Orbit

Orbit, in Astronomy, is the path described in space by a heavenly body in its revolution round its primary. The path so described is of an elliptic form, and would be accurately an ellipse were it not for the disturbing influence of the other heavenly bodies (see PERTURBATIONS). The complete determination of a planet's orbit is of the last importance to astronomers, as it enables them to predict the planet's place in the heavens at any period, and thus determine the exact date of eclipses of the sun and moon, of transits and occultations of the planets, and of the appearances and disappearances of comets. For the determination of a planet's orbit it is necessary to know three things: (1) The situation of the plane of the orbit in space; (2) the position of the orbit in this plane; and (3) the situation at a given epoch, and rate of motion, of the planet in its orbit. Since the plane of the ecliptic is for convenience taken as the reference plane, the position of the plane of a planet's orbit is known when (1) its inclination to the plane of the ecliptic and (2) the line of intersection of the two planes are known.