Sextant use

[CEFeighn]

How about an article on the construction of a brass sextant and the proper use of a sextant.

[Ian McColgin]

Ocean Navagator. ?

[genglandoh]

My boys and I build a sextant using cardboard, a protractor, a mirror and a glass slide.
Not the best but it worked.

The key is the glass slide (microscope slide)
1. You look thru the slide to see the horrizon.
2. The mirror reflects the image of the star on the slide.
It is like when you look out a window in your house you can see both the outside and your reflection.

[CEFeighn]

Ocean Navagator?
Not at this time, hope to be soon. I flew planes and I liked the "toys" needed to plot a course and I was good at it, winds, compass corrections, air speed. I belive that navagating in the water should be much the same, only you need to THINK a little further out when making a turn or any other course correction, water currents are a little more forcefull than air currents.
I could buy a sextant, but I would like to try my hand at making one. I have the skills, I just need to find the proper plans.
Background, at 15 I ground my first 8" telescope mirror, I was an armorer for the DOE, I have also worked in the acritectural sheet metal industry, and as a hobby I made jewelery, so I can do the fine work.

[Brian Palmer]

You can also find instructions on-line to make a sextant from a CD and case, and Lego blocks.

Brian

[J. Dillon]

Here's a plan for one taken fom the Sea Scout manual.






I made one when a teenager. It worked fine for noon sights and I noticed they had moved the roof top from which I was shooting about 10 miles.

JD

JD

[CEFeighn]

J.D. Thank you for the post, that is very close to what I am looking for.

[Ian McColgin]

By Ocean Navigator, I meant the magazine. Check it out.

[CEFeighn]

Ian, sorry.
I will be at the book store on Monday and will look for the magazine, Thanks.

[George Ray]

Rather nice free books and software on the topic by a generous hobbyist.

http://mysite.verizon.net/milkyway99/id1.html


Links:
Book on celestial nav: http://mysite.verizon.net/milkyway99...tionTeacup.pdf
Book on building an Octant: http://mysite.verizon.net/milkyway99...ley_octant.pdf
Also available are free Excel and Visual basic tools for celestial

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Good One Stop Shopping link page for celestial:
http://www.celestialnavigation.net/

[MiddleAgesMan]

I've always been intrigued by the aviator's sextant with it's built-in horizon, not the correct term but it eliminates the need to sight the real horizon. I take it that's the sort of device you used when flying.

Could you explain how that self-contained horizon worked?

[Ian McColgin]

An artificial horizon is essentially an ultra-refined bubble level. You can make one for use on land, practice sites or wilderness navigation, with a pan of water. The type inside sextants is a spirit bubble in oil viewed through a magnifier that projects it up to your view. The aircraft versions are "self-averaging" on a wind-up clock mechanism. You start it and keep moving the fine adjust screw to keep the horizon line with the star. The mechanism takes the average of the time you made the site in, usually ten seconds as I recall, and you mark your time as half way from the sight start to sight finish.

The Air Almanac was made for quick sights from fast moving planes and is thus much simplified from the Nautical Almanac. Air navigators also pioneered what's now taught at all levels - pre-computing your LOP and just moving it with the actual sight data. Between the simplicity of the calculations and the speed of resolving them on paper with the pre-computed LOP, Dad could, on the old flying boats, shoot half a dozen stars and give the Captain a good fix in under five minutes start first sight to position. That's not counting pre-sight time of star selection and pre-comping the LOPs, which took another five minutes.

That ease also makes the Air Almanac totally wonderful for the small boat sailor. The boat's all over the place anyway and going through the whole Nautical process takes more time for utterly spurious "accuracy". A small boat might also benefit from an artificial horizon, which makes getting a rehabed airplane sextant a good investment.

[SchoonerRat]

A small boat might also benefit from an artificial horizon, which makes getting a rehabed airplane sextant a good investment.

As I understand the problem, a bubble sextant relies on a relatively stable acceleration like you get on a plane. There is a correction to compensate for the speed of an aircraft. On a boat, unless you're in a complete calm, the movement is too large and random to allow you to compensate for the acceleration.

Bubble sextant not so great on a boat unless you have some kind of computer controlled compensator.

[Ian McColgin]

The artificial horizon needs correction not exactly for what we normally think of as acceleration, like of the plane, but rather of the cross between the plane's motion and the earth's rotation, rather like the ballistic calculation needed to toss an artilliary shell fifty miles or so, especially on a north-south line. In practice on a boat, it can be ignored.

