ORBIT (from hat. orbita, track of a wheel. course. orbit. from orbis, circle). In astronomy, the path described in space by a heavenly body. The paths described by the different planet. are of elliptic form, and would be accurately an ellipse. were it not for distill-1611g influences of the other heavenly bodies known as perturbation; ( q.v.). The complete determinat ion of a planet's orbit is of the greatest importance to astronomers, as it enables them to predict the planet's place in the heavens at any period, and thus to determine the exact date of eclipses of the sun and moon. of transits aml occultations of the planets. For the determination of a planet's crldt. it is necessary to know three things: (I) 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 (q.v.) is for convenience t ikon as the reference plane. (he position of the plane of a planet's orbit is known when its inclination to the plane of the ecliptic (1) and the line of intersection of the two planes (21 are known. The orbit cannot lie wholly above or below the plane of the ecliptic. but must cut it in two points. called nodes and the position of the line of intersection, or line of nodes, is gen erally given in terms of the longitude of the as cending node. The situation of a planet's orbit
in its plane is determined when we know its shape (3), magnitude (4). and the position of its major axis or line of apsides (5). The shape and ina!mitude of the orbit depend upon the length of its major and minor axes, but astrono mers prefer to employ the major axis and eccen tricity (see Emrsn) : and the position of the major axis is known by determining the helio centric longitude of its pribetion. To com plete knowledge of a planet's motion. a 11 now require are the epoch of its appearance at sumo determinate point of its orbit. say at the perihelion (6), and the velocity of its motion in its orbit (7 ). for when this last is known, the law areas, as given in Kepler's ellabk, 11, to determine the position of the planet in its orbit at any future period. These seven facts. the possession of which gives us a com plete clue to a planet's motion, are called the seven 'elements' of a planet's orbit. What has been here stated concerning the planetary orbits is similarly true of the comets and satellites.
though in the ease of the latter the of dis• torbing forces is so great as to produce a con siderable change of the elements in one revolu tion. See Et.LNIENTs.