another thread on sextant

[Peter]

Further to the recent thread about sextants or not. As a back up to my GPS, main means of plotting my position, I want to learn how to use a sextant. In "back up" mode what is the idiots way of plotting your position. What training, ie books etc, is available to one who wants to learn how to use a sextant (located in Cumbria). And finally are the plastic sextants any good and what is a good make.

 

[Mirelle]

Easy. Click here....

http://www.opensacandaga.com/celnav.htm

This is written by Captain Frank Holden who sometimes posts on this bb and who when not commanding a passenger ferry between Tasmania and the rest of Australia sails a Westerly Oceanlord and sometimes writes for YW.

It's the best account I've seen.

 

[Guest]

Plastic sextants are fine for back up purposes... they are susceptible to distortion with different temps.

I found Mary Blewitt's classic book pretty good...

Andrew

 

[AndrewB]

The very simplest method ...

The methods given in all the standard astro-navigation texts, such as Mary Blewitt, the current RYA syllabus, and the site advocated by Mirelle, are really dauntingly complicated.

They are designed to give a level of accuracy of a tenth of a mile, and require you to carry large, expensive volumes of the ephemeris (which must be renewed annually) as well as sight reduction tables, if the power failure that wiped out your GPS also means you cannot use a computer based reduction. Unless you keep practicing the calculations, they are hard to get right. Such methods are not really suitable as a backup.

There is a much simpler method that requires you to carry just a couple of pages of tables, giving the daily declination; the 'equation of time'; and an adjustment for the sun's altitude and height of eye (which can be combined). This method relies only on determining the exact time and height of the lower edge of the sun at local noon. You still need an accurate watch set to GMT, but with that it is possible to get a position within 3 miles, well within the accuracy of a cheap sextant. The method is described for example by Moitessier in several of his books, and is the most suitable for a backup.

 

[Guest]

Another way

Book a course with a guy call Stokey Woodall who runs Astro courses on the Pride of Bilbao whenever he's in the country.

He doesnt teach RYA method but a "by rote" system for getting accurate (<1mile) fixes regularly without having to understand the "why". If you can do that, then the why becomes easier to understand while you are already practicing the art.
My first sight ever took me about 10 minutes to reduce to a position and it was within a mile of the GPS position, so hes method DOES work.

 

[Guest]

I use Astro as backup, generally a Sun run sun (& Meridian Pass) every other day and a star fix every three or so. Fraid I use Sight Redudction Tables for Air Navigation and the Nautical Almanac. Its not difficult.

Frankly amazed that people get <1Nm accuracy on a sunsight fix, given the need for an accurate log for the transferred position line. Can see it possibly on a four star fix - on a good day and on a flat sea..... Maybe I need to find the Pride of Bilbao

 

[AndrewB]

Impressed!

If Stokey Woodall can teach a method of astro-nav by which a beginner can reduce a position within 10 mins, to an accuracy 1nm, then he has to be a genius! Even the simple noon-sight method I described cannot be done quicker without practice, and certainly doesn't have that accuracy.

 

[Guest]

Re: Impressed!

I'm sorry if you are rather sceptical about my posting. I'm only reporting what I experienced taking the course.

Subsequent to that trip I havent had much opportunity to keep myself in practice but I have managed some sights, mainly star ones, and the worst has only been 3 miles out.

As for the speed of reduction, 20 years of accountancy tends to sharpen up the numeracy skills BUT most of my classmates were reducing 4 stars in less than 30 minutes.

As for actually understanding the mechanism of Astro, well I've read a number of books since the course and it's now starting to make sense as to WHY it works. That is the difference to the standard RYA courses - SW just shows how to "fill the boxes" and yes that could cause someone to believe their result is "gospel" rather than question it's validity....but then that can happen with the traditional method!

Given that elsewhere there are postings about carrying 3 or 4 GPS's rather than a sextant, at least there are a few people keeping the old practices alive. If there are simple ways to learn the basics then there will always be some who will be inspired by the subject and learn/develop into the more knowledgable Astro-yachtsman like you.

So from one 3 day course alone, 10 people learnt enough to be able to get a decent basic fix with a decent sextant and at least 3 to my knowledge have become inspired to further their learning.

Is that so bad ?

 

[Guest]

Peter
years ago I was totally confused, I read lots of books on " how to use a sextant"none of them proved to be very easy to follow. In the end I bought 3 books on celestial navigation and spend the next three months learning how to use it. I ended up writing myself a simplified version of all my knowledge gained.using the site reduction tables and the nautical Alminac
after successfully taking a Sun site I kept a copy of my site reduction form and a copy of the nautical Alminac with the pages suitably marked and the reductions on that page are used marked in pencil.
this method I only use for sun sites and is fairly straightforward, providing you don't need to know the reasons While the Sun and stars move around the planet's.

I dug it out last year, and after a short while was once again able to use my sextant. I must admit I do use a calculator to add and subtract degrees, but I can do it manually, I'm just a lazy sod at heart.
E-mail me if you like, and the next time I'm down on the boat. I'll dig out my notes, and let you have a copy
a lot of people out there, would have you believe it's a black art, that is a lot of nonsense. I myself use a Yacht sextant.
all the best
Dave
Ps I'm in the middle of moving house at the moment, so I am not sure when I will be next on the boat.

Dave