A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2013 Apr 2, 10:52 -0400
In an earlier message Frank Reed wrote:
Speaking of treating dip as an unknown, for any of you who were following the thread about measuring dip, you can measure it with an ordinary properly adjusted sextant as long as your goal is just collection of data for statistical analysis (as opposed to live navigation). You measure altitudes from a known location and clear them treating dip as unknown (set to zero). Then the error in the sights is equal to your dip.
That's under the assumption that ALL other errors are zero. That's a mighty big zero to shoot for. Any error will be treated as an estimate of dip.
In principle, this concept has merit. We know everything else precisely, so the unknown is the dip. Ah, but the human must then get the altitude precisely correct. Easy to say, not so easy to get right.
Bruce, regarding your dip measurements you wrote:In case these measurements will be done in view of producing a larger data set allowing (hopefully) to arrive at an improved way to estimate the dip, then the "etc." may eventually also stand for e.g:
Wave height, temp, wind, time, tide etc will be observed/recorded. Knowing how variable waves can be, an effort will be made via the angle measurements to estimate wave height where sighting.
-> sea surface temperature (SST possibly from http://ourocean.jpl.nasa.gov/ see G1SST) or/and sea temperature from a buoy. My Hs measurements show indeed a correlation with the temperature difference between SST and air.
-> Cloud cover. You possibly ask now why also cloud cover. For his studies in Archaeoastronomy Victor Reijs had the idea to relate terrestrial refraction to the atmospheric Stability Classes. You find more on this on his Web-page(s) http://www.iol.ie/~geniet/eng/stabilityclasses.htm The Stability Classes could possibly indeed contribute to an improved dip estimate.
It may eventually also be useful to know:
-> Nearest meteorological station (having comparable conditions to observer location) providing archived data records (via e.g. http://english.wunderground.com)
-> Nearest radio sounding station (having comparable conditions to observer location) providing archived data records (via e.g. http://weather.uwyo.edu/upperair/sounding.html or via http://www.ncdc.noaa.gov/oa/climate/igra/index.php?name=coverage )
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