The line of the equinoxes is the imaginary diameter of the celestial sphere which joins them.
The ecliptic itself is the great circle in which this plane meets the celestial sphere.
It is also defined, but not with absolute rigour, as the apparent path described by the sun around the celestial sphere as the earth performs its.
The conception of the co-ordinates we have defined is facilitated by introducing that of the celestial sphere.
Taking as origin the position of an observer, the direction of a heavenly body is defined by the point in which he sees it in the sky; that is to say, on the celestial sphere.