What is the point on the celestial sphere that is diametrically opposite to the zenith called?

The point on the celestial sphere that is diametrically opposite to the zenith is known as the nadir. The zenith is the point in the sky directly above an observer, while the nadir is the point directly below them, making it the lowest point on the celestial sphere. Understanding this relationship between zenith and nadir is essential in fields such as astronomy, navigation, and surveying, as it helps to define a complete understanding of an observer's position in relation to the celestial sphere.

The other terms relate to different concepts: the equator refers to the imaginary line that divides the Earth into the Northern and Southern Hemispheres, the horizon is the boundary where the Earth meets the sky as perceived by the observer, and the meridian is an imaginary line that runs from the North Pole to the South Pole, marking the position of both humans and celestial bodies in a specific longitude. Hence, while these terms are valuable in their contexts, only the term nadir specifically identifies the point directly oppositional to the zenith on the celestial sphere.