What is the term for the point directly below an observer in the celestial sphere?

The term that refers to the point directly below an observer in the celestial sphere is "nadir." In the context of celestial navigation and astronomy, the celestial sphere is an imaginary sphere that encompasses the Earth, and it is used as a model to visualize the positions of celestial objects.

The nadir is the point on this sphere that is diametrically opposite to the zenith, which is directly above an observer. Understanding the concept of the nadir is important in various fields such as geodesy, navigation, and astronomy, as it helps in locating objects or coordinates in relation to an observer’s position.

The other terms listed represent different celestial concepts. The zenith, for example, is directly overhead. The North Celestial Pole is the point in the sky that aligns with the Earth's North Pole, and the First Point of Aries is a specific point in the celestial coordinate system marking the start of the astrological zodiac.