Peter
Member
Feeling bored with lockdown and having acquired a sextant, plastic one, so going to have a go learning how to use. So any recommendations on easy to use books, guides etc for the learning process.
Thanks Peter.
Thanks Peter.
How to use a sextant
- Thread starter Peter
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pandos
Well-known member
Youtube Tom Cunliff...!
john0740
Member
There are many online resources, of course. I am re-learning this at the moment, it being almost 20 years since I did the 'ocean' course. I have found the tutorials at Marine Navigation Books by Dominique Prinet to be good.
The workbook can be purchased which allows working through questions and it contains the necessary table extracts.
The workbook can be purchased which allows working through questions and it contains the necessary table extracts.
johnalison
Well-known member
I have an Ebbco sextant but never used it in anger, though I used to play with it. The best explanation of the geometry I saw was in Hitchcock's 'Voyaging under sail', though the workings-out will now be obsolete.
jdc
Well-known member
The first question to ask yourself is whether you (i) just need a way to just make it work by religiously following a procedure, or (ii) actually want to understand it.
The vast majority of practitioners fall into category (i), for which Cunliffe's book / videos and approach is excellent. However, if you have the skills (something a little above O level but not so far as A level maths) then I recommend approach (ii). This latter means working out a great deal for yourself, but it'll sink in and you'll never be muddled by any of the steps again. The best on-line source of the maths I have found is by Henning Umland.
The second question is how purist you insist on being while leaning. The books tend to insist on absolute 'purity' from the get-go. To my mind the problem with this is it involves four aspects:
(i) using the sextant to derive an altitude of the heavenly body - mostly a physical skill plus a few minor corrections needed such as height of eye above sea level.
(ii) looking up the position on the surface of the earth at that exact time where that heavily body is directly overhead - mostly simple looking up and applying interpolation for the exact time, but lots of tedious steps.
(iii) From (ii), and where you guess you are, calculating what the altitude would be where you exactly where you guessed yourself to be - quite mathematically heavy
(iv) differencing the result of (i) and (iii) to give an adjustment and then plotting this on a scrap of chart you've constructed for the purpose - graph paper, and simple subtraction so not hard.
While you certainly have to do all 4 eventually, it means a single trivial error in any one means the result is wrong which can be disheartening when just starting out. I prefer to use a computer program to do steps (ii) and (iii), which means 10 mins after one has first clapped eyes on a sextant one has a position line! Then only when confident moving on to looking up the lat (called Greenwich Hour Angle) and long (called Declination) of the sun or star etc. I use this one.
The vast majority of practitioners fall into category (i), for which Cunliffe's book / videos and approach is excellent. However, if you have the skills (something a little above O level but not so far as A level maths) then I recommend approach (ii). This latter means working out a great deal for yourself, but it'll sink in and you'll never be muddled by any of the steps again. The best on-line source of the maths I have found is by Henning Umland.
The second question is how purist you insist on being while leaning. The books tend to insist on absolute 'purity' from the get-go. To my mind the problem with this is it involves four aspects:
(i) using the sextant to derive an altitude of the heavenly body - mostly a physical skill plus a few minor corrections needed such as height of eye above sea level.
(ii) looking up the position on the surface of the earth at that exact time where that heavily body is directly overhead - mostly simple looking up and applying interpolation for the exact time, but lots of tedious steps.
(iii) From (ii), and where you guess you are, calculating what the altitude would be where you exactly where you guessed yourself to be - quite mathematically heavy
(iv) differencing the result of (i) and (iii) to give an adjustment and then plotting this on a scrap of chart you've constructed for the purpose - graph paper, and simple subtraction so not hard.
While you certainly have to do all 4 eventually, it means a single trivial error in any one means the result is wrong which can be disheartening when just starting out. I prefer to use a computer program to do steps (ii) and (iii), which means 10 mins after one has first clapped eyes on a sextant one has a position line! Then only when confident moving on to looking up the lat (called Greenwich Hour Angle) and long (called Declination) of the sun or star etc. I use this one.
Yellow Ballad
Well-known member
I watched these during the first lockdown and everything clicked in place, well worth investing the time in watching.
neil_s
Well-known member
The little booklet that came with your Ebbco is a good start. It tells you how to use the sextant and do a noon sight and work out your latitude. Beyond that, Step by step sextant user's guide by Andrew Evans will get you going - Google will find it for you!
Sandy
Well-known member
A very recent thread
Celestial Navigation by David Burch
I loved the simple step by step layout of the book and sections if you wanted to delve deeper.
Celestial Navigation by David Burch
I loved the simple step by step layout of the book and sections if you wanted to delve deeper.
KAM
Well-known member
I'd second Tom Cunliffes book although he does skip over a couple of bits too briefly particularly the moon sights but all you need is there. Much more fun not using calculators and computers. The online almanac is very good. The trick is working out which pages to print out. You don't actually need that many. Very satisfying when you get a 5 point star fix in the back garden.
LadyInBed
Well-known member
reeac
Well-known member
I've never used a sextant but something over 20 years ago I thought that I'd like to understand this area so I bought a secondhand book on the subject. Trouble was the notation ... the book included "A Short Glossary of Terms used .." which ran to 8 pages and 126 different terms. Furthermore these terms were far from self explanatory. eg. the Zenth Distance is actually an angle whilst the Declination, which is actually an angle is described as a Great Circle Arc. In the end I just worked out that a noon sighting reduced the geometry from 3 dimensions to 2 so one could just draw a triangle to explain things. The same 2 dimensional method can be used for nighttime sighting of Polaris. I stopped at that point.
