A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding
From: Frank Reed
Date: 2018 Jul 30, 19:53 -0700
Thanks for the links, Marcel and Murray. I couldn't find it anywhere myself. I read through the whole book. It's relatively typical of chapters on lunars in other navigation manuals in this period. In fact, some of it seems borrowed from similar English-language books. That's not a fault -- really, that's to be expected. A few interesting items that I can recall right now:
- There's a nice short history of the concept of lunars from a French perspective.
- He talks about the conflict between Maskelyne and Harrison and notes that the way this was played out in public was directly responsible for bringing the whole issue of "finding longitude" to wide public attention. That's an interesting notion since it was the personal drama and protagonist/antagonist (hero v. villain) conflict in Sobel's "Longitude" 20+ years ago that brough the issue of longitude to a broader public in our era.
- He describes a much too fussy three-person procedure for shooting lunars that was common in "armchair" descriptions of lunars in this period. This definitely seems like a borrowing from another source.
- After describing the three-person circus, he then describes a simple method for one observer to take a lunar. This is also a common feature in navigation manuals in the late 18th and early 19th centuries.
- He carefully notes the difference between civil time and the astronomical time found in the almanacs. There's no mention of the "sea day" which may well have been primarily a component of British (and later American) navigation culture.
- He describes the various altitude corrections explaining what causes them in basic terms and then as a practical matter advises the navigator to use the table in the almanac for the values (like any standard navigation textbook, then and now).
- He cites a number of reasons for preferring Sun-Moon lunars over star-Moon lunars, and as I have noted, this is what you find in the logbooks, too: a preponderance of Sun-Moon lunars. Generally celestial navigation was a daytime activity.
- The method outlinef for calculating local time would have been out-of-fashion not long after the publication of this book.
- As described, the French lunar distance tables in this period were reduced to the meridian of Paris using a "hack". The French astronomers borrowed the British tables and changed only one feature: the hourly headings were adjusted from 0h, 3h, 6h, etc. to 0h+x, 3h+x, 6h+x where x is the time difference between Greenwich and Paris. It was a cheap trick that allowed them to maintain the fiction that the tables respected the Paris meridian.
- There are several methods outlined for clearing lunars. In particular he mentions the Cambridge Tables (prepared and edited by Lyons, et al.) and says he's not going to detail them further because a navigator can simply read the instructions in the tables themselves.
- The only method he outlines with detailed steps is Borda's. Since Borda was French there's a natural link here, but they were shooting themselves in the foot. While the British and Americans rapidly shifted to series methods ("corner cosine" methods) which were much easier to work, the French apparently lost interest in lunars (at least according to the Baron de Zach and other commentators at the time), stuck as they were with Borda's method.
- Of note, in the section on interpolating to get the Greenwich Time, proportional logarithms are not mentioned at all. Some lunarian enthusiasts become quite obsessed with proportional logs, believing them to be somehow critical to the process. Here we have a direct historical example showing that one can easily live without them. They are, after all, just common logarithms subtracted from a constant value (the log of three hours).
- It's a short book, that's true, but it was probably too short. Books like this, devoted strictly to lunars and omitting the necessary tables were reduced to nothing more than a chapter in a more typical navigation manual like Moore's "New Practical Navigator" or Bowditch's "New American Practical Navigator" (which was derived from Moore). And those navigation manuals also included the tables that enabled the work itself.