However, it's true that on a bouncy boat, keeping your eye on the artificial horizon is a huge pain and often impossible. You can cheat if you have the real horizon and it's rough because you can turn the sextant upside down and crank the horizon up to the star. Even then, however, it really helps to have traditional microscope or marksmanship training where one eye has the detail while the other is on macro.

On that latter, had Cheney that skill he'd never have hit that lawyer since his left eye (guessing but he'd right handed I think) would have been taking in the whole scene. Something similar could be said for his focus in foreign affairs.

Keep your eye on the target, for sure, but keep the other eye on everything else.

[George Ray]

Nice thing about aircraft bubble sextant is the ability to take sights in-your-yard/anywhere and not have to mess with an artificial horizon that are at their best, difficult and limiting.

If you have the chance, get a copy of John Letcher's book, available on Amazon for $5+$4_ship.....

http://www.amazon.com/Self-contained.../dp/0877420823


He used an bubble sextant, but it was one that had the option of switching to from bubble to natural horizon. The long term almanac is now out of date, but even so, he writes more eloquently and clearly than most. The point was to have a totally self contained book with instruction, almanac and sight reduction tables in one place that would do sun and stars. (Long term almanacs usually do not provide moon and planet data)

*************
Another one stop shop is Kobe's long term almanac.

http://www.amazon.com/Long-Term-Alma...3&sr=1-1-fkmr0

There is a sight reduction method/tables included with the book and you are ready to shoot sun and stars.


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Mark V , sextant that John Letcher used and discusses in his book (I have one and removed the averager).
Rather pricey fully restored/calibrated from Celestaire.com ... $750 , can be found on Ebay for less.

http://www.celestaire.com/Aircraft-S...store.tpl.html


An extremely versatile sextant. It is particularly suitable for marine use, can be used in aircraft; and is perfect for back yard practice. Its great versatility lies in the fact that it is a combination marine and aircraft sextant. As such, it utilizes both an artificial (bubble) horizon, and a visible (sea) horizon.
The bubble horizon is most useful on land when a see horizon is not available. It does, however, require dark adapted eyes at night since the radium illumination has diminished over the years.
The Mark V includes: a 2x telescopic optical path, a selectable astigmatizing lens, and 4 sun filters. The elevation scale reads from -10° to +100°. A fine scale micrometer drum indicates to 2 minutes of arc with interpolation easily possible to within less than one minute. The sextant is made mainly of brass and aluminum.
The MARK V was manufactured with some models incorporating a chronometric averager. The averager records 60 different readings over a two minute period and supplies an average reading. It was designed for use aboard aircraft where accelerations produce erroneous bubble positions. Sextant weight with the averager installed is 6 lbs., and without the averager is 4 1/2 lbs. Because of the weight penalty, we recommend the non-averaging model for marine use.
The Sextant comes in freshly, overhauled condition, with comprehensive instruction manual, and calibration report. It does not, however, come with a carrying case. SW 7 lbs.

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Some good reading;
http://sextantbook.com/

[Dave Wright]

What frustrated me after learning the "long" methods was that I would forget them and have to put in much effort to refresh my mind after a few years went by. My recommendation for most foks is to learn a simple method that is much less easy to forget, in fact you can learn and understand it and never forget it.

Start out with "Noon Position" by Arthur Piver, 1963, 31 pages, includes a good enough 4" by 4" chart of L.M.T of Noon for any day of any year. Piver also covers the aircraft bubble sextant here. Old stuff but still available in reprints.

BUT BE VERY SURE TO ALSO GET: "Latitude and Longitude by the Noon Sight , Including a 30 year Almanac of the Sun" by Hewitt Schlereth, 1982, 304 pages. This is the book that will enable you to learn and thoroughly understand the technique permanently. Schlereth also covers Bowditch Tables 29 and 30 which give you a method for when you miss noon by 30 minutes or so on either side.

Schlereth has done a number of books that are quite good and I recommend them.

[Redeye]

Rather nice free books and software on the topic by a generous hobbyist.

http://mysite.verizon.net/milkyway99/id1.html


Links:
Book on celestial nav: http://mysite.verizon.net/milkyway99...tionTeacup.pdf
Book on building an Octant: http://mysite.verizon.net/milkyway99...ley_octant.pdf
Also available are free Excel and Visual basic tools for celestial

That book is great!
I read it last night, and now have a far clearer understanding. I'd hate to have learnt Celestial without actually understanding what's going on when you do it...