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    Re: Advice concerning sextants
    From: Frank Reed
    Date: 2010 Dec 29, 21:21 -0800

    Hello Patrick.

    Congratulations on your sextant purchase!

    You say you got a latitude that differed 23 miles from the known position. I would say off-hand that 1here's a good chance that you did much better than this and there may well be an error somewhere in your calculations. Twenty-three miles error in latitude by Noon Sun is necessarily equivalent to 23 minutes of arc error in the observation, but the apparent diameter of the Sun is around 32 minutes of arc so, if this were due to observational error, it would mean you had a gap between the Sun images (or an overlap) equal to more than two-thirds the apparent size of the Sun's disk. It wasn't that bad, was it?? Could you post the details of that sight? Did you record the actual angle (double the altitude) as taken from the sextant? And what was your index correction?

    By the way, taking sights with an artificial horizon is definitely harder than taking sights with the true horizon. If you feel like going for a little drive, there are lots of places near you that have true horizons for over 180 degrees of azimuth. For example, just poking around on Google Maps, there's a beach labeled "Grandview Natural Preserve" a little east of Langley AFB. If you can get on the beach there (there's a little road visible but it may not be open), you have clear horizons from about 15 degrees west of south, through east, to about 30 degrees west of north for a total of about 225 degrees of horizon. There's land in almost every direction if you project a line far enough from that spot, but it's all over eight miles away. That means that if you're, say, sixteen feet above sea level the horizon is in front of the land beyond (the visible horizon is at a distance in miles roughly equal to the square root of the height of eye in feet). So you can do Noon Sun sights from there even though you're looking right at Norfolk. That's just an example location. If I remember right, there are also parking areas right out in the middle of Chesapeake Bay near the tunnel entrance.


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