🔍 Key Concepts

• Refraction of light at very low altitudes • How sextant accuracy changes at small angles (near the horizon) • What modern sight reduction tables typically cover for altitude ranges

💭 Think About

• When the observed body (like the sun) is very low, what happens to the light as it passes through more atmosphere? • Is the main concern optical instrument limitations, table limitations, or atmospheric effects? • Which option describes something that can be especially unusual or unreliable when the body is close to the horizon?

✅ Before You Answer

• Compare which options involve instrument limitations versus atmospheric effects. • Ask yourself which condition becomes significantly worse as the altitude approaches the horizon. • Consider what typical sight reduction tables (like HO 229/249) say about minimum usable altitude and why that limit exists.