NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Calibrating a sextant scale
From: Frank Reed
Date: 2007 Nov 25, 23:07 -0500
From: Frank Reed
Date: 2007 Nov 25, 23:07 -0500
Alex, you wrote: "Of course, there would be no difficulty to detect such large errors using either Lunars or stars distances. By the way, this shows that the sextants you experimented with were of poor quality. The arc error of a good sextant is not supposed to change by 1'8 even if you ship it or transport in a car, is it?" The sextants in question were both over thirty years old. I have no idea what their history was. I don't know if they were thrown around like so much loose baggage, or were they treated with care and respect. That's the problem for most of us on this list. We are much less likely to have new sextants that we have had under our complete control from day one than practicing navigators a few decades ago. We are much more likely to buy used sextants, thirty years old, sixty years old, even a hundred and sixty years old. They often come from ebay merchants who claim they've cquired them at estate sales, but we really have no idea where they've been. There's no reason whatsoever to believe that the certificate which accompanies an old sextant represents its current condition. That doesn't mean a sextant is "poor". It just means the certificate may be irrelevant. And you wrote: "When, answering George's question, I said that "I failed", I was talking about STAR-STAR distances only. Which are less precise than the Lunars." Yes, and I agree. Based on my personal experience, star-star distances are less effective than lunars for judging arc error at the finest level of observation (tenths of a minute of arc). But this is all a matter of "degree". Some people aren't worried about that very fine level of measurement. Mike was talking about a plastic Ebbco sextant. For an instrument like that, star-to-star angles should provide all the necessary accuracy. I'm assuming here that the normal inaccuracy with a plastic sextant from one sight to the next is about 1 minute of arc (that's based on my own experience --your mileage may vary). So we don't have to worry about any differences smaller than that. So in his case, star-star distances will serve nicely. Also, if you're not worried about differences under 0.6 minutes of arc, you can also ignore stellar aberration which changes the star-to-star angular distances during the course of the year. And: "My Sun- and Jupiter- Lunars(taken in long series and averaged) generally agree with the most recent certificate I have within 0'2. As you can see from the statistics I posted on my web site. Determining the arc correction from Lunars is a long-term enterprize, this cannot be done in one night." It's certainly not something that you could do *every* night, but yes, it could be done in a relatively short period of time (a night or two?) maybe once every five or ten years by using special apparatus (e.g. higher power scope, equatorial mount with clock drive, etc.) or a special location (a site with four distant lighthouses spaces ninety degrees apart, etc.). When I purchase a sextant, which is "new" to me, but invariably "used", I don't assume that the certificate can be trusted. And you wrote: "So this procedure would be pretty useless in practice, if the arc error changes during a car transportation." Obviously, I wasn't suggesting that sextant adjustment changes *every* time you stick one in a car. Otherwise, I wouldn't drive with them at all, right? But I do believe that it's fairly common knowledge that rough treatment will *eventually* change the adjustment of a sextant and require re-certification. Does anyone have a sense how often this was recommended historically? I remember reading a report of an eclipse journey by Newcomb in the 1880s or somewhere around then where he brought along a sextant to measure lunar distances for longitude up in central Canada. He made a point of re-measuring its arc error, stating that it had not been tested in twenty years (if I remember correctly), before hitting the road. But that's just one case. And you wrote: "Fortunately, in my sextant it does not seem to change: I transported it a lot by cars, trains and airplanes (in the luggage). Even IC does not seem to change much. It is between -0.3 and -0.5 since I bought it." I'm not surprised. One of the things that I really like about the SNO-T is its very sturdy design. I do expect that your sextant could survive more banging around than average sextants. -FER --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---