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    Re: Doing my homework
    From: Frank Reed
    Date: 2016 Oct 31, 09:38 -0700

    "Could you please give me some raw data for finding a fix - as if I just (or on some date within 2016) completed sighting of three different objects."

    You can generate such cases yourself ...and learn a lot in the process. Download and install Stellarium, if you have not already, at stellarium.org. Learn the basics of using it. Then click on a star or planet to get its "Az/Alt" (in the middle of the block of data that appears at the upper left). There's more precision here than you need for celestial navigation. Ignore the "Az" (azimuth) and round the "Alt" to the nearest minute of arc or tenth if you prefer. Record the time. Then advance two minutes or so in time, and select another star. Get its altitude and record the time. Two bodies is sufficient for a fix. Add as many more as you like. Note that the altitudes generated by the Stellarium software have no dip (or height of eye is zero) and no index error. As long as the atmosphere is "on", the altitudes do include refraction, and of course they include parallax. If you check an altitude for the Sun or Moon, it's the altitude of the center (no semi-diameter correction required).

    Suppose you want to make a game out of this, where you don't know your position, and you want to see how close you can get. Get a friend to randomly select a position in mid-ocean. You can just click on the little map in Stellarium that allows location selection. If necessary, hide the tag in the lower left corner of the display that shows the selected lat/lon. Your friend can provide you a DR by taking the true position and adding/subtracting 0-30 minutes of arc for both latitude and longitude. Note that the altitudes generated in this way will assume that the vessel is dead in the water. With a little work, you could throw in some motion, too.

    Frank Reed

       
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