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In celestial mechanics, a planet’s orbital speed is governed by its distance from the Sun. Closer to the Sun, a planet travels faster; farther away, it slows down. This principle applies to any elliptical orbit, where the planet’s speed peaks at perihelion and dips at aphelion.
While the Sun and each planet technically orbit one another, it is common—and sufficiently accurate—to model a planet as orbiting the Sun alone. As the planet moves along its path from perihelion to aphelion, the variation in distance determines how much its speed changes. The smaller the difference between perihelion and aphelion, the rounder the orbit, and the more constant the speed.
Eccentricity quantifies the "roundness" of an elliptical orbit: 0 represents a perfect circle, and values approaching 1 describe increasingly elongated shapes. A circular orbit would produce no speed variation, but all planetary orbits are slightly elliptical. Earth’s eccentricity is 0.017, ranking third lowest in the Solar System. Neptune follows with 0.011, while Venus boasts the lowest at 0.007.
Venus’s near‑circular path means its orbital speed remains remarkably steady compared to other planets. With the smallest eccentricity, Venus experiences the least change in speed throughout its year, making it the planet with the most uniform orbital velocity in our Solar System.
