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    Re: Simulating platform instability on land?
    From: Frank Reed
    Date: 2013 Jul 26, 10:03 -0700


    I've been thinking about this simulation issue. I'm not sure you would be able to simulate vessel motion in any useful way. Bouncing around on a mattress simulates bouncing around on a mattress. The connection with motion at sea is at best a weak analogy. Where do you live? Are there any regularly scheduled passenger ferries nearby? You could at least experiment on the water. But as I see it, the big problem is that you would be simulating the first fifteen minutes at sea. Even sailors with a lot of experience take a few hours to acclimate to the motion of the vessel and get their "sea legs". Unless you plan to camp out for a few days in your simulator, I'm not sure you could simulate anything approaching real observing conditions.

    In your original post on this, you wrote:
    "The idea would be to stand on a platform that's being moved around, either by an assistant or by motors/actuators. It would be nice if the motion could be dialed from "calm" to "stormy"."

    This sounds really familiar! Strangely, I think it's reminding me of an episode of "Gilligan's Island" from my childhood. They built a raft simulator that could pitch and roll, and if I remember correctly they simulated wind with a big fan made of palm fronds, and they threw buckets of water for simulated ocean spray.

    Setting aside tv farce, one of the biggest problems with the simulator idea is something Bill B mentioned already. You need six degrees of freedom. There's roll, pitch, and yaw, but among the unpleasant feelings that might lead to sextant sighting inaccuracy and certainly lead to seasickness, the linear accelerations are all-important. That feeling of being pushed sideways or driven upwards can be most unpleasant if the frequencies are just right. The linear accelerations will not affect the direct pointing of the sextant relative to the Moon and the other object, but they would affect your ability to stand.

    You also wrote:
    "Where can I find model parameters, or actual time-series recordings, of the orientation of the deck underfoot under various weather conditions and type of craft? This must be well-known stuff, but my searches aren't coming up with much."

    I don't know, but I would enjoy seeing a source on this, too. I'm sure it is "well-known," as you say, but it's obscure data. More importantly, what type of vessel do you want to simulate? And under what conditions? There's a substantial difference between sailing vessels in the first half of the 19th century, when lunars were common enough, and steamships in the latter half of the century which I remember reading somewhere (anyone remember who described this?) rolled more. A sailing vessel is held over at a relatively constant angle by the wind. It pitches but it doesn't roll much (relatively speaking). Meanwhile a steam vessel of the same general size (and moments of inertia, etc.) will roll as well as pitch. Lunarians were generally advised to shoot from the middle of the vessel where the rotational motions and related accelerations were minimized. And historically they could easily do so since the work was considered vital. But today, a lunarian at sea may well have to find some secluded "corner" from which to shoot sights while the crew watch in amusement.

    You also wrote:
    "I've read that sextant users at sea anticipate the swells. Would platform orientation include a large Fourier component representing the swell, plus a smaller noiselike component?"

    This all depends on the size of the vessel relative to the waves and swells. If you're in a 20-foot catboat, you will bounce about on the smaller waves at a relatively high frequency and also roll about on the swells at a much lower frequency. A larger vessel simply won't "see" those smaller waves. I'm sure you know this, but just in case someone else finds it misleading, the advice to anticipate swells and "wait for the top" applies to altitude sights and doesn't matter for lunars. With altitudes, it can be important to wait for the top of a swell to ensure that the horizon is clearly visible.

    You concluded:
    "Of course hopping on a boat would be an option, but the hassle factor is much higher (for me anyway), and the conditions are less controllable."

    I agree with Greg here. Make some sailing friends! They'll take you out in trade for a quick celestial lesson. As for conditions being less controllable, aye, that thar be the sea, matie! (beware pirates). How can you simulate "real" conditions when all conditions are different? How about simulating lunars in the doldrums on a miserably hot day when the Sun is near the zenith, and your hands are slippery with sweat! And let's not forget simulating lunars on a day with nice weather and easy seas. Lunars were not a daily necessity. Navigators could wait a day or two if necessary. In all such talk of simulations, there is one thing we cannot simulate. And that is reality itself. If you don't really "need" your lunars, how careful will you be in observing them? Life-or-death necessity inspires accuracy. On the other hand, if you have no need for them, you would never feel the pangs of doubt that lunarians sometimes reported historically. Could we really be 200 miles east of where we thought we were? Do I really trust these crazy lunars?? That's always the biggest problem with historical simulation. It's only a crude approximation.


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