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I wonder if a virtual reality planetarium would be better, somehow
adapting the VR goggles so they would work with a sextant. I can see
the planetarium is great for learning stars, and indeed have had the
great privilege of seeing Frank demonstrate the one at Mystic Seaport.
He mentioned the same limitation. It would be very interesting to hear
if Jon Sutcliff had an extra trick.

Bill
On Sun, 11 Nov 2018 at 23:24, Bob Goethe  wrote:
>
> >>unless the student is precisely at the center of the dome (sphere) of the 
planetarium screen, severe angular errors are introduced.  The angle measured 
is directly affected by the baseline or distance.<<
>
> I observed this problem when I assessed my local planetarium as a teaching 
site.  I could take an altitude on a particular star, then move to the seat 
immediately next to me, and get an altitude that was 2° different.
>
> Other issues that I observed:
>
> There is a certain amount of backlash in the gears/motors that control the 
planetarium display.  If your curriculum involves using the sky as it would 
appear on such-and-such day/hour/minute/second, the altitude of stars could 
be different from one time to the next, depending on whether they were 
advancing the projector or dialing it backwards.
> There was no unambiguous horizon to deal with.  In the dark, it was hard to 
see the edge of the dome.  I asked management if they could project a laser 
around the base of the dome to give a clear, unambiguous horizon to work 
with, and they didn't have the capability to do that.
> Our planetarium is 40 years old.  While the dome may have been a perfect 
hemisphere when it was built, the apex of the dome has sagged by several 
centimeters over the years.  So even if you could sit in the precise center 
of the room (which would be awkward, as that happens to be the same spot 
where the projector is), your results would be inconsistent because of the 
dome being shaped inconsistently.
>
> I could imagine that - maybe - if you had a brand new planetarium 
purpose-built for instructing CNav, you might be able to make it work.  But a 
regular planetarium found in a civic science center is not up to the 
challenge.
>
> If problems inherent in the planetarium itself could generate results that 
could be off by several degrees, then as Brad said, sextant results can be so 
wildly inconsistent that a student would have no idea whether he was using 
proper technique or not.
>
> In teaching individuals to use an artificial horizon, I have a couple of 
times had somebody get a altitude that was 32' from what it should have been. 
 I could look at that result, and infer that their observation was good, but 
they were using the upper limb of the sun rather than the lower limb...and 
were getting results that were one solar diameter wrong.
>
> Identifying an error in observational technique would be all but impossible 
in a civic planetarium environment.
>
> If Jon Sutcliff has solved these problems, I would love to dialog with him 
about just how he accomplished what he did.  I have searched Google, but can 
find no indication that he published a paper on his techniques.
>
> Bob
>
> 

   

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