A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2020 Oct 10, 18:00 -0700
David C, you wrote:
"Now that computations are "free" the Sumner chord method has (IMHO) advantages over both the Sumner tangent method and the intercept method."
Indeed. And that has been a fundamental component of my "Modern Celestial Navigation" classes for the past ten years. That is exactly what I teach, and the reasoning that you mention is very much why I teach it. I should warn you, though, that the terminology you're using here is meaningless to all but those very few who are already familiar with it. Calling it a "Sumner" method causes many navigators to assume we're talking about something antiquated and obsolete. The expression "Sumner line" has travelled along an interesting story arc. At first it meant something specific to Sumner's unique methodology. By the early twentieth century "Sumner line" was just the way they referred to any general celestial line of position. That is, they didn't say "let's get the Betelgeuse line of position." They said "let's get the Sumner line for Betelgeuse." The expression had become very general, especially so in the US since Sumner was American. But by the late 20th century, "Sumner line" had become a term of historical derision, something to be avoided, something antique.
Also, all that talk about tangent versus chord or secant in the old discussions of "Sumner lines" causes still more confusion for modern navigators, and I would avoid it like the plague! The problem is that many modern navigators are familiar with trigonometry terminology but only dimly aware of the sense of words like secant and tangent applied to the pure geometry of the problem. When they hear that there is a "Sumner tangent method", they immediately assume that there will be tangent functions in the math, and that this is the origin of the name. The key distinction here is a "two point"line of position versus a "one point plus azimuth" line of position. From a modern perspective, these are nothing more than algorithmic differences that lead to the same end.
As for this damn "lon by chro" thing, I suppose I need to reiterate, repeat, say one more time... that "longitude by chronometer" once upon a time (mid-19th century) was a completely valid expression, and you'll see it in most logbooks. A navigator could determine and record in a logbook a "longitude by a/c" (account, meaning DR) or a "longitude by lunar" or a "longitude by chronometer". In both of the latter cases, the "by" part of this referred to the method for acquiring GMT or absolute time, and it went without saying that you had to take a sight for local time, turning your sextant into a sundial. This was so obvious that it hardly required a name. It was not a euphemism or slang for a local time sight as it became in the early 20th century in British navigation culture, and its presence in those earlier sources does not somehow validate much later slang usage that turned the whole thing on its head. It might be interesting to follow this changing meaning... Perhaps there are hints of it in logbooks and letters written by navigators... or maybe it's lost to unrecorded oral history, and we'll never know.
One thing that we can say for certain is that the odd --and oddly British-- inversion of the sense of "lon by chro" is not an aspect of history that you will learn by studying textbooks and navigation manuals. The history of textbooks is not the history of navigation. If you want to know the history of navigation, you have to seek primary sources. Primary sources get us as close as we can to the thoughts and practices of real navigators. And fortunately we do have vast resources in the form of logbooks, sea journals, letters among navigators, and marginal notes.