Definition check: The altitude of a celestial body is its angular distance measured along the vertical circle through the body, above which fundamental plane?

Introduction / Context:
Precise terminology is vital in astronomical surveying. Altitude and azimuth form the local horizon system, while declination and right ascension form the equatorial system.

Given Data / Assumptions:

• Altitude is measured along the vertical circle passing through the body.
• Local horizon is taken as the reference for vertical angles.
• Angles increase upward from the horizon.

Concept / Approach:
Altitude (h) is defined as the angular distance of a celestial body above the observer’s horizon, measured along the vertical circle through the body. It is related to zenith distance z by z = 90° − h. The equatorial plane and ecliptic belong to different coordinate systems (equatorial and ecliptic) and are not the reference for altitude.

Step-by-Step Solution:
Identify the correct reference plane: the observer’s horizon.Recall the relationship h + z = 90°.Therefore, the correct option is “The visible horizon.”

Verification / Alternative check:
Instrumentally, altitude is what a theodolite or total station measures as vertical angle above the horizontal sight line.

Why Other Options Are Wrong:
Celestial equator / ecliptic / pole are references for different coordinate systems; they do not define altitude.“None of these” is incorrect because the horizon is explicit in the definition.

Common Pitfalls:
Mixing horizon-based and equator-based coordinates; always clarify the reference plane when handling angles.

Final Answer:
The visible horizon.