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    Re: Zenith Fixes
    From: Frank Reed
    Date: 2019 Mar 26, 11:15 -0700

    Sure. It works. For a home version, get yourself a piece of PVC tubing, and attach a bubble level (two-dim) in a position so that you can see it when looking up the tube. Maybe even string a couple of wires across the top end to make "crosshairs". At night, look up the tube and observe the zenith. Calibrate the level from a known location. Accelerations will spoil the leveling, as they do with any bubble sextant, for that matter, but you can get a fairly decent fix this way. This technique depends on one tool and one skill that many navigators do not possess: a decent star chart or star chart app (with stars down to at least fourth magnitude) and strong pattern recognition ability. The expected position accuracy is around half a degree under typical conditions and as good as a couple of tenths of a degree repeatably with excellent conditions.

    But the stars are over-rated. Heh. :) Assuming you're sailing in summer (and if you're not, why not?!), you've got the Sun and probably no other celestial body for fifteen hours or more, close to two-thirds of every day. No zenith tube will help then... (except at those rare moments in the tropics when the Sun is close to the zenith). If we could see stars near the zenith at any hour, celestial navigation could be trivial (one solution: always navigate above 60,000 feet ...then you've got the stars all day!).

    Here's a post about this technique from thirteen years ago:

    Vega over Chicago
    From: Frank Reed CT
    Date: 2006 Sep 27, 20:05 EDT

    Last night at 7:07:00 Central Daylight Time, I  was standing next to a streetlight here in Chicago. Looking up the pole, I  noticed Vega very close to he zenith. So I pulled out an index card ruled with  lines spaced 0.25 inches apart and measured the distance between Vega and the  zenith. I judged the position of the zenith by visually extending the sides of  the pole to the point where they met. I knew the distance to my hand at arm's  length looking up was 16 inches so it was a simple matter to get the distance in  degrees. I found that Vega was 3.3 degrees south of the zenith and 0.8 degrees  west of the zenith.

    Almanac data: Vega's Dec: 38d 48'. Vega's GHA at  1900 CDT (0h GMT): 86d 21'.
    So at 1907 CDT, the GHA would be 88d 07'.

    I  can calculate my latitude right away. It's 38.8+3.3 or 42.1 North. Next I have  to convert the angle west of the zenith to Local Hour Angle. For small angles,  that's easy, I think. I divide the measured angle, 0.8 degrees, by the cosine of  my latitude. That gives 1.1 degrees. So my longitude must be 88.1-1.1 or 87.0  West.

    My actual position at the time of this observation was 41d 56'  50"N, 87d 39' 47"W so the error is about 0.15 degrees in latitude and 0.65  degrees in longitude. I'm fairly confident that I was able to estimate the  angles accurate to +/-0.2 degrees so I attribute the error in longitude to a  tilted lamp post. :-)

    Frank Reed

       
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