The Vertex: where the prime vertical, the great circle through the zenith
and the east and west points, meets the ecliptic in the west. It is the
descendant of the colatitude, taken on the western side of the meridian. At
the equator the prime vertical is the celestial equator, and the Vertex is
the equinox west of the meridian. Where the ecliptic passes through the
zenith, the two circles meet on the meridian and neither point is west.
The Vertex: where the prime vertical, the great circle through the zenith and the east and west points, meets the ecliptic in the west. It is the descendant of the colatitude, taken on the western side of the meridian. At the equator the prime vertical is the celestial equator, and the Vertex is the equinox west of the meridian. Where the ecliptic passes through the zenith, the two circles meet on the meridian and neither point is west.