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Sextant and finding out where you aren't

11K views 68 replies 21 participants last post by  titustiger27 
#1 ·
Its old fashioned and archaic and allegedly unseaman like.
Is anyone interested in learning how to use one.?
It's not hard.
There are books, on line courses, and schools who teach celestial navigation. I am not an instructor or a teacher. Just an old fart in an armchair with a computer.
If there are a few people out there who are interested, I can try and explain it. Answer questions and help you to figure it out.
 
#51 ·
Neither mirror appears to be adjustable. 😕

Sent from my SM-G900W8 using Tapatalk
 
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#53 · (Edited)
A quick look at the $20 brass decorative sextant shows that it is only 4 inches tall. :D

Adjustable mirrors are a must! If my Davis Mk3 comes without adjustments (it won't) it is going straight into the trash.

The micrometer grooves on the arc of my Davis Mk15 are much more robust, I can't imagine those "barely there" stamped, pre rounded off, soft brass teeth will last very long. Real sextants are not made of (probably Indian very soft) brass, they are made of much harder bronze or quality aluminum, most of which have a bronze arc for the machined worm gearing. Or of course plastic, but with robust gearing cut into it. It's hard to appreciate the level of accuracy that has to go into machining the micrometer grooving on the arc. It's one of the only things that cannot be adjusted.

I don't see a quick pinch release. I can't imagine trying to screw your way across that arc with the micrometer. Perhaps the backside thumbscrew is high speed?

Personally, for decorative purposes I'd prefer a drawing of a quality instrument, or a working plastic one.

You can certainly navigate with the plastic sextants. If my metal instrument (aluminum and bronze) is good for roughly 2 miles, my Davis is easily good for 10 mile accuracy, perhaps more with real care. I've never seen the cardboard build it yourself models, but wouldn't be surprised if they were better than the 4 inch tall decorative brass for navigation, though not sure about adjustments.

https://www.celestaire.com/marine-sextants/cheap-sextant.html

Having to adjust the mirrors often on the Davis models is good for losing your fear of adjusting. They have thumbscrew adjustments and turn (too) easily so you don't even need a tool. Personally, I've been collimating expensive astronomical telescopes since my youth and I have no fear at all of adjusting my sextant, I need to in order to be accurate.
 
#55 ·
There is really nothing wrong with the Davis plastic sextant as long as you are aware of their idiosyncrasies. Before you take a run of sights you will probably have to adjust out errors. They are very temperature sensitive. Still, I managed to do better than 1 mile reductions with my Mark 25 for my USPS JN and Nav courses. Remember as well that when one will be using sextant sights, you will be out of sight of land and a few miles error in the middle of nowhere is not that important.

I own both a Mark 25 and an Astra III and yes I prefer the brass over plastic for three reasons; larger mirror, greater magnification and much more stable.
 
#56 ·
I can totally understand the fascination of celestial navigation. It is interesting from a variety of points of view.

I had a sextant crossing the Pacific from 2006-2008. It was in a "mahogany" box. Hate to say I don't even remember the make/model. I had the tables, etc. Never took it out once to even try to take a sight. The main reason I didn't was because I had several perfectly good GPS's that were much easier to deal with.

I took a real multi-week course in Miami years before that and what I remember the most was how hard it was - getting a good sight, visibility problems from weather, need for an accurate chronometer, extreme complexity of calculating everything using the tables. It was error prone and time consuming and I never got very good at it. However I can appreciate that many have the knack for it and it is a learnable skill.

I sold the sextant in New Zealand at a marine sidewalk sale at the marina to an eager young Kiwi. I kept the box and use it to store odds and ends. I won't get another, I don't think, to go across oceans, which we plan to do. I might reconsider if there were a way to electronically do the math and table lookups, i.e. just enter the sights to a program/app. I thought someone would have identified one on here by now (not pointing your phone at the sky). I do think about what I would need to do if the GPS satellites all went down though. Basically it would be to use the compass to sail whichever direction the wind would take me where I "thought" there might be land in that direction and be very careful at night. Hope that never happens.
 
#58 ·
I can totally understand the fascination of celestial navigation. It is interesting from a variety of points of view.

...what I remember the most ... extreme complexity of calculating everything using the tables. It was error prone and time consuming and I never got very good at it...
The error and time can be reduced. I downloaded a standardized sight reduction form in PDF, then uploaded it to a converter to word. I opened that in Open Office and corrected the funny parts. Then each time I did a sight reduction I customized the form to my tastes. I chopped out what I didn't like or need. I added little notes of where and how to look in the tables, and what other info to grab that I would need later while on a certain page so I didn't have to come back again. At first I found looking up in tables to be the most difficult part by far, but now my form does a lot of the work, it's all laid out like a flow chart. It gets easier. I found it to be a brain breaker when I started.

...I do think about what I would need to do if the GPS satellites all went down though. Basically it would be to use the compass to sail whichever direction the wind would take me where I "thought" there might be land in that direction and be very careful at night. Hope that never happens.
That will work much better than nothing, but you could do even better with some more fairly basic knowledge. The fussy stuff is to produce quite an exact location. Sailing a latitude line is a great old fashioned method.

We all hope that never happens, but lightning can hit anywhere and I've seen it do some weird things. If electronics are knocked out I would not have big hopes for the computer/calculator method coming through unscathed. Or even the digital clock.

Personally, I'm on land amusing myself with celestial nav - and a digital watch :) . Certainly can't hurt and at below zero temps in Maine sailing is out for me. It's a way to feel closer to our traditional origins. Hey, don't we sail instead of fly or powerboat for similar reasons? We don't sail because it's the latest greatest.
 
#57 ·
The best description of an ocean passage "long periods of tedium, punctuated by episodes of terror."

Giving myself the task of getting fixes relives a lot of that tedium, an presented some purpose in the form of a mental challenge.
 
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#59 ·
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#60 ·
Thank you Jack, I just learned or a new site from your post.

Astro Navigation: All you need to navigate by the Sun; Almanac, Sight Reduction Tables.

It is interesting to me to see your modified form, it is so different from mine, forces me to do the hardest of all things..........think!

I have saved many links and purchased a number of books to get different points of view on C-nav. I feel that helps me as I'm learning at home with no teacher. (insert smiley with gears and smoke for a brain).

I'd read Foolish Muse's ebook Singlehanded Tips. Great guy and a big thank you to him for sharing his insights. He is on this site for those not familiar. I naturally downloaded his Step by Step Sextant User's Guide but have not actually read through it, too much reading to keep up. I did glance through it and saw his form, which yours may be based on. I used a different form entirely and then modified it. I not sure I have the right to post it, and I don't know how anyway - doesn't it need to be hosted somewhere? But I recommend to anyone to start with a form and modify it to fit your own techniques.

I used this one for my basis:

https://my.vanderbilt.edu/astronav/files/2015/05/Sight-Reduction-Form.pdf

I got the site from a thread here on sailnet (ivy league celestial nav course or something like that). All these forms are hard to struggle through the first time, but then it gets easier and easier.
 
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