Might be time to buy a sextant

[zoidberg]

And if it's fuel distribution in a war zone you want to discuss I can do that too:

Ah, the TonkaToy 'somewhere over Kuwait'......

Not a nice night for sipping some 'getyouhomejuice'.

 

[zoidberg]

Aparently its also called a jiggle pump but the principle remains the same. Shifts 25l of diesel in two or three minutes.
Hyfive Jiggle Self Priming Syphon Hose Barrel Tank Siphon Pump Fuel Transfer 5060361850305 | eBay

I must have a dozen of those.
Bought another one from the stallholder at every So'ton Boat Show for more than a decade - 'cos I could never find where I put it 'where I'd find it when I need it'....

 

[srm]

The talk of precise machining comes from old men with a fetish, not from the genuine need.

Not a fetish but a genuinee need. As a hydrographic surveyor pre GPS I could take a hundred or so horizontal sextant angles a day. Microwave ranging instruments capable of dynamic measurements were just coming on the market when I was training. The accuracy of the charts we produced depended on the accuracy of the instruments we used.
Sextants were used for ocean navigation and again the more accurate the instrument the more reliable the position line. Yes, the Arabs navigated the east coast of Africa by the pole star and a marked piece of wood on a loop of cord aroud their neck, so you could improvise a sextant with a protractor, and get a position line to +/- 60 miles.

Oh yes, an accurate sextant is also a very useful instrument in pilotage situations.

 

[lustyd]

And how many people here are making charts? Most learn as a hobby if they even get around to taking it outside. If a protractor and pencil can do 60 miles, don't you think that cheap or ornamental sextants (which have all the working parts) could do a little better?

 

[capnsensible]

Yes I know what they were talking about, and most of those would navigate just fine. The talk of precise machining comes from old men with a fetish, not from the genuine need. Sure, a highly engineered one is more accurate, but to say the others are useless is utterly ridiculous and calls for a visit to the national maritime museum to observe what has been used throughout history. If you're close enough to something that precision matters, switch to pilotage because your sextant isn't the appropriate instrument at that point.

Weel, apart of course anyone in the global maritime industry who is licenced as Master 200 tons and hugely upwards....

Part of the prerequisites.

 

[Buck Turgidson]

Ah, the Tonka Toy 'somewhere over Kuwait'......

Not a nice night for sipping some 'getyouhomejuice'.

Balkans.

 

[lustyd]

Well yes, but they'll be expensing one I'd imagine so probably not looking on Ebay and probably not getting advice from forums. Still doesn't mean that the slightly less well engineered objects can't be used as a sextant.
My post was simply questioning the "these cannot be used for navigation" stance. Just because it's not accurate enough to draw cartography doesn't mean you couldn't find America with it!

 

[zoidberg]

Balkans.

.....And the same to you, sir!

Oh, I see what you mean.....

 

[srm]

don't you think that cheap or ornamental sextants (which have all the working parts) could do a little better?

Yes the angle will be better. However, are you seriously suggesting that someone should risk their eyesight looking at the sun through the coloured glass fitted to something that is sold as an ornament?

 

[lustyd]

Yes the angle will be better. However, are you seriously suggesting that someone should risk their eyesight looking at the sun through the coloured glass fitted to something that is sold as an ornament?

Now you're suggesting the glass used in proper sextants is unavailable on the open market and has magical properties that are understood only by grand master navigating wizards?
I feel like you're the one buying all the gold plated HDMI cables for the better signal on the TV ?

 

[FWB]

If GPS ever did go down Worldwide, then there would be no point in worrying about your position.
Since you won’t be there much longer. ?

 

[[194224]]

................................... The talk of precise machining comes from old men with a fetish, .................

Well I'm guilty as charged, an old man. But there is nothing fetishistic about my interest in navigation methods and techniques past, present and future. I went to sea in the 60s and the use of the sextant for celestial and terrestrial observations was essential. I maintain that interest to this day and would feel very confident about applying my knowledge successfully if it were called upon remote that need might be. I taught my son astro when he signed up as a deck cadet as it was part of the syllabus. He now has his Master (Unlimited) licence and even he still takes sights and reduces them but just for practice.

As matters have improved/progressed I've fully embraced all the wonders that more modern technology has brought and even though I've still got my Decca Interpolator (!) I follow very closely modern developments in navigation. I welcomed the affordable availability of GPS receivers for non-professional use but it didn't cause me to lose my respect for what we absolutely relied on for our safety in previous years.

Interest and respect do not equate to fetishism.

 

[Mirelle]

In response to "lustyd”:

I have been poking around on ebay.

You can buy a good real sextant from the fifties and sixties for about the same price as a fake. Out of interest I bought a WW2 Tamaya for £87.00 It had spent the post-War decades in the care of a P&O officer as he progressed from Cadet to Master.

