Horizon definitions in celestial surveying: the circle formed by a plane tangent to Earth at the observer’s location intersecting the celestial sphere is known as which horizon?

Introduction / Context:
Multiple horizon definitions exist in astronomy and geodesy. Distinguishing them is essential for reducing observations and applying corrections.

Given Data / Assumptions:

• A plane tangent to Earth at the observer's point is considered.
• The celestial sphere is the imaginary sphere on which celestial bodies are projected.

Concept / Approach:
The sensible horizon is the small circle on the celestial sphere formed by the intersection of the observer's tangent (horizontal) plane with that sphere. The visible horizon is the apparent line where sky meets terrain/sea; the true (rational) horizon is an ideal plane through Earth's center and the observer's zenith–nadir line; the term celestial horizon is often used synonymously with the rational horizon, not the local tangent definition.

Step-by-Step Solution:
Construct the tangent plane at observer's location.Intersect that plane with the celestial sphere.Identify the resulting circle as the sensible horizon.

Verification / Alternative check:
Standard astronomical surveying texts define sensible horizon precisely in this manner.

Why Other Options Are Wrong:

• Visible horizon: depends on obstructions and refraction.
• Celestial/true horizon: refers to the rational horizon, not the local tangent circle.

Common Pitfalls:

• Interchanging visible and sensible horizons in reductions.

Final Answer:
sensible horizon