In the celestial sphere, which term refers to the point directly beneath the observer?

• A. Nadir
• B. Zenith
• C. First Point of Aries
• D. North Celestial Pole

Answer: (A)

The key idea is that the celestial sphere uses opposite points for directions through the observer. The point directly above you is the zenith, and the point directly beneath you is the nadir. So the nadir is the location on the celestial sphere that lies opposite the zenith, directly below the observer. The other terms refer to different things: the zenith is overhead, the First Point of Aries is a reference position on the celestial equator used for coordinates, and the North Celestial Pole is the point in the sky around which northern stars appear to rotate.