Plastic Fantastic Sextant

[Rowana]

Anybody got a plastic sextant lying around that they don't use?

I'm not looking for great accuracy or something to go sailing the Atlantic with, I just want something to faff about with and see if I can do the sums. Keep what grey matter is left from completely seizing up, really.

[Nick]

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I have to say that shooting a sight is relatively easy but the sums and plotting did my head in. I did a correspondence course while we were in the Canaries and after a lot of brain-wrenching grief got as far as being able to do a sun-run-sun, but it took me several hours. I abandoned the course half way through, and wouldn't attempt it again unless I had a live teacher.

I'm keeping the sextant and the idiots guide on how to use it in an emergency and will have them on board if we ever do go bluewater again - I reckon being utterly lost would concentrate the mind wonderfully - but short of a lightning strike I find it very unlikely I will ever need it. (We usually have at least three and sometimes four GPS systems on board).

And - in the event of a lightning strike - how many of us could handle the arithmetic to plot a sight without a calculator, which would presumably also be fried. Not me anyway, and if you are dependent on a calculator you might as well have a programmable that does the calclations for you.

Keep us posted as to how you get on - I am sure you ahve a more mathematical brain than mine (most people do!)

[Mark]

And - in the event of a lightning strike - how many of us could handle the arithmetic to plot a sight without a calculator,

Or an accurate time which really means quartz, or a time signal from a Radio - both of which would also be fried.

I'm reading "Dove" at the moment and he says the maths involved in Astro wouldn't tax a 10 year old. Is he using some kind of tables whereas you were working it out from first principles?

My own experiments with Astro have been few and a bit Heath Robinson.

In the Northern Hemisphere I can get a half decent Lat with absolutely minimal kit.

I've worked out longitude from the Sun only once in my life, on a flat calm Solent. It was 100 miles off which I was well chuffed with. If it had been for real it would have put mein the Channel somewhere which would be plenty good enough. I just worked out Local midday and the time relative to GMT midday was my long, give or take a bit. I'm sure there are all sorts of refinements but I didn't make any.

Here's a question. How much less accurate is someone who hasn't learned Astro Nav then someone who has? I'd get a Lat within 60 miles of someone who could to it properly. I don't know how far off my subtracting midday from midday is from a properly calculated long but how far off can it be?

[Mark]

Here's a question. How much less accurate is someone who hasn't learned Astro Nav then someone who has?

I asked on YBW.

Feisty lot over there, apparently my "assumptions" are "inconsistent".

They still seem to have answered my question.

[smartcom]

The maths isn't too difficult, but it is one of those things you need to repeat often enough to get into the swing of it. Or you could make life easier by using a tool like WinAstro (www.winastro.com), which saves the cost of annual tables too.

For time, any digital watch is likely to be accurate enough to see you across the Atlantic, when you take into account the other errors in using a sextant on a small boat.

If you're using a plastic sextant, be aware that it does expand and contract a lot more than a metal one, so check the index error often, and don't leave it on deck in the sunlight before taking your sight!

[Rowana]

I still haven't got my hands on a sextant as yet, so it looks as if my grey matter won't be taxed any time soon! I'm not in any great rush as it's more of a "nice to know" thing rather than a necessity. The GPS tells me where I am withfar more accuracy and certainty than ever I'll be able to muster anyway!

If anyone happens to come across a cheap sextant, please let me know.

[Mark]

I still haven't got my hands on a sextant as yet, so it looks as if my grey matter won't be taxed any time soon! I'm not in any great rush as it's more of a "nice to know" thing rather than a necessity. The GPS tells me where I am withfar more accuracy and certainty than ever I'll be able to muster anyway!

If anyone happens to come across a cheap sextant, please let me know.

If it's just about doing the sums why not just guess the angles or use a protractor or something? It would add to the experiment to use really crude gear. The satisfaction of getting a roughly accurate position with a protractor and a bit of string would be Astro-nomical! (Did yer see what I did there??? Did yer? Did yer?)

[smartcom]

Or there are a few sites telling you how to make a sextant out of a spare CD, such as http://www.zetatalk3.com/docs/Navigatio ... d_2004.pdf