NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sextants, vernier and micrometer.
From: Alexandre Eremenko
Date: 2006 Oct 29, 10:43 -0500
From: Alexandre Eremenko
Date: 2006 Oct 29, 10:43 -0500
George, Thanks for your very informative letter. > It's a pity Alex and I are on opposite sides > of the Atlantic. He would Actually I am planning a visit to England next summer:-) I hope to be able to find a good vernier sextant on e-bay by that time, but my general impression is that in the last two years, the number of real sextants in e-bay sharply decreased. (As opposite to the number of decorative imitations that sharply increased). Two years ago I could see 5-10 reasonable sextants on e-bay every week. > introduction of the micrometer sextant. I clearly remember reading somewhere that the wormscrew was suggested by R. Hook, as early as in the beginning of XVIII century. Unfortunately, I do not remember WHERE did I read this. The text said that it is unclear whether the idea was actually tested at that time. We had a short discussion of this on the old list about 2 years ago. There is no doubt that the idea of a wormscrew was not new. And the reasons why it was not implemented in XIX century still has to be discovered. If I understand correctly, the first "dividing engins" (late XVIII century) used the wormscrew principle. BTW, the very first one is apparently located in Smithsonian now, but it is not on display and I was unable to see it. I also remember reading somewhere the following argument in favor of the drum/worm: Easy reading permits you to make more observations in the given time. Then you average them and achieve a better result. On my experience in reading the silver scale (on my pocket sextant): it is very relevant from which direction the light falls. To read the first observation, I just turn around and turn the sextant to find the best position. At night I use a small but very bright LCD flashlight (of the type sold in Celestaire) and it works very well. In Bremerhaven museum I saw some very interesting reflecting circles (by Pistor and Martens), equipped with detacheable oil lamps on each vernier:-) Alex. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---