Orbit, the path of a celestial body or other object in space, governed by the gravitational attraction of other bodies. An orbit, as the term is commonly used, is the relative path of one body with respect to another. For example, the orbits of most artificial earth satellites are said to be elliptical (like slightly flattened circles) since they are defined with respect to the earth. If these same orbits were defined with respect to the sun, they would be said to be spirals, because the earth moves in its path around the sun as the satellites move around it.
When two celestial bodies differ greatly in mass, as in the case of the sun and one of the planets, the smaller body travels around the larger one. When the two bodies have approximately the same mass, as in the case of some double stars, both bodies orbit around a common point.
The orbits of the planets are governed primarily by Kepler's laws of planetary motion. These laws are oversimplified to the extent that each planet is considered to be controlled only by the gravitational attraction of the sun. In reality, the orbit of each planet is influenced by the other planets to a small degree, as well as by the sun. The influences of the other planets are called perturbations, and must be taken into account in calculating planetary orbits.