On the celestial sphere used in astronomy and surveying, what is the name of the point that lies directly underneath the observer (i.e., vertically below the observer's position)?

Introduction / Context:
The celestial sphere model identifies two special points relative to an observer: the zenith and the nadir. Correct identification of these terms is essential for celestial observations and astronomical surveying.

Given Data / Assumptions:

• Observer stands at a point on Earth.
• Vertical line through the observer defines up (zenith) and down (nadir).
• Other options are standard astronomical terms.

Concept / Approach:
The zenith is the point on the celestial sphere directly overhead; the nadir is the point directly beneath the observer. Celestial poles are the projections of Earth’s rotation axis; equinoctial points are where the ecliptic crosses the celestial equator.

Step-by-Step Solution:
Identify the definition that matches “vertically below the observer”.Nadir exactly matches that description.

Verification / Alternative check:
Draw the local vertical line; the upper intersection with the celestial sphere is zenith, the lower is nadir.

Why Other Options Are Wrong:
Zenith is overhead, not below.Celestial pole and equinoctial points are unrelated to the observer’s vertical direction.

Common Pitfalls:
Confusing zenith and nadir due to their opposite meanings along the local vertical.

Final Answer:
Nadir