Science / Astrology / Nadir: The point in the celestial sphere directly opposite the zenith; the point directly beneath an observer on Earth; the low point.
Science / Astrology / Zenith: The point in the celestial sphere directly overhead; opposite the nadir. The zenith and nadir are the poles of the horizon. MORE
Science / Astrology / Prime Vertical: The great circle that rises vertically from the east point of the horizon and passes through Earth's zenith and nadir. MORE