NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Refraction for beginners
From: Bill B
Date: 2004 Jul 9, 15:06 -0500
From: Bill B
Date: 2004 Jul 9, 15:06 -0500
Jeez guys, all I asked about was what corrections to make to a bubble-horizon observationsIf I put it all together the refraction issue relating to HO229 comes out something like this: Dr. Carney, meteorologist and Head of Purdue's Aviation states aviation "altimeter" pressure has adjusted local station pressure to sea level plus 10 feet (hmm) and most broadcast sources have adjusted the local "station pressure" to sea level. As a sanity check, if the pressure in inches mercury is above 29.8" in normal weather, it has been adjusted. His rule of thumb was plus/minus 1" mercury per 1000 ft change. Quick formula to adjust station pressure to sea level: Station pressure + (.0011 x height above sea level in feet) If I understand the earlier posts, if I were sitting in Denver or on a mountaintop, the air would be thinner and therefore less refraction. As a skier, I can attest to the former. So it is not just what happens at the "boundary" of outer space and Earth's atmosphere, but also how much air (distance) there is between the boundary and observer, and how dense it is. Watching the color temperature of light change at sunset/sunrise and the high refraction corrections for a low Hs seems to validate this. So sea-level pressure works out fine if you are at sea level, but a barometric pressure report in Denver that has been adjusted for sea level is not a big help. Getting back to Great Lakes sailing, it would therefore behoove me to use the ship's barometer or adjust the broadcast pressure (that has been adjusted to sea level) back to local "station pressure." Since we are only talking about .6" of mercury, that would keep me within the limits the temp/pressure table. If I were in Denver and used local pressure (maybe 25" mercury), I would be outside the table's parameters anyway. Would also guess that the parallax change etc. by moving closer or farther from the Earth's center because of tides is minimal, but might encounter that and other problems with a mountaintop bubble shoot and HO229. After all, the cover of my HO229 does say, "Sight Reduction Tables for Marine Navigation." Sounds like HO249 (which I have not explored yet) is the way to go for mountaintop bubble shoots. Do I got it? Bill