NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2013 May 6, 09:26 -0700
Jeremy, you wrote:
"I tend to agree with Byron. It's easiest to observe and compute the amplitude on the celestial horizon."
William H wrote that he had been taught to observe the Sun when it was 1 semi-diamter or 16' above the sea horizon. Byron said that the rule he learned was 2/3 of the Sun's diameter or about 21' above the sea horizon. The (presumably) nearest to correct rule would be dip + 34'(for mean refraction) - 16'(for SD) or in other words dip + 18' (and to be yet more accurate, +/- a few minutes of arc for non-standard temperature and pressure). Considering you're shooting from a greater than average height of eye, do you have a rule of thumb that incorporates a higher dip value? For a height of eye of 100 feet, dip+18' is nearly 1 Sun diameter (especially since the vertical diameter is reduced by refraction). Also, do you normally sail in the tropics? For low latitudes, the specific choice of altitude matters very little. You could use 16' or 32' and in the tropics this should only change the observed azimuth by an eighth of a degree at most (not enough to worry about, right?). But in latitude 45, any difference in altitude yields nearly the same difference in azimuth: half a degree is half a degree. These rules then really only matter in higher latitudes.
Do you have your crew get their azimuths by table or by calculator? Why not have them measure the Sun's altitude, even with a crude instrument, for a more accurate azimuth? If they can work it up electronically, the clearing process is no more complicated. That way if clouds obscure the horizon for a degree or two, you could still catch it a bit higher, right? I understand that the sighting process becomes much more difficult at somewhat higher angles, but is it difficult at two or three degrees altitude? I'm just asking...
-FER
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