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    Re: Not exactly "pinwheeling"
    From: Frank Reed
    Date: 2011 Oct 21, 13:12 -0700

    Randall, you wrote:
    "My actual position, from a web site based on a street address, is W119*5', N35*19'. (I don't own a GPS) "

    You don't really need it, but just so you know, you can get a position for your observing site more accurate than this from Google Maps. In case you haven't done this before, go to Google Maps here: http://maps.google.com. In the search box, type in 35.3, -119.1 and click search. That will take you to your neighborhood. Then zoom in to whatever level allows you to see your exact observing location. Then right-click with your mouse. A little menu pops up and the second option from the bottom says, "What's here?" If you select that, the search box updates with the latitude and longitude at a precision that's considerably greater than necessary. Take the first three or four digits after the decimal point in each coordinate and multiply by 60 to get minutes of arc.

    You also wrote:
    "The sextant is a Davis Mark 15, using a levelled mirror a.h and a digital level only accurate to 1/10 degree."

    That Davis sextant should have a standard deviation of measuring error on the order of 3 to 5 minutes of arc. The digital level you're using may not actually be as accurate as it claims to be. My guess would be that a big source of error is unknown tilt in the "artificial horizon".

    You added:
    "The intercept distances ranged from 0.3 miles to 32 miles. "

    It's been said before, but let's say it again. The intercept distances do not matter. They are just a component of the plotting process. When you try to gauge your success, you want to measure the distance from your actual position to each LOP. The rough center of your plot of LOPs seems to be about 5 nautical miles north and about 20 nautical miles west of your estimated actual position. That could be due to entirely to the tilt of the a.h. The scatter is more problematic. The error circle on this position is at least 20 nautical miles in diameter which is considerably worse than you should expect.

    You also wrote:
    "My moon sights always seem to be the worst of the bunch."

    There's not necessarily evidence for that in this plot. Of course, moon sights require a somewhat longer calculation, and many people mix up the corrections. If you find that you have trouble with them, just skip the Moon entirely! Many seasoned navigators avoid the Moon because they worry they will not do the calculation correctly. Also, when you shoot the Moon with that a.h., how are you doing it? Are you superimposing the two images of the Moon? Or lining them up limb to limb? The corrections are different depending.

    -FER


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