eccentricity changes slowly over long timescales as gravitational interactions with other planets reshape its orbit. Those changes are one part of the Milankovitch cycles, alongside axial tilt and precession. Eccentricity does not cause the seasons by itself, but it can change how strongly seasonal sunlight patterns differ between hemispheres when combined with the timing of perihelion and aphelion.
Why it matters
Eccentricity is a compact idea with wide reach. It helps explain planetary motion, comet behavior, satellite design, exoplanet climates, and long-term changes in Earth's orbit. Because orbit diagrams are often exaggerated for clarity, the number itself is often more trustworthy than a picture: two orbits can look dramatically different on a page while their actual eccentricities are modest.