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    Re: Help for beginner
    From: Mark Coady
    Date: 2015 Dec 20, 20:16 -0800

    If you stay at this you have just begun a fun journey.  

    I found the internet a "compost heap" of CNAV information.  A fair bit is incomprehensibly explained, and or partially wrong, or simply done in ways far more complicated than necessary. Local instruction I have have obtained here has been a huge help at distilling the compost into sensible methodology.

    I am no expert, but a fellow traveler a bit farther along.  My own journey is recent enough to genuinely appreciate your start.  Not sure where you are at in the learning phases so a couple of quick thoughts. If you email me on the side I can send you names of a few texts that explained well and got me quickly working right answers from shots, before I started to dig deeper. I started with "I need to find my way around" and rapidly got to "I want to understand how this all works".  I am just scratching the surface two years later. 

    1. The noon shot provides latitude and reasonable longitude without the mathamatical gyrations we call a sight reduction. Its harder to get longitude right because it is pretty damned hard to get the sight at exactly local noon, especially without pre-calculation. 

    2. Remember local clock noon (zone time) is not actual solar noon due to the naughty little "equation of time" (a simple correction really) and your actual longitude vs local time zone time.   You can 1. Calculate local noon for your estimated or Dead reckoning positon multiple ways from the alamanc to plan your sight in advance, or 2. take multiple sights of equal altitude before and after local noon, and thus average your way to it and thus longitude.  Like many I learned first what i now consider the hard way. finding apparent local noon with the sextant directly. I know my latitude is 41.19 on my boat, so I can look up declination and figure out with quick check sights when I have to be getting close. If you don't know lattitude, and there are puffy clouds about...its a total Pain in the a.....

    3. Without a bubble horizon for a handheld sextant, once you get to the point your artificial horizon won't do, you have two options:  Find a decent size lake of known length off a map and use "dip short" correction to correct for lack of full horizon, which is a simple formula. Or learn to take lower altitude sights and do sight reduction.  On my first shore sights I played with dip short calculations becuase I had an island a couple of miles away in my way and got quite acceptable accuracy, but I hate to add any additonal error risk with extra math.  Sight reduction is intimidating looking, but really is just a lot of little steps of arithmatic or lookup done with either tables or calculator.  THere are of course programs that do it all for you. 

    4. A single off noon sun sight reduction gives you a "line of position" you must be on vs a postional fix. Multiple sun sights from the same location over a period of hours (say morning and afternoon) gives you crossing lines and thus a fix.  You can move of course, and dead reckon your way from Line of position to line of position, using your good DR to update your spot on the next LOP.  If your dead reckoning is good, you'll be pretty close to where you think you are in a few sights.  If you do get a horizon on a lake, of course, the moon if visible can be used to create crossing LOP "fix". I found dip short very difficult to attempt in twilight on stars/planets/moon, as the backlight isn't available on the horizon with land in the way. 

    5. With a plastic sextant, as they have more slop in the gears("backlash") than metal sextants, always try to bring the body down to the horizon.  If you go past, just back up agian and come back down agian. On my old Davis it knocks out several tenths of backlash error. 

    Agian, no expert here...wishing you the best...

     

       
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