in astronomy, the interval of time in which a celestial body completes one full revolution in its orbit, one of the elements of an orbit. Depending on the point chosen to mark the beginning of each revolution, various types of periods are distinguished. Two of these types are the nodical period, which marks the beginning of each revolution of the celestial body in its orbit as the ascending node, and the anomalistic period, which has as its starting point for bodies orbiting the sun the perihelion and for those orbiting the earth the perigee. Because orbits are continuously changing owing to perturbations, periods of revolution of the same body, measured from different starting points, may differ somewhat. In the case of an unperturbed orbit, all the periods are equal.