Which great circle on the celestial sphere passes through the celestial poles and the observer's zenith?

• A. Celestial Equator
• B. Prime Vertical
• C. Horizon Circle
• D. Observer's Meridian

Answer: (D)

The main idea is the line of sight that passes through the zenith and the two celestial poles—the observer’s meridian. It’s defined by the local vertical plane that contains the observer and the Earth’s rotation axis (the line through the celestial poles). The intersection of that plane with the celestial sphere is a great circle, and it uniquely passes through both poles and the zenith.

The celestial equator is a different great circle that runs between opposite points on the sky and is 90 degrees from the poles, so it does not generally pass through the zenith. The prime vertical passes through the zenith but not through the celestial poles. The horizon forms a circle by a tangent plane to the sphere, which is not a great circle. Thus, the observer’s meridian is the one that passes through both celestial poles and the zenith.