Basic sextant use

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Babylon

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I want to learn the practical basics of sextant use (setup, sun-sights, distance-off, etc) but not have to sign up to the the whole Ocean Yachtmaster caboodle at this stage. Once I've got the hang of it, then I reckon I can learn the rest from books and practice in my own time.

Would anyone be able to teach me this, or reccomend someone who can?

Solent or South Coast would be more convenient than Gib or The Orknies.
 
Babylon said:
I want to learn the practical basics of sextant use (setup, sun-sights, distance-off, etc) but not have to sign up to the the whole Ocean Yachtmaster caboodle at this stage. Once I've got the hang of it, then I reckon I can learn the rest from books and practice in my own time.

Would anyone be able to teach me this, or reccomend someone who can?

Solent or South Coast would be more convenient than Gib or The Orknies.
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If you happen to "sign up to the the whole Ocean Yachtmaster caboodle", you might be disappointed anyway.

Actual practice with a sextant is very limited, with most of the time given over to 'sight reductions' - the difficult sums bit + other stuff of course.

This might help http://www.google.co.uk/search?sour...&resnum=7&ved=0CFEQqwQwBg&fp=484139f58e92cc35
 
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Just start with noon sights. They're the simplest of the lot. It'll give you practice at using the sextant, bringing the sun down to the horizon, and rocking the sextant to ensure you've got it on the horizon.

There's a few fair small books on the subject available. I've got Mary Blewitt's & Tom Cunliffe's and prefer the latter.

I'm pretty inexperienced with the sextant myself, but I can get noon sights down to about 1 or 2 miles, I'm nowhere near as accurate when the sun is moving faster.

If you can find anywhere where you have a southern horizon available to you, you're laughing, as you can play with the sextant on land regularly, which should really allow you to get more skilled at the sextant use. I've found the roof of a building that requires an 8' correction, but I have to sit in exactly the same place everytime (so it's only any use for noon sights).

There's calculators available on the web, so you can leave the maths bit till later. Even when you get around to the maths, it is all tabulated and if you develop a decent proforma for yourself you can get it down to a pretty simple level.
 
While I'd be happy to show you the ropes I'm a bit far away! There's no reason you shouldn't teach yourself the basics then maybe get someone to go over it later to give you some of the finer points.

Start with the sun, it's probably all you'll ever need for simple navigation. Put all the shades down, set the sextant to zero and point it at the sun then flip back the shades one at a time starting with the most dense until you can see the sun clearly without discomfort. You are now reasonably safe and can experiment.

First thing to do is practice capturing the sun in the field of view. Some prefer to set the sextant to zero and point it at the sun then move the arm forward as you lower the sextant to the normal position, others make a guess at the setting and wave it around a bit till they see the sun flick across the field of view.

You must determine the index error which is easily done by setting the sextant to zero and looking at a distant horizontal line then tweaking the adjustment until the line is continuous.

Handling the sextant correctly is important. Never pick it up by holding the moving arm, put your fingers through the holes in the main frame and hold that. Never put it down anywhere except in its box

Now you've got the basics it's a matter of practising bringing the bottom edge ('lower limb') to sit on the horizon, first on land then at sea.
 
john_morris_uk said:
I suspect that the OP might need to check for side error and parallel errors etc on the sextant if he has only just got it. Mind you if the images come down anywhere near each other, it will be good enough to practice with.
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That's what I meant by bringing in the finer points later. As long as the sextant hasn't been badly abused it should be capable of giving some decent results, at least within the limits of the beginner's ability.

Once he has mastered the basics someone needs to show him how to check and adjust before venturing out to sea to do it for real.
 
I too would be interested in a local south coast (between Brighton and Hamble) short course (perhaps one day on a weekend) on sextant use. I have several books and manuals on sextant adjustment and use, including sight reduction tables, but it's always useful to have someone knowledgeable show you how to do this in practice.
 
Sgeir said:
We carry one, but have only ever used it as practice for distance off, which is very accurate even when the boat is moving under sail. Much better than using a handheld compass.

There are several free easy-to-follow manuals that may be downloaded;

Ebbco - http://fer3.com/arc/img/109211.ebbco-sextant-manual.pdf
Andrew Evans - http://www.minitransat650.com/Sextant_Users_GuideV6.pdf
Davis - http://www.davisnet.com/product_documents/marine/manuals/00026-710_IM_00025.pdf
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I can't seem to get the Andrew Evans link to work?
 
I notice that no one has yet suggested using an artificial horizon for practice - use the reflection of the sun in a saucer of engine oil and divide the altitude by 2. It's ok at this time of year, but mid summer, twice the noon altitude will probably exceed the the sextant's arc.
 
Neil said:
I notice that no one has yet suggested using an artificial horizon for practice - use the reflection of the sun in a saucer of engine oil and divide the altitude by 2. It's ok at this time of year, but mid summer, twice the noon altitude will probably exceed the the sextant's arc.
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I avoided mentioning that in my reply because I don't really think artificial horizons are helpful to someone learning how to use a sextant for the first time. As a source of data for practising sight reduction they have their uses but they are more difficult than the 'real life' situation.
 
snowleopard said:
I avoided mentioning that in my reply because I don't really think artificial horizons are helpful to someone learning how to use a sextant for the first time. As a source of data for practising sight reduction they have their uses but they are more difficult than the 'real life' situation.
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No question, especially trying to see the reflection of stars and planets in the oil! However, the OP might find the mechanics of taking a sight easy enough to master but reducing the sight might need more practice.
 
This kind of thread pops up several times a year, and there are always a dozen or so of the usual suspects eager to help out with their brand of acquired know-how.

I have a dimming memory that Merchant Navy Cadets were required to keep a logbook of their sights and reductions, typically with several hundred entries, for inspection by the 'Lords of Creation' at London BoT Examinations for their 'tickets'. Royal Air Force student navigators had to do much the same, using astro cabins out on the airfield, and again a well-filled Sights Logbook was demanded.

Each cohort of aspirant navs was taught and tutored by acknowledged experts. It was accepted that 'the first few hundred sights were just to get familiar with the procedures, and the next few hundred were to build accuracy and reliability....' There were many tricks of the trade passed on by the professional instructors, some of which hairy old nav John Goode will touch on if you corner him at a boatshow, such as tricks and tweaks in confined waters like the Gulf of Suez...

So, hands up those who have been taught to use a sextant - and use the results - by a qualified professional, and 'fess up those who have both taught and examined themselves..... :D
 
Lady Campanula said:
So, hands up those who have been taught to use a sextant - and use the results - by a qualified professional, and 'fess up those who have both taught and examined themselves..... :D
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I would certainly fall into the latter camp!

However, I did have many years experience of using scientific and optical instruments, Cunliffe's book (and this forum!) for reference and the examination of my reduced sights by reference to GPS. Let's face it, it's not rocket science. Still have to do it on a boat tossed about in a F8, with the sun glimpsed for 2 secs through a break in the clouds, though. That'd sort the men from us boys.......
 
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Isn't it about a basic level of competence and confidence? We are not asking people to invent the wheel or discover a new planet, after all. Just be happy.

Yeah, there are millions of people better at it than I will ever be, but I just spent 2 weeks bouncing down to the Canaries and back from Gib, with 4 more enlightened converts, plus a good workout for me.

Its leisure and pleasure, after all !!

Dont forget, an ex is a has been and a spert is a drip under pressure.....
 
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