Science / Weather / Nadir: The point on any given observer's celestial sphere diametrically opposite of one's zenith.
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