Which term denotes the great circle that passes through the nadir and the south celestial pole?

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

Answer: (B)

Think about the celestial sphere from the observer’s location. A great circle is formed by a plane that passes through the center of the sphere. The circle that naturally runs from zenith to nadir and through both celestial poles is the observer’s meridian circle. If you require the circle to pass specifically through the nadir and the south celestial pole, you’re selecting the southern meridian, which is still the meridian circle by definition.

The horizon is a small circle—the intersection of the horizontal plane with the celestial sphere—not a great circle, so it doesn’t pass through the poles. The vertical circle is a circle in a vertical plane through the zenith and nadir but not defined by passing through a celestial pole. The hour circle relates to the position of a particular object at a fixed hour angle and is not characterized simply by nadir and a specific pole.