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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Testing sextants in 1885
From: Frank Reed
Date: 2009 Jul 15, 19:06 -0700
From: Frank Reed
Date: 2009 Jul 15, 19:06 -0700
A couple of months ago we were talking about the required quality of sextant mirrors. I found a nice description of a good practical test today in "The Sextant" published in 1885 by H. Wilberforce Clarke of the Royal Engineers. This guy has a way of making simple things complicated, but he is also extremely thorough in his writings (he has another book on "lunar distances" which is the epitome of "difficult" lunarian texts). His advice for testing the mirrors: "To test the reflectors, look with a small telescope obliquely and separately into each reflector, and observe the image of a distant object. In every part of the reflector the image should be clear, distinct (not streaky), single, and well defined about the edges." This idea of looking at a reflection at a very shallow angle is a good basic test. I tried it today with a sextant that I know has a slightly concave index mirror (it's on a Davis plastic sextant so not unexpected), and the imperfection of the reflections was very obvious. He also offers a suggestion for measuring arc error: "Imperfect centering may be detected by comparing the distance between two(N.A.) stars measured by the sextant under trial with the same distance observed with a standard sextant, or with a circle; or with the same distance deduced from computation. The error being found for certain places on the arc, the correction for any angle may by proportion be obtained." ...with a footnote showing how to calculate the correction for refraction correctly --albeit by a long method. The idea of comparing the observed angle with two sextants, one as a standard, is an obvious way to avoid the math work (so obvious, even I didn't think of it!), and it also strikes me as a good "sanity test". There are some other good ideas in the book. They're primarily intended for explorers and surveyors working on land. Clarke shot many, many lunars and other sextant sights on the Upper Nile in the 1870s/80s, but most of the advice also applies to accurate work at sea. "The Sextant" is a quick read, only 45 pages in length. It's available here: http://books.google.com/books?id=epAoAAAAYAAJ -FER I was browsing on google books today (I needed a statistic: there are 2,160 books from the 19th century indexed with the word "sextant" in their text). --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---