I checked the three errors: perfectly usable. Since I already had a Plath that I bought forty years ago, I passed it on to a friend who holds command in my own former employers’ fleet and it’s back at sea again. He doesn’t need it, his ships are well supplied both with electronics and with their own chronometers and sextants, but it goes nicely with a copy of Lecky and he can have a bit of fun.

Unless it has been hurled across a wheelhouse, a sextant from that era is very likely to be almost as accurate now as it was when new.

I don’t see any reason to buy a fake.

There are plenty of good books on sextants. Here’s one:

 

[coopec]

I see this said all of the time as if the sextant is some unbelievably sophisticated contraption. It's a protractor and a stick, sometimes with some lenses and shades. They were making them a thousand years ago, I'm pretty sure even a cheap one is adequate and quite straightforward to check with an actual protractor!

Are antique sextants any good? - Cruisers & Sailing Forums

 

[Mirelle]

The old books are full of warnings about buying second hand sextants. When I bought my first one I had it re-certified by the venerable firm of nautical instrument suppliers from whom I bought it. That is practically impossible now. The makers are out of business and only a firm like Potters (who had an upstairs room devoted only to sextants
as recently as 1982), could ever have afforded a collimating instrument and the skilled staff to operate it.

My other one hasn’t been checked since new but since it came unused from German Navy stores I reckoned it was OK.

I think that if a sextant depends essentially on:

Four things that can’t be fixed:

1. The accuracy of the centering of the arm pivot.
2. The accuracy with which the worm and thread have been cut (for a micrometer sextant)
3. The accuracy of the gradations and of the clamping screw (for a vernier sextant)
4. No distortion in the frame

Three of these are manufacturing errors and the last one is what happens if it’s drop kicked across a steel deck in which case there will be other visible damage.

Four things that might be fixed.
5. The absence of collimation error in the telescope(s).
6. The condition and quality of the index and horizon mirrors
7. The condition and quality of the telescope lenses and prisms
8. The state of the box and accessories
5 and 7 depend on seeing the instrument, 6 and 8 can be judged from a picture on line.

And finally the three errors which the user is expected to check and correct himself anyway…

Then if you keep these things in mind you can probably take an educated guess about the condition of a sextant that you see for sale on line.

 

[lustyd]

An interesting list for those planning to create their own charts, use for their work, or those fetishising these simple objects. As with kitchen knives though, those first learning the skills don't need the ultimate tool.

 

[AntarcticPilot]

The sextant was invented in the 18th century to allow measurements of lunar distance, not "thousands of years ago". It can measure angles of up to 120°, on an arc only 60° long. Because of this, it requires precise construction - errors on the arc are doubled; an error of half a degree on the scale becomes an error of 1 degree in the measured angle. It replaced the quadrant, which required less precise manufacture but could only measure angles up to 90°.

There were earlier devices to measure the sun's altitude, but these were of three types:

1. Devices such as the kamal that allowed a specific altitude to be checked, allowing the latitude of a known point to be followed.
2. The astrolabe, which was never a practical sea going instrument because its horizontal reference is obtained by hanging it from a fixed point - you can imagine how well that would work on a ship at sea!
3. Backstaff and crossjack, which were difficult to calibrate because the scale is non-linear, could only handle angles up to about 60°, and in the case of the crossjack required the user to look directly at the sun without shades!

Basically, to achieve an accuracy useful for even the coarsest navigation requires the arc of the sextant to be divided with an accuracy of half the expected accuracy of the angle measurement. So, to achieve an angular accuracy of 1° (60nm in latitude) requires that the arc be divided with an accuracy of 0.5°, and so on.

 

[capnsensible]

An interesting list for those planning to create their own charts, use for their work, or those fetishising these simple objects. As with kitchen knives though, those first learning the skills don't need the ultimate tool.

Do you buy all your tools at Poundworld?

 

[coopec]

An interesting list for those planning to create their own charts, use for their work, or those fetishising these simple objects. As with kitchen knives though, those first learning the skills don't need the ultimate tool.

I notice you put your location down as " . "

Is that because you are still trying to work it out using a home-made chart and and Indian made "sextant" bought on eBay? ?

 

[lustyd]

The sextant was invented in the 18th century to allow measurements of lunar distance, not "thousands of years ago"

The concept us much older than 18th century. As I said, try the national maritime museum in Greenwich.

Do you buy all your tools at Poundworld?

I did when I was learning, for sure. Did you buy the premium version to learn with every time you tried something new? Was your first compass fully calibrated or a POS out of a Kellogs box?
Have you tried one of these terrible quality protractors of death? Usually a master can make do with inferior tools so I'd be interested to see how good of a fix could be managed by someone with good experience.