A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Greg Rudzinski
Date: 2013 Apr 4, 09:56 -0700
The problem of reading a sextant off by a whole degree is easy to do if observations get rushed such as when taking a series of ten in a row for averaging. The error should be easy to discover when reviewing the series of observations. Care must also be taken when reading the micrometer drum. It is easy for beginners to read from the wrong side of the vernier causing a 9' error which is harder to discover. (see attached micrometer image)
Analog watches cause me trouble still as an experienced CN navigator. The digital watch solves the problem but for those that like analog then mark observations soon after the second hand passes 12 o'clock.
Practice sights and sextant reading basics
From: Frank Reed
Date: 2013 Apr 4, 08:54 -0700
Lu, regarding checking students' observations (immediately after they've taken them) with a bit of software or the USNO site on an iPad, you wrote:
"What a great way to provide instantaneous feedback to the person about the quality of their sight!"
Yes, it really does help. As early as 2005, I was clearing lunars on my cell phone (NOT a smartphone) with students so they could get instant feedback. Of course, if it's Sun sights, you can just print out a table before you head to your practice location. I do that with lunars, too, though I worry about biasing my observations.
I discovered an interesting problem this past weekend with my "Easy Introductory Celestial" class. I've been teaching people how to read sextants for years, but there's always something new. Many of these students had never used a sextant before (interesting, because nearly all brought sextants to class, some quite expensive, which they had owned for years). The altitude of the Sun around noon was close to 52° 52'. That's confusing, no matter what. But I noticed that several of them were writing down 53 degrees and 52'. The pointer on the index arm was directed nearly at 53, so they just took that as the proper number of degrees. I couldn't even think that way. I suspect it's related to a familiarity with digital readouts. I realize now that I am going to have to include a lesson on reading the sextant like an "exotic analog clock" so that they understand that the index arm pointing just a bit to the right of 53 doesn't mean we should record an angle of 53. It's "just like the hour hand on a clock, and the micrometer is like a minute dial" (I'm hoping that some language like that will help in future classes).
Frequently, the sextants that students bring to classes are badly misaligned or even damaged. It's not uncommon to find a brand new plastic sextant with the horizon mirror so wildly askew that the "side error" is wider than the field of view. If that happens, you can't even superimpose two views of a nearby object like a lighthouse. The index error may well be several degrees. One student had a nice Astra IIIB which was giving him trouble. He asked me why we didn't talk about the correct direction in which to turn the micrometer, which puzzled me, and at first I though he might have read about backlash somewhere. After a while, we discovered that his micrometer knob had come completely loose: in one direction, and eventually both, the knob was just spinning freely with no resistance and not turning the screw at all. I had no idea how to adjust that one (never seen it on an Astra before), so I just started pulling on things until I discovered the endcap on the micrometer drum pops off and exposes a screw. Tightened up, he was back in business. Still another student had a quartermaster friend who had let him borrow an old USN Mk.II sextant, with a fair amount of green corrosion here and there. Those are good sextants, and I started to adjust the horizon mirror, but then I heard a little clattering noise and realized that the index mirror itself was barely attached to the index arm. It was rocking back and forth and of course the sextant would not be usable until it was repaired. It's a fine sextant in principle, and with a little love and care it will be again, in fact.
It's always somethin'...
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