Celestial Navigation · Refraction at Horizon
Horizontal refraction (a body observed on the visible horizon, as in an amplitude sight) is approximately:
- A0', it can be ignored
- BAbout 5 degrees
- CAbout 34' (and the Sun's center is taken on the horizon allowing for it)✓ Correct
- DEqual to the dip
Explanation
At the horizon refraction is at its maximum, roughly 34 minutes of arc — about one Sun-diameter. This is why a body is geometrically below the horizon when it appears to be on it; amplitude rules account for it.
Authority: The Nautical Almanac; Bowditch (Pub. No. 9), refraction
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