ORB, ORBIT. The word orbis signifies the circumference of a circle or of any round body ; orbs among the ancient astronomers meant the vast crystal spheres in which the heavenly bodies were supposed to be placed, and with which they revolved; hence orb came to be used for a sphere, as when the sun is called the orb of day. The word orbit now means the relative path [MoTioN] in which a planet travels round the sun, or a satellite round its primary.
We should perhaps rather say that by the orbit of a planet, technically speaking, is meant the approximate path, circular or elliptic, in which the planet may for a time be supposed to move without sensible error. Thus, when for some particular purpose, as the explanation of the seasons, we say the earth moves round the sun in a circle, we make an orbit which is exact enough for that purpose ; and when, to explain the equation of time, we are obliged to have recourse to a more exact supposition, that of an elliptic motion, we are still said to speak of an orbit of the earth. But if we were to speak of the closely-folded interlacing spiral in which the centre of the earth actually moves round the sun, usage (and nothing else) would require us, if we would be understood, not to call this the orbit of the earth, but its real path, or real orbit, or some such distinctive term; for it is generally understood that the word orbit applies at most to the ellipse, which for a time does not differ sensibly from the real path.
Theoretically speaking, this ellipse only touches the real path in one point ; and the ;ellipse which most exactly tends to coincidence with the real path, consistently with satisfying other desirable conditions, is called the instantaneous ellipse. [GRAVITATION.] The elements of the orbit are those quantities by which the position and magnitude of the (pro tempore) orbit are fixed, such as the major axis and eccentricity, which determine its magnitude ; the longitude of the node and inclination of its plane to the ecliptic, which determine the position of that plane in apace ; and the longitude of the perihelion, which determines the direction of the major axis. One more deter mining quantity is tacitly supplied in the condition that one focus of the ellipse is always to be in the centre of the sun. The only remaining element is the periodic time of the planet, or its time of revolution round the sun ; this however is not a separate clement for each planet, but, all the other elements being given, is known for every planet when it is known for any one.