Orbit is the path described by any heavenly body with reference to the body which controls- it.
The orbits of all the members of the solar system are ellipses with the sun in one focus, and since we have every reason to believe in the universal application of the law of gravitation, we assume that every other heavenly body travels also in an elliptical orbit. When the ellipse is of enormous length, as in the case of a comet, the part of its.' path which we are able to investigate is indistinguishable from a parabola. The orbit with which we are most familiar is, of course, that of the earth, and the great circle in which this orbit cuts the celestial sphere is called the ecliptic. The positions of other orbits are usually defined with reference to . the ecliptic.