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    Re: Astro Navigation at HMS Collingwood
    From: Frank Reed
    Date: 2018 Nov 12, 07:44 -0800

    Bill Lionheart, you wrote:
    "I wonder if a virtual reality planetarium would be better, somehow adapting the VR goggles so they would work with a sextant."

    Yes, exactly. Of course, it is then no longer a planetarium at all. Such a system could be used almost anywhere for sextant training. I've thought this through in some detail over the past few years, and this would be quite expensive to implement (a minimum of perhaps $50k to get it launched, which isn't much if you have large numbers of customers... but you don't). Perhaps a military partner could be persuaded to fund something like this.

    There's an option to use something like a VR sextant in conjunction with a planetarium display, which might be effective. Imagine: rather than goggles, you replace the scope on a sextant with a VR viewer. It needs to have some electronic pickup for the position of the index arm and possibly shade positions, also. When you look through this VR sextant you see what an observer should see when using a sextant looking at the real sky at "infinite" distance. When used under the planetarium dome, you should be able to aim the sextant at a star and see that star properly through the VR sextant. But dome parallax remains a major problem. The angular altitudes and azimuths will be radically different for observers near the edge of the dome on opposite sides of the planetarium theater. That means that even these VR sextants could not be properly "aimed" at the planetarium dome except by users close to the center. Either that or the VR sextant would need to know the observer's exact position under the dome and exact orientation in order to provide an appropriate view. Or it would need some robot vision capability (and low-light camera sensors) so that it could see the dome as the student sees it and provide the proper star view when aimed correctly.

    A simpler option is just to ditch the whole planetarium side. Combine the VR sextant with a common monitor display, perhaps projected on a wall in front of each student. Then the direct view and the VR sextant view can be synchronized. We could easily add vessel motion to both views and get the student properly seasick... just for fun.

    Another way to create a planetarium-like display for sextant training has been discussed before. Two years ago, I think, we talked about a US Air Force dome system, long since demolished, that used collimators to show the brighter stars. These would probably have been parabolic reflectors projecting single bright star-like points from behiind the dome. The sky would be projected from outside the dome and the entire dome rotated for latitude and time changes. Since there's no central projector and since the collimators cover a relatively large zone, a handful of students could sit near the center of the dome and take sights closely resembling true sextant observations. It's crazy expensive, but hey, you have Cold War funding!

    You added:
    "It would be very interesting to hear if Jon Sutcliff had an extra trick."

    I highly doubt it. Does your source say anything more than 'he used the planetarium for celestial navigation education'? Is there any indication that they used real sextants under the dome?? There are numerous aspects of celestial navigation that benefit from a little dome time. Unfortunately, as with any expensive tool, the properties of the tool itself can drive the curriculum if you're not careful. It's better to plan what you want to teach and how you would display that and then use the planetarium only when appropriate. And today there are a great many software and app simulation tools that rival a traditional planetarium's capabilities, and they're often preferable to a traditional projector and dome. The best aspect of a planetarium remains the "Ooh and Aah" moment. It excites the imagination using simple technology. That's worth something.

    In my years of association with the Treworgy Planetarium at Mystic Seaport Museum, both as an employee and as an independent contractor, I have delivered over 3500 planetarium programs demonstrating and discussing aspects of celestial navigation in nearly all of them. Just two days ago, on Saturday, we spent half an hour under the dome, learning some stars (primarily for azimuth reference, not for sights) and also watching the daily motions of the Sun and stars, which is one of the best features of a planetarium display. In this particular instance, since there were only a couple of students registered for the class, I had them stand during the presentation, hanging onto the low wall around the planetarium projector at the center of the room. This, of course, minimizes, but can never eliminate, the dome parallax that distorts the appearance of the constellations in the planetarium.

    Just one example of a simple misunderstanding that can arise from the planetarium projection:
    The projector at Mystic Seaport can put up a meridian line marked off with angular altitudes. It's a nice way to define the concept of the meridian, and we can follow the Sun as it rises from the eastern horizon and reaches the meridian at local noon and then read off that noon altitude (approximately). But I have discovered over the years that a great many people who see this meridian projection get the wrong idea, precisely because of the dome parallax problem. The meridian should be a straight line running from due south to due north passing through the zenith. But anyone not seated exactly under the meridian as projected sees an arch, like a rainbow or like the Gateway Arch in St. Louis, Missouri. Having seen the meridian in the planetarium, they can end up thinking that they should imagine something like a rainbow, deflected to one side of the sky, rather than a simple straight line passing straight overhead. Fortunately, this one is easy to explain away, but educators accustomed to the planetarium frequently cannot imagine the problem. They don't realize that this grand tool can also create serious misconceptions.

    Frank Reed

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