The horizon's pole, mathematically. The Prime Vertical (q.v.) and Meridian circles pass through this location straight overhead. Every location has its own zenith, and the closer a planet is to it, the greater its effect. The cusp of the Tenth House, which is technically speaking merely the point of the zodiac or ecliptic through which the meridian circle passes, is frequently referred to as the term. The Celestial Sphere is the fifth of the seven celestial spheres.
