A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2018 Dec 17, 10:11 -0800
Lars, you wrote:
"Your favorite 19th century blogger has a typo in the half sum of logs."
Heh. Yeah, it's not a textbook, that's for sure! Hart's book is bizarre. As I have noted before, he reminds me of a modern blogger (or... yikes... I should day he reminds me of a blogger from a decade ago ...when blogging was popular), ranging widely over disparate topics mixing interesting firsthand observations with strong opinions and cosmic musings. It's an oddity to find the details of a time sight spelled out, and with such clarity, despite the typos, in a book like this. Naturally, if you want to get it all right, you turn to the various navigation manuals, like Bowditch and Norie, or you find a local expert aboard ship (and to his credit, turning to Bowditch or a local expert is exactly what Hart recommends). The "key detail" here is that you can spot this highly-standardized logarithmic method by looking for the instructions to calculate the "half-Sum". Once you find that, you're probably in the right section.
There's another curious detail in Hart's description that matches practice and demonstrates the "rote" nature of these calculations. He describes how to calculate the polar distance and works the calculation to seconds of arc. That's two orders of magnitude more than necessary. Minutes of arc were quite sufficient for the task. And I have seen this in other primary source calculations. It wasn't just Hart. That was the recipe, and navigators followed it. Again, navigation is cultural, and I suspect this was American commercial practice only, but i haven't investigated further.