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    Re: Practice sights and sextant reading basics
    From: Lu Abel
    Date: 2013 Apr 4, 10:19 -0700
    Good points, Frank!   Last time I had a group (about eight people) out for sun sights, we had an assortment of sextants.  As might be imagined, a bunch of Davis plastic ones.  The most interesting one, though, was an old Plath that the guy bought on eBay for a realtive song.   The IE on the thing was almost a full degree!!!   But it was rock solid -- we measured the IE, corrected for it, and most of that person's sights were tighter than the folks with the plastic sextants!

    And, yeah, teaching people that if the sextant reads 52' you don't assume the degrees where the index mark is almost resting, you go down a degree.   Greg, I really like your idea of telling people that it's like reading a clock!!   I'll have to remember that for the next time I teach.


    From: Frank Reed <FrankReed@HistoricalAtlas.com>
    To: luabel@ymail.com
    Sent: Thursday, April 4, 2013 8:55 AM
    Subject: [NavList] Practice sights and sextant reading basics

    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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