A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2016 May 18, 15:17 -0700
Stan, you wrote:
"In our 19th Century class, you teach adding 12' to the sextant altitude (corrected for index error) to account for semidiameter, dip, and refraction. In this example, it appears that 12' is added to account for semidiameter, parallax, and dip, and a separate 4' is subtracted to account for refraction. Please explain. "
Navigation is cultural. Methods are employed because they are learned and passed on, with occasional corrective from respected authorities and textbook authors like Bowditch. Practice in the 19th century was not monolithic, of course. The advice in Bowditch here is slightly more detailed than the common "add 12 for LL, subtract 20 for UL". Bowditch was only one among many navigation resources available, and as a textbook it is only a clue to practice, not a reflection. In real history, you never find laws. There are always exceptions to every rule you can write down about historical navigators' behavior, and there are always variations from one culture of navigation to another. Any navigator worth his salt understood that there were really various separate corrections. But in practice, the advantage of consistent calculation methods seems to have been more important. That's just speculation on my part. The only thing we can say honestly is that "navigation is cultural". Patterns come and go, and only the primary source evidence --logbooks, navigation notebooks, marginal notes, scrap paper-- can really help us see those patterns with any certainty.
By the way, I don't know if you were there when I said this in the portion of the class you attended, but Lecky himself in his 'Wrinkles' mocked navigators who were so lazy that they didn't calculate dip, refraction, and other corrections separately (meaning navigators who were still using the 12/20 trick). The first edition of 'Wrinkles' was published in 1883 if I remember correctly, which suggests that this was still a practical issue, perhaps waning, for British navigators at this late date. But I haven't studied any logbooks or other primary sources for British navigation on that, so, as I say, it's just suggestive. Certainly as late as the 1890s on American vessels, the 12/20 altitude correction was still common. It's interesting that some navigators used this easy correction for latitudes but were slightly more careful for longitude.