In the celestial sphere, what is the great circle that passes through the nadir and the south celestial pole called?

The great circle that passes through the nadir and the south celestial pole is known as the Observer's Meridian Circle. This circle is fundamental in celestial navigation and astronomy because it represents the path that the sun or celestial bodies take as they rise and set relative to an observer at a specific longitude.

The Observer's Meridian Circle is particularly significant as it is aligned with the north and south celestial poles, effectively dividing the celestial sphere into eastern and western halves. This division allows for the determination of the positions of celestial objects at specific times and locations on Earth. Additionally, on the celestial sphere, an observer's meridian circle helps establish local noon when the sun is at its highest point in the sky, which is a critical aspect in timekeeping and understanding solar movements.

Other terms in the context do not represent this specific great circle. For instance, the observer's horizon pertains to the boundary between the visible sky and the Earth terrain, while the observer's vertical circle includes all points at equal angles above and below the horizon but does not specifically relate to the celestial poles. The hour circle, while related to measurements of time and celestial positions, does not define the great circle through the nadir and the south celestial pole as clearly as the observer's meridian does.