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Loading contentThe small departures of a real orbit from a perfect two-body ellipse — caused by the pull of other bodies, a planet's equatorial bulge, atmospheric drag, and radiation. They accumulate over time, which is why orbits must be tracked and recomputed continually.
Facts on this topic will be cited from these primary and reference sources.
Orbital data, ephemerides, and small-body parameters for planets, asteroids, and comets.