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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Bubble Horizon Altitude Corrections
From: Frank Reed CT
Date: 2004 Jul 5, 20:35 EDT
From: Frank Reed CT
Date: 2004 Jul 5, 20:35 EDT
George H wrote:
"I would agree with Howell's view here, not with Frank. Because those
corrections are so very small, and because the inherent accuracy of a
bubble=-sextant (even on a steady platform with a steady hand) is so much
worse than that of a sextant using a sea-horizon or liquid
artificial-horizon, then such temperature / pressure corrections will have
little practical effect on the answer, except under extreme climatic
conditions."
Saying that they MAY be ignored because they have little practical effect is certainly reasonable. But notice that this would get you in trouble in a backyard in Denver again. A 15% change in refraction should not be ignored even with a low accuracy sextant sight. Of course, the Nautical Almanac refraction tables were never intended to be used at high altitudes. The Air Almanac refraction tables included a separate entry for altitude above sea level (which is equivalent to removing the sea level calibration from the barometric pressure).
Also the section in Sue's book is specifically describing artificial horizon sights which can actually be *more* accurate than standard sights using a sea horizon. So her instructions to ignore the temperature correction are definitely incorrect. It's an excellent introduction to celestial navigation, but unfortunately Sue never had the opportunity to revise it thoroughly.
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
"I would agree with Howell's view here, not with Frank. Because those
corrections are so very small, and because the inherent accuracy of a
bubble=-sextant (even on a steady platform with a steady hand) is so much
worse than that of a sextant using a sea-horizon or liquid
artificial-horizon, then such temperature / pressure corrections will have
little practical effect on the answer, except under extreme climatic
conditions."
Saying that they MAY be ignored because they have little practical effect is certainly reasonable. But notice that this would get you in trouble in a backyard in Denver again. A 15% change in refraction should not be ignored even with a low accuracy sextant sight. Of course, the Nautical Almanac refraction tables were never intended to be used at high altitudes. The Air Almanac refraction tables included a separate entry for altitude above sea level (which is equivalent to removing the sea level calibration from the barometric pressure).
Also the section in Sue's book is specifically describing artificial horizon sights which can actually be *more* accurate than standard sights using a sea horizon. So her instructions to ignore the temperature correction are definitely incorrect. It's an excellent introduction to celestial navigation, but unfortunately Sue never had the opportunity to revise it thoroughly.
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
