Atmospheric refraction corrections: which statements correctly describe how refraction corrections vary with the altitude of a celestial body?
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ARefraction correction is zero when the celestial body is at the zenith
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BRefraction correction is about 33 minutes when the celestial body is at the horizon
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CRefraction correction depends on the altitude of the celestial body
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Dall the above
Answer
Correct Answer: all the above
Explanation
Introduction / Context:Atmospheric refraction bends incoming light, shifting the apparent altitude of celestial bodies. Accurate astronomical observations require altitude-dependent refraction corrections.
Given Data / Assumptions:
- Standard atmosphere; observations near horizon are most affected.
- Altitude measured from the sensible/visible horizon upward.
Concept / Approach:Refraction increases sharply as altitude approaches the horizon and diminishes to negligible at the zenith. Practical tables show approximately 33 arcminutes near the horizon and nearly zero directly overhead; in general, correction follows functions proportional to cotangent of altitude.
Step-by-Step Solution:At zenith: path through atmosphere is minimal → correction ≈ 0.At horizon: longest path → correction ≈ 33′.Between these, use altitude-based tables or formulae to interpolate corrections.
Verification / Alternative check:Refraction tables in nautical almanacs/survey manuals confirm the quoted limiting values and altitude dependence.
Why Other Options Are Wrong:
- Each individual statement is correct; thus the combined choice is appropriate.
Common Pitfalls:
- Neglecting refraction at low altitudes, leading to large altitude errors.
Final Answer:all the above