Sextant

[TeeDee]

I keep seeing ( or maybe I'm looking for it...) a lovely Brass Sextant for sale on FB MP

I quite like the steampunky brass look of it but interested if they navigational use of it has a relevance. I guess as a historical ( and maybe also present ) tool its certainly does the job.

Anyone played with one or taught oneself how to use it ?

 

[Seagull]

I retired from the Sea, many moons past and, I continue to take observations of the celestial bodies using a home-made artificial horizon.
Using real instruments which have served me very well and re-playing, once more, the necessary techniques, it is possible to ascertain a position line within 0.1 of a nautical mile.
My peepers are not as precise as my younger days but I can manage a 4-body set of stars and have them resolve into a positional accuracy of, at best, 0.6 of a nautical mile.
Observations specifically for Longitude, by equal altitudes either side of Transit, are, at best, 0.25 nautical miles off True ( taken from Google Earth) .

These days, rather than resorting to the long-winded Haversine method, I calculate via an ordinary school grade calculator, using the sine/ cosine formulae, though I still purchase the annual Nautical Almanac.....an Almanac of Sorts, also exists on-line, it is usually extremely co-incident with the Nautical Almanac, and the only reason I don't use it, is because of its cumbersome layout. There are also some very good, easy to use and freely available on-line spreadsheets.

I also keep a watchful eye on the kind of devices being sold on the well-known auction site and, most of what takes up the bulk there , are not working instruments at all, but mere decoration items.

Oh, these suchlike replica's are interesting insomuchas they do reflect a level of contemporary likeness to the real thing and, indeed the brass effect is certainly eye catching. Visual appearance is very representative of type and are convincing, for what they are.
However, the necessary Devil for the required details needed for accurate measurement, just isn't there at all....nor was it ever meant to be.

As a member of NavList, I can state that there are current posts concerning the search for a circular devise known as a" Pocket Sextant".
These were made, large hand size with a scale accuracy resolved to One minute of arc, and were mostly just about capable of that sort of fine-ness, though the optics did not always provide a small 'scope and were thus a naked-eye instrument.
Real one's are as rare as Hen's teeth, and always exchange hands for a great deal of cash.
Unfortunately, the many replica's of this type of instrument, are as convincing as ever, just as their full-sized brethren.

The replica market is fraught and there has been a significant rise in selling prices, that have been jacked-up just enough to convince the less knowledgable into believing that such a thing is possibly a pukka deal.

If one is tempted by what seems a bargain, then just browse diligently through the whole of the listing and there MAY be a note to the effect of being a replica, some of the sellers do just that.
Thankfully, a sure way to tell replica's , is that they always come with a smart and glossy wooden box...or an unmarked valise of leather.
...why this should be so, when it is obvious that a distressed-looking case will inevitably add greatly to the artifice....I don't know for sure, though I do like to think that the 'not doing so', is a glim ***** of conscience

Regards All
Ceeg

 

[Ozmundo]

It's a long time since I tried any astro. Do you need an accurate altitude for land navigation with a sextant/octant?

I feel the need to get the brasso out now.

 

[Seagull]

I would comment on your post, Oz

However long it has been since your last dabble, the process is the same...as is the satisfaction of a result just as valid as ever it was....more so, in that any land based Observers can easily check their accuracy offset and compare it against known data.

Yes, a well calibrated instrument is a necessity if an accurate result is the aim
At Sea, the most common errors of the actual altitude measurement, aside of the 3 adjustable errors, were due to a vessel movement , rolling, that changed the value of the correction for height of eye. There was also the uncertainty of just where the horizon was....I know it sounds weird, but the truth is that the effect of refraction is rarely the same all around the horizon. Mostly, this factor was assessed via one of the many bog-standard formulae....but we never knew for sure .

The Math/Trig and its process, were more than sufficient to deal with sextant measurements and any permanent scale error...it should be remembered that Marine Celestial Nav was essentially an ocean-passage navigation system and was "capable" to a seamanlike degree........However, the one big limitation was the necessity of having a clear view of the heavenly body and also the need of a clear Horizon, though through the use of a Wrinkle, a suitable horizon could sometimes be accounted...

Land based use of a standard sextant, was used for many years by the various Surveyors and Explorers of old. And, today, with our modern timepieces, we can be absolutely certain of out time quotient , in a way that the Old Un's could not. This means that modern, land based Ob's can be a good deal more accurate, saving the ever present problem of atmospheric refraction.

From my own Ob's's, I have detected changes, in my local refraction and the causes are many...f'r instance,a measure taken over a sight-line over a roof top or a tree in leaf, can be variable in its effect on results...and can change seasonally.
For land based stuff, an artificial Horizon is easily 'fabbed'-up, using a dish of water under a protecting tent of glass.
Conversely, a good thickness ,plane surface mirror can be levelled as evidenced by up-market spirit levels and used for the same thing...........levelling it can still be a pain in the proverbial.
Both these methods work extremely well and , besides wooly error of refraction, the only downside is the tendency to a thermal rolling of the water medium, or other liquid.....(I personally use a modern creosote) which can slightly distort the lower image.

Incidentally , I watched the whole of the recent Rogue Hero's...hoping to see sight of re-enactment of the CelNav that they used.......which would have been , at best, a Tavistock Theodolite. These, having a fitted levelling system, would be used to measure Stars/Planets, Not an altitude to any horizon, but , the dead opposite of the altitude i.e they , having found the local vertical, could measure from their Zenith Point, downward to the object being, 'taken'. For the calculation, I wouldn't be in the least surprised if they used the cylindrical, Bygrave slide rule ...which was equivalent to a slide rule of around 40 feet long.

Anyhow, I have managed to alert myself to the fact that I have been 'waffling-on' about my favourite Hobbyhorse.
Can't help it, I'm afraid, always was a bit of a stargaser.

Rgds All
Ceeg

 

[Pattree]

I know very little about using a sextant and longitude is a dark art but:

the cylindrical, Bygrave slide rule

I used to drool over one of those in the window of Flatters and Garnet in Manchester. I’d seen them being used to six significant figures.
I was stuck with the usual foot long guessing stick that told me that 9x9 = 80.96

 

[demented dale]

I keep seeing ( or maybe I'm looking for it...) a lovely Brass Sextant for sale on FB MP

I quite like the steampunky brass look of it but interested if they navigational use of it has a relevance. I guess as a historical ( and maybe also present ) tool its certainly does the job.

Anyone played with one or taught oneself how to use it ?

What does one even do? ive seen them on films and I think Patrick Moore had one in his scruffy house when he was presenting the Sky At Night. x

 

[Ozmundo]

Thank you Seagull for waffling on!

Sadly my shipboard navigation of late has been limited to leaning around the skipper and peering hopefully at the chart plotter and sounder trying to spot a u-boat and such!

I’ve always been curious about land use of sextants and similar.

I had few a journeys with native boatmen and skippers who navigate purely on the stars and seamarks just by eyeballing it.

I always liked the local fisherman in one of Patrick O’Brien’s novels who knew where he was by the bottom composition from casting the lead.

Had a dig in the book shelves.



Something to start off plus found some brass to clean!