What is the name of the great circle that passes through the nadir and the south celestial pole?

• A. Hour Circle
• B. Observer's Horizon
• C. Observer's Meridian Circle
• D. Observer's Vertical Circle

Answer: (C)

The name you’re looking for is tied to the plane that contains the observer and the celestial poles. The meridian plane is a vertical plane that passes through the zenith, nadir, and both celestial poles. When you slice the celestial sphere with that plane, you get a great circle—the observer’s meridian circle—that includes the zenith, nadir, and both poles.

Since the nadir lies opposite the zenith along that same vertical plane, it lies on the meridian circle, and the south celestial pole, being one of the celestial poles, also lies on that same circle. So this great circle naturally passes through both the nadir and the south celestial pole.

Other circles don’t share this exact property: an hour circle is defined for a specific celestial object by its hour angle and isn’t fixed to pass through nadir and a pole; the horizon circle comes from the horizontal plane and doesn’t generally include the nadir or the poles; a vertical circle depends on a chosen azimuth and isn’t guaranteed to contain both poles.