Celestial Navigation · Twilight — Civil Twilight
Why are star and planet sights normally taken during nautical twilight rather than full darkness?
- AThe almanac only tabulates positions during twilight
- BStars are brightest during civil twilight
- CRefraction is zero during twilight
- DBoth the horizon and the brighter stars are visible at the same time, allowing accurate sextant altitudes✓ Correct
Explanation
Accurate altitudes need a clear sea horizon and a visible body. During nautical twilight (sun 6°–12° below the horizon) the horizon is still distinct while the navigational stars and planets are visible, giving the short window for the star round. (Civil twilight is brighter but only the few brightest bodies show; astronomical twilight is too dark to see the horizon.)
Authority: Nautical Almanac, twilight tables; Bowditch (Pub. No. 9)
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