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#280516 - 05/01/16 05:31 AM Re: Found an interesting new thing to learn... [Re: haertig]
AKSAR Offline
Veteran

Registered: 08/31/11
Posts: 1233
Loc: Alaska
Originally Posted By: haertig
.......and I've gotten a few chapters into Burch's book "Celestial Navigation".

I haven't gotten into celestial navigation, but I've used David Burch's "Fundamentals of Kayak Navigation", and it is a superb book!
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"Toto, I've a feeling we're not in Kansas any more."
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#280517 - 05/01/16 06:59 AM Re: Found an interesting new thing to learn... [Re: haertig]
M_a_x Offline
Veteran

Registered: 08/16/02
Posts: 1203
Loc: Germany
Originally Posted By: haertig
A sextant would obviously have to be accurate, but I don't know what level of precision is needed for ocean navigation, or how precise sextants can actually get. But I imagine there's a point of diminishing return spending more money to get more precision.


Bobby Schenk - a person who did a lot of blue water sailing and navigation - stated in one of his books that the accuracy of a shot with a sextant would be around 5 minutes of angle. Being able to read it with more precision may be nice but does not really improve the accuracy of your fix.
Cassens & Plath is one of the leading manufacturers of nautical instruments for both recreational and professional navigation. Their most precise sextants are state of the art equipment which also serve to show how good Cassens & Plath is.
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If it isnīt broken, it doesnīt have enough features yet.

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#280521 - 05/01/16 12:53 PM Re: Found an interesting new thing to learn... [Re: M_a_x]
Russ Offline
Geezer

Registered: 06/02/06
Posts: 5357
Loc: SOCAL
I have two sextants, both plastic and I consider the least precise model -- the Davis Mark 3 -- to be good enough and it weighs a whole lot less than a precision instrument in brass. Something to keep in mind is that on a small boat at sea you are not standing still; the boat is rolling and pitching always, it's what they do so the line you choose as your horizon is moving as they waves move you in the vertical plane.

Time is also a factor, you need precision in the time aspect of your shot because the look-up table expects you to know when the sighting occurred and it's better to take the shot at the time in the table rather than between times forcing you to interpolate. Really important when you shoot local apparent noon.

They sell cheap brass knock-offs online, but I wouldn't use one for actual navigation without extensive testing to see what it can do. The Davis Mark 25 is for if I ever buy that boat. Meantime, I can take sightings on sun, moon, stars and the occasional planet (Mars & Venus when visible) using the Mark 3 and really, the Mark 3 works.

Does the cardboard sextant come with sun filters?

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#280523 - 05/01/16 03:07 PM Re: Found an interesting new thing to learn... [Re: haertig]
gonewiththewind Offline
Veteran

Registered: 10/14/08
Posts: 1517
You can do basic celestial navigation without the need for the sextant or charts and books. It will not give you a precise latitude or longitude, but if you are not completely unfamiliar with the area you are in, it can keep you oriented and going in the right direction. There are many books and videos on the subject:

Finding Your Way Without Map and Compass

Natural Navigator

These books include many other techniques than just celestial navigation as well.

Ron Hood (RIP) has some useful techniques on video:

Primitive Navigation

The sextant and charts method is interesting, and not a bad thing to know, just difficult, resource intensive, and dependent on on a clear sky.

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#280524 - 05/01/16 04:32 PM Re: Found an interesting new thing to learn... [Re: Russ]
M_a_x Offline
Veteran

Registered: 08/16/02
Posts: 1203
Loc: Germany
Originally Posted By: Russ
They sell cheap brass knock-offs online, but I wouldn't use one for actual navigation without extensive testing to see what it can do.

I handled a few. They are Ok for ornamental use. I wouldnīt use any of the ones I handled for actual navigation. All had alignment errors that could not be adjusted.

Originally Posted By: Russ

Does the cardboard sextant come with sun filters?

Yes, the cardboard one comes with a sun filter.
_________________________
If it isnīt broken, it doesnīt have enough features yet.

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#280525 - 05/01/16 04:41 PM Re: Found an interesting new thing to learn... [Re: gonewiththewind]
haertig Offline
Pooh-Bah

Registered: 03/13/05
Posts: 2322
Loc: Colorado

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#280535 - 05/02/16 01:26 PM Re: Found an interesting new thing to learn... [Re: haertig]
Russ Offline
Geezer

Registered: 06/02/06
Posts: 5357
Loc: SOCAL
If the shipping charges are reasonable, those are very good prices.

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#280536 - 05/02/16 03:27 PM Re: Found an interesting new thing to learn... [Re: M_a_x]
bws48 Offline
Old Hand

Registered: 08/18/07
Posts: 831
Loc: Anne Arundel County, Maryland
Originally Posted By: M_a_x
Being able to read it with more precision may be nice but does not really improve the accuracy of your fix.


The distinction between accuracy and precision is one the crops up in many places, is often not understood, but remains key.

In my experience, accuracy is usually more important than precision.
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#280537 - 05/02/16 05:55 PM Re: Found an interesting new thing to learn... [Re: bws48]
Russ Offline
Geezer

