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

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

Answer: (C)

In the celestial sphere, every direction from you has an opposite point on the sphere. The point directly overhead is called the zenith. The point directly below you, on the opposite side of the sphere, is the nadir. The north celestial pole marks a fixed direction in the sky related to Earth's axis, not your vertical position, and the First Point of Aries is a reference point used for celestial coordinates rather than a vertical position. So nadir is the correct term because it is the antipode of the zenith, lying directly beneath the observer.