Celestial Navigation · Sextant Corrections — Refraction
At which observed altitude is atmospheric refraction greatest, and what is its sign?
- ARefraction is constant at all altitudes
- BNear the zenith (high altitude); subtractive
- CAt 45°; additive
- DNear the horizon (low altitude); refraction is always subtractive✓ Correct
Explanation
Refraction bends light from a body, raising its apparent altitude, so the correction is subtractive. It is greatest near the horizon (where light passes through the most atmosphere) and approaches zero at the zenith — a reason low-altitude sights are less reliable.
Authority: Nautical Almanac, altitude corrections; Bowditch (Pub. No. 9), refraction
Practice the full Celestial Navigation bank
Free spaced-repetition quizzing across 2190 USCG exam questions — it schedules your reviews so the ones you miss come back until they stick.
Related Celestial Navigation questions
- Sextant Corrections — Refraction
Atmospheric refraction of a celestial body's light is greatest when the body is:
- Sextant Corrections — Refraction
Atmospheric refraction makes a body appear HIGHER than it really is, so the refraction correction is:
- Time
The Nautical Almanac tabulates the positions of celestial bodies against which time standard?
- Time
A vessel in west longitude has a zone description of +5. To convert zone time to UT you must:
- Coordinates
How is the GHA of a star obtained from the Nautical Almanac?
- Coordinates
Sidereal Hour Angle (SHA) is best defined as: