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    Re: accuracy of Cook's lunars
    From: Frank Reed
    Date: 2012 Dec 31, 19:44 -0800

    Tom, you wrote:
    "Yes, sorry I should have addressed it properly."

    You did. It was quite clear that you were responding to my message. But with NavList messages, it pays to be doubly clear. :)

    In your earlier message, you wrote:
    " Now I am feeling challenged. I will need to get my AH out and do some land based lunars."

    Aha! But one of the nice things about lunars is that you don't need a horizon, artificial or real, to shoot them on an experimental basis. Historically, altitudes were taken primarily to get the local time and secondarily to clear the lunar of the effects of refraction and parallax. But if you're shooting "backyard" lunars, you don't need altitudes --you can calculate them for your "DR" position, and they will be close enough. Do note the carefully-worded caveats in my earlier post. You cannot expect the level of accuracy I described unless your sextant is properly adjusted. Over the years, I've found that the biggest issue here is surprisingly the simplest: it's index correction. Normal celestial navigation sights are considered accurate if they're good to +/-0.5' and so an IC that's accurate to, let's say, +/-0.3' is just fine and an error in IC in that range would be essentially undetectable. But for lunars, you have to get a rock solid value for the index correction. Indeed, one very practical way to do this is to shoot a lot of lunars!

    Tom, many of your posts end with that famous little tagline, "Sent from my iPhone". You may remember a little noise a couple of years ago when Apple introduced iPhones with a "retina display". The implication was that the pixels were so small and so close together that they were finer than the resolution of the human eye (more specifically, the human retina) when the device was held at a range of about 18 inches. This resolution for normal vision is right around 1' of arc or somewhat better, as good as 0.7' in excellent lighting. If the pixels are smaller than that, then the display is smooth (excluding notorious hyper-acuity tasks which will allow Apple to introduce even higher resolution displays with similar hype at a later date!). Similarly in celestial navigation, if that's the limiting resolution of the eye, then obviously we can't see differences in alignment of celestial objects smaller than that. This, of course, is where the sextant's telescope comes into play. With a 7x scope, you can resolve actual angular differences as small as 0.14' or 0.1' (just dividing the previous numbers by 7). That's the "ultimate" limit on these observations. If you can't resolve it, you can't see it. But notice that the standard deviation I previously stated with a 7x scope is considerably larger than this. If you had a "perfect" sextant with excellent lighting, then you could expect a standard deviation on individual observations of a tenth of a minute of arc. I am reasonably happy with a quarter of a minute of arc. If you've only managed a whole minute of arc of error (in the one standard deviation sense), then you can certainly expect better as long as you're using a 7x (or better) scope.


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