Sextant...

[cagney]

Originally Posted by jbowering

More than one set of angles is obviously better. In an anchorage set up your sextant with a horizontal angle between two points and if it changes - you are dragging.

Yes "Horizontal Sextant Angles" are rely useful and very accurate, but
you need at least 2 angles between 3 different objects for a position.
With one angle between two objects you get one position "line", that is actually a circle. Hence, it is possible to drag without a change in angle. Even with 3 objects and two angles, you have to be careful exactly how you choose your objects:

Avoid a selection of objects which results in a "revolver" or "swinger"; that is, when the three objects observed on shore and the ship are all on, or near, the circumference of a circle (...). In such a case the ship's position is indeterminate by three-point fix.
Bowditch 1977 Vol I Chapter XI p 309.

Chapter XI in my edition of Bowditch is called "Use of Sextant in Piloting" and is a good source. I failed to find this subject in a quick search of the present, on line, Bowditch.

[RBEmerson]

jbowering hits several good points. Buying anything on eBay is something of a gamble. However, in all the buying I've done from guitars to motorcycle parts to boat gear, I've only been really stung once or twice, had a handful of disappointments (not what I expected but not bad enough to gripe about), and had a number of truly pleasant surprises. Among the wins are my present Plath, originally bought in Panama and known to have done some traveling. But, yeah, eBay is something of a coin toss.

The current price for the Astra IIIB at Celestaire is $575. Defender is selling it for $569.99 (a whacking great $5.01 saving! ). For the $5, I'd stick with Celestaire and be glad of the pre- and post-sales support.

"[A] relatively cheap Plath or equivalent"... they are few and far between. Even badly corroded Plaths go for reduculous amounts ($400+ isn't uncommon). There's a (purported) ship breaker in India who claims he's selling off sextants found in ships that are to be broken up. However, his prices are surprisingly high and, of course, there's the matter of the real provinence and condition of these items. I had some correspondence with this seller over some other items and the answers were ...ah... ambiguous (and not just a matter of language complications - the replies were written in quite fluent English).

Frankly, if dropping $500+ on a sextant is out, wait for a Davis (even most of the good ones will go for at least $150-200, or close to retail) but understand it does have its limitations (the optics issue).

Um, for horizontal angles, a good, simple fluxgate (e.g., the old Autohelm fluxgate hand compass) or even hand bearing compass is more practical. But for those wanting to turn a sextant on its side, I highly recommend John Budlong's "Shoreline and Sextant" (and, of course, his "Sky and Sextant" if the sextant's held vertically). Although out of print, they're available through the used book web sites and search engines such as ABEBooks.com and UsedBooks.com. (Noting Cagney's comment about not finding something in the on-line Bowditch, use these sources to find older, printed editions of Bowditch - the newer editions have been trimmed significantly to fit into one volume. Granted the older editions are dated in some regards, nonetheless, much of the material is still as relevant and useful as ever.)

Added:
I find it ironic that sextants are clearly not in use to any great extent, but their prices act like those of a commodity of limited supply. The number of for sale notices that include "thought I'd use it but haven't" are countless. Makes ya wonder... [/headscratch]

Disclaimer:
I have no connection with Celestaire, Defender, Davis, or C. Plath save as a customer.

[btrayfors]

Originally Posted by cagney

Yes "Horizontal Sextant Angles" are rely useful and very accurate, but
you need at least 2 angles between 3 different objects for a position.
With one angle between two objects you get one position "line", that is actually a circle. Hence, it is possible to drag without a change in angle.

A useful technique for position fixing is to combine a horizontal sextant angle (which gives you an arc) with a compass bearing to a plotted object ashore (which gives you a fix on the arc).

Also, a single horizontal angle can be useful in keeping you outside a plotted obstruction -- like a reef or rock or wreck -- provided that you choose the two objects carefully and plot the arc on a chart. Then, any sextant angle greater than XX degrees signals danger...you're inside the arc.

Bill

Bill

[alan2]

RBEmerson, and anyone else interested:

I have both of Mr. Budlong's books. They appear to be interesting, one of these days I intend to read them or at least read through them. Seems I used to get a lot more done, BEFORE I retired than I do these days, when I have unlimited time.

As for taking bearings with a sextant, certainly it can be done, however I would think that for routine use, most sextants are more delicate than is needed fore this use.

The Astra 111B, according to Celestaire's 2009 Catolog, is $599, call it $613 with domestic shipping. Seems never-the-less that respecting that instrument, one gets a handful of sextant at a quite reasonable price, compared to Plaths and Tamayas, the scope is "sold separately" with the latter.

Astra sextants seem to be "regular" items on E-bay, some claimed to be ex Mechant Marine Academy equipment. Prices often start out on the low side, but escalate toward listing's end date. Re a difference of $100 or so, one might be better off with a new item, as they come with a warranty. Pay your money and choose.

[RBEmerson]

Budlong is good reading and he supplies a range of things that are handy to know. There are, of course, tons of books of the subject of navigation, and a variety of approaches within the collection. For the die-hard anal, I recommend Precision Cruising (something of an oxymoron, at least in some regards, IMHO). One of my faves for general instruction is, unfortunately, only available in German as Navigation nur zum Ankommen (Navigation Enough to Arrive) by Bobby Schenk.

Sextants shouldn't be used to drive nails, of course, but treated with respect, they'll put up with an amazing amount of nonsense. Horizontal bearings won't bother a good sextant.

$599 for a IIIB? Oops... I was working from the web site's quoted price of $575.

I own both a IIIB (one of the older models with the "faux leather" on the scope) and a C. Plath from 60's. Some days the lighter Astra feels like the right sextant to work with and some days I like the extra heft of the Plath. It helps that the Astra will take Plath scopes, so I can strap on the Plath monocular. The Plath bubble horizon would work, too, if I could figure out how to power the light in the horizon (needed to illuminate the bubble, day or night) without chewing up the wiring on the Astra or the Plath unit. Decisions, decisions, decisions...

[alan2]

Re the lighted Professional Bubble, they stand $950, almost twice the price of my Astra. The Celestaire PBH is 59, and not lit. If all else fails, there is always daylight, though it's tricky to use, or is it just my limitations?

[RBEmerson]

Bubbles are an exercise in patience and slow-motion stability. For morning and afternoon shots, you can always use a reflecting surface (Davis, of plastic sextant fame, makes one). In this case, you use the sun's reflection in the artificial horizon as the "horizon" and bring the sun's image, via the index mirror, down the horizon or reflected sun image. Divide the altitude by 2, and use that as Ho, with no need for adjustments for dip or height of eye. You can do the same with the moon's image, although it moves fast enough to make the shot something of a challenge. See the Davis Mk 25's instruction sheet ( click here ) for more on this.

[alan2]

RBEmerson:

Vis-a-vis the long history of Celestial Navigation, and books thereupon, one would think that just about anything there was to say on the subject would already have been said, several times over. Notwithstanding this conclusion of mine, quite possibly wrong, people still produce wordy tomes thereon, and people, myself included, buy them. Go figure.

On sextant sales and Ebay, Russian surplus instruments appear continually, sometimes with a single scope, sometimes with 2. From what I've read of these, they are good instruments, appearing to be copies of the Frieberger line of sextants. Problem with the Russian instruments is the telescopes, some of them being astronomical, that is they present an upsie down image.

Re payment, the sellers of these seek payment by wire transfer, which according to a banker I spoke with amount to the following. Your money, once ent, is gone. dditionally, the bank charges a fee for wire transfer, and shipping costs can add $100 to the deal. Should it turn out that there are problems with your purchase, recovery could prove "difficult" to say the least, the seller being overseas. Prices of these Russian instruments have gone up too.

[alan2]

RBEmerson:

I have one of the Davis Artificial Horizons, also one of the Celestaire Practice Bubble Horizons. See my question headed Artificial Horizons, can anyone help, and or the same as follows.


In latest edition, 1994 edition of Mary Blewitt’s Celestial Navigation for Yachtsmen, page 53 under Practice Sights, one finds the following, re the Davis Type Artificial Horizon, essentially a dish of water or other liquid, with it‘s own sun shields, side panels and clear wind shields. “Corrections for index error and refraction must be applied to arrive at observed altitude.”
Fair enough until one looks at page 22 where one finds the following. Nautical Almanac tables already contain refraction correction. Also, index correction is obtained from “zeroing” one’s sextant, then viewing the natural or sea horizon through the sextant’s optics. If there is a “break” in the horizon line, turn the micrometer wheel until it’s leveled out, and read the vernier. The number of minutes shown, if any, is the index correction. “If it’s on, take it off, if it’s off, put it on”, as the saying goes. Unless one has previously taken shots with this sextant, how does one arrive at the referenced index correction, or might this be a printing or editing error that got by?
Re the Celestaire Practice Bubble Horizon, page 13 of Celestaire’s 2009 catalog, see page 12 for Davis unit, one finds the following instruction for determining the BC (Bubble Correction). Using the Known Position Method, “take several observations from a known geographic position (GPS does serve a purpose), and compute the lines of position normally. You may attribute the average error to the bubble, and subtract as a correction (BC) for future use”. My questions follow.
Re the several observations mentioned, I assume the following. One takes several sets of sun shots. Reduce each set of shots, and plot them. You have already plotted your Known Position. Measure the displacement between calculated position plots and the plot of Known Position.. Say you took three sets of shots, the average error, displacement between their plotted positions and that of Known Position being say 12 nautical miles, the BC would be 12 minutes of arc, 1 minute of arc equaling 1 nautical mile. Next question is, re “subtract this as a correction (BC), subtract the obtained BC from what number. I use the USPS SR 96a form for reducing sights.
If it turns out that I’m way off base here, having seriously misunderstood the directions, someone please explain, so that I might correct the error of my ways. Thanks for reading through the above, and for the benefit of your wisdom, whomever you might be.

[RBEmerson]

The books... the basic business of celestial navigation hasn't changed a lot in at least the last 20-30 years. The equipment has changed some (for example C. Plath was bought up by Litton and tossed down the rat hole, IMHO) and the reduction process has gotten much easier thanks to better programs on PC's and calculators. So there's some grist for the book mill. But I think what happens is at some point either a writer or editor figures out that all the books to date haven't really quite gotten the concepts across, that people keep saying "isn't there a book I can understand?!?" and one more crack is taken at writing the The Great Celestial Navigation Book. And people, who for one reason or another, find the whole business of sextants and so on compelling, buy the books in hopes It Will All Make Sense.

I've never fooled around with a "SNO" or "CHO" sextant but I suspect they're decent enough to satisfy the demands of the Soviet fleet. And it's probably no surprise they look at lot like a Plath (Freiberger sextants are something else). There's nothing wrong with an inverted image. The goal is to get the object to kiss the horizon, not sightsee. If the sun has to come up to the horizon, sobeit. But, yeah, I can understand that it feels funny.

I dealt with one Russian seller and the process really is a little scary. The truth is, the overall situation is there's so much corruption that short of meeting in person (brown paper bag full of rubles, anybody?), only a bank wire has a reasonable chance of reaching the seller. But it sure is a huge leap of faith to fire off $300-400 and hope for the best. In my case, I made out very nicely, and got my horizon and monocular for relatively little money. But I can't say as I plan to do it again.

[RBEmerson]

Originally Posted by alan2

RBEmerson:

I have one of the Davis Artificial Horizons, also one of the Celestaire Practice Bubble Horizons. See my question headed Artificial Horizons, can anyone help, and or the same as follows.


In latest edition, 1994 edition of Mary Blewitt’s Celestial Navigation for Yachtsmen, page 53 under Practice Sights, one finds the following, re the Davis Type Artificial Horizon, essentially a dish of water or other liquid, with it‘s own sun shields, side panels and clear wind shields. “Corrections for index error and refraction must be applied to arrive at observed altitude.”
Fair enough until one looks at page 22 where one finds the following. Nautical Almanac tables already contain refraction correction. Also, index correction is obtained from “zeroing” one’s sextant, then viewing the natural or sea horizon through the sextant’s optics. If there is a “break” in the horizon line, turn the micrometer wheel until it’s leveled out, and read the vernier. The number of minutes shown, if any, is the index correction. “If it’s on, take it off, if it’s off, put it on”, as the saying goes. Unless one has previously taken shots with this sextant, how does one arrive at the referenced index correction, or might this be a printing or editing error that got by?

First off, I have to say that I've never been exactly blown away by Blewitt. The book just doesn't sing for me. Also, I only have a 1975 printing - the references to p.22 and p.53 don't work at all with my copy.

OK, no question about apply index error and refraction even with an artificial horizon. Sometimes it can be adjusted out and sometimes it's just plain back and there's no time to adjust it out. Shooting with a strange sextant without checking for side error, index error, etc. is risky. But if you have reason to believe all the other errors, save index error, have been adjusted out, just look at the horizon, line it up, and read off the index error. Easy-peasy.

Height of eye and dip are not applicable with an artificial horizon but additional refraction corrections (see the table on A4 of the Nautical Almanac) may be needed. Otherwise, it's my understanding the tables assume standard pressure and temperature in their contents.

Revised:
I found a later copy of Blewitt that does match up your p.22 and p.53 references.

P.22 is a fair demonstration of what I don't like about Blewitt. It's somewhat misleading to say "[t]he navigator who uses The Nautical Almanac need not be concerned with refraction, semi-diameter, and parallax as such." In fact, of course that all matters. The good news is the people who compile the almanac include the adjustments in their computations and the navigator doesn't have to do it on the worksheet. Blewitt says it's in the computations but handling of the issue, as your comments show, is not as clear as it might be.

On to p.53... my comments above fit the situation seen in the discussion in Blewitt. That is, correct for the sextant's index error and then, if atmospheric conditions are outside of standard temperature and pressure, apply the table on A4, which does twek the refraction component of the corrections. This matters more when using an artificial horizon as the sights, because of the limited altitude available (i.e., 1/2 the sextant's maximum angle measurement), will be relatively low altitude sights.

Re the Celestaire Practice Bubble Horizon, page 13 of Celestaire’s 2009 catalog, see page 12 for Davis unit, one finds the following instruction for determining the BC (Bubble Correction). Using the Known Position Method, “take several observations from a known geographic position (GPS does serve a purpose), and compute the lines of position normally. You may attribute the average error to the bubble, and subtract as a correction (BC) for future use”. My questions follow.
Re the several observations mentioned, I assume the following. One takes several sets of sun shots. Reduce each set of shots, and plot them. You have already plotted your Known Position. Measure the displacement between calculated position plots and the plot of Known Position.. Say you took three sets of shots, the average error, displacement between their plotted positions and that of Known Position being say 12 nautical miles, the BC would be 12 minutes of arc, 1 minute of arc equaling 1 nautical mile. Next question is, re “subtract this as a correction (BC), subtract the obtained BC from what number. I use the USPS SR 96a form for reducing sights.
If it turns out that I’m way off base here, having seriously misunderstood the directions, someone please explain, so that I might correct the error of my ways. Thanks for reading through the above, and for the benefit of your wisdom, whomever you might be.

I'm baffled on the Bubble Correction thing, too. Frankly, I'd just let Celestaire explain this one - send 'em an e-mail and see what happens.

[alan2]

RBEmerson:

Re your response to my comment on books, I expect that here we see that eternal human quest for perfection or Hope Springs Eternal at work.

Contacting Celestaire is likely a good idea. I understand that Ken is "good people" as they say.

As for C. Plath being bought by Litton and "ratholed", corporate America at work here? Were they worried about competition? Strange as there haven't been sextants made in the U.S. since the last of the Navy Mark 3's sometime in the 1970's, as I understand.

As for the artificial horizon itself, given that I likely have no need at all for position finding on land, GPS would serve for that, the atrificial horizon would serve strictly as a practice tool. Problem with the Celestaire PBH is holding the bubble level with the reference line, while "bringing the sun down to the horizon". that sir is a bear, at least it is for me.
Re the German text you referenced, my German literacy, technical German especially, is no way nead adequate to the task, but thank you anyhow. I stopped at feirknockingwheelerinluftbenzinemotor, pardon my failing phonetics, that all one word (double overhead camshaft gasoline engine).

[RBEmerson]

Yep, hope (and manuscripts?) spring(s) eternal.

Ken is good people indeed.

Litton was interested in a NATO contract for navigation equipment for the German version of the F-104 (the ones that tended to drop out of the sky too often). However, to qualify, they had to have a German company to work with so they bought C. Plath (which had gotten into a lot more than sextants and compasses). The "bean counters" at Litton messed with the goose that lays the golden eggs and... pfft! Their parting gift was the Navistar, which looked more like a Star Trek prop than anything else. And then there was the Navistar Professional, with a calculator built into the frame - prototyped but, AFAIK, never sold.

I hear you on the artificial horizon issue. And not much call for bringing the sun down to the horizon while rafting down the Monongahela, is there?

Bobby Schenk is one of those prolific writers of things to do with boats. It's hard to miss his name even when browsing the shelves of Buch Haber in Wiesbaden (well, there is sailing and boating on the nearby Rhine, I guess). He wrote an interesting book about celestial navigation, too, (of course) and it's another book that might do well in English. But my translations skills are nowhere near that good.

[alan2]

RBEmerson:
Wasn't the Navistar a combination sextant/calculator/computer, all in one unit mentioned by the late William Buckley in one of his sailing books? If memory serves, he mentioned the Navistar or something by the Plath people. I think he also made mention of HP software and a navigational calculator with navigation software that loaded into the old HP 65. Fine when it woked, but he supposedly had problems with one he had bought.

You slso noted : I hear you on the artificial horizon issue. And not much call for bringing the sun down to the horizon while rafting down the Monongahela, is there?

Not much call for celestial navigation whilst on the Mon, or likely most other rivers either. As I had mentioned, the artificial horizon is most always a PRACTICE DEVICE, handy for those removed from that sea horizon or largfe laks, which might do too, something with which one might "keep their hand in". If the practice becomes overly complicated, it could become "offputting". I seem to have read something somewhere about the following. During Desert Warfare in WW2, both Rommel and the British, who had units running about in the desert might well have used "Celestial Navigation" , sun and or moon shots, taken with something similar to the Davis apparatus (a dish of liquid, likely oil) to find and chart their locations, for that hoped for return to base. They likely also used "dead reckoning" and 60D=ST too, recording compass headings, speeds and time, woirking with such maps/charts as might have been available.

I will forward my question about the PBH to Celestaire, for their consideration, worst that can happen is they won't bother.

[dacust]

It would seem to me a bubble horizon would also come in handy if there was enough haze, rain or clouds where you can't see the horizon, but overhead you have a break in the visibility where you can see the sun. Limited occasions where this would be the case, but as the "Petite Flower" said: "It could happen!".

-dan