Astron. the arc of the ecliptic measured eastward from the vernal equinox to the point where the ecliptic is intersected by the great circle through the star, planet, etc. and the poles of the ecliptic
See celestial longitude in American Heritage Dictionary 4
The angular distance eastward from the vernal equinox along the ecliptic to the great circle drawn through the pole of the ecliptic and a celestial body.