🔍 Key Concepts

• Atmospheric refraction and how it changes with altitude of a celestial body • How a light ray’s path through the atmosphere differs when the body is low vs high in the sky • Relationship between sextant altitude and size of refraction correction in sight reduction tables

💭 Think About

• Think about the thickness of the atmosphere that the light from the celestial body must pass through at different altitudes. When is that path longest? • Compare the typical size of refraction corrections listed in tables (like the Nautical Almanac) for low altitudes vs high altitudes. • Ask yourself: when does the apparent position of a star or the Sun differ the most from its true geometric position because of bending of light?

✅ Before You Answer

• Check how refraction values in the Nautical Almanac correction tables change as altitude increases from the horizon toward the zenith. • Verify that refraction is always negative for celestial navigation altitudes and note when its absolute value is largest. • Confirm which choice corresponds to the situation where the light passes through the maximum amount of atmosphere before reaching the observer.