Great circles on the celestial sphere – identify the one through zenith, nadir, and the poles Which great circle passes through the observer’s zenith, nadir, and the celestial poles?

Introduction / Context:
Great circles define fundamental reference planes on the celestial sphere. Recognizing which points each passes through is essential for interpreting transits, azimuths, and altitude measurements in astronomical surveying.

Given Data / Assumptions:

• Zenith and nadir are the upward and downward vertical directions.
• Celestial poles define Earth’s rotation axis on the celestial sphere.

Concept / Approach:
The observer’s meridian is the north–south great circle through the zenith and nadir that also intersects the celestial poles. While any vertical circle also passes through zenith and nadir, only the meridian additionally goes through the poles; the prime vertical goes through east and west points instead of the poles.

Step-by-Step Solution:
List defining points for each candidate.Meridian: zenith, nadir, and poles → matches the requirement.Prime vertical: zenith, nadir, east, west → does not include poles.General vertical circle: may not pass through poles.

Verification / Alternative check:
Standard celestial diagrams show the meridian orthogonal to the prime vertical, both passing through zenith and nadir; only the meridian includes the poles.

Why Other Options Are Wrong:

• Vertical circle (general) lacks poles.
• Prime vertical lacks poles (passes through E and W).
• “None” is invalid because the meridian satisfies the condition.

Common Pitfalls:
Confusing prime vertical with the meridian; assuming all vertical circles pass through poles (they do not).

Final Answer:
Meridian