Celestial Navigation · Twilight — Observation Window
Star and planet sights are taken during morning and evening twilight because that is when:
- ARefraction is exactly zero
- BBoth the sea horizon and the navigational stars are visible at the same time✓ Correct
- CThe stars are at their highest altitude
- DThe almanac only lists star positions then
Explanation
A sextant altitude needs a sharp sea horizon and a visible body. Only during twilight (best during nautical twilight) are both the horizon and the brighter stars simultaneously visible, giving a short window for a round of star sights.
Authority: Bowditch (Pub. No. 9), The Day's Work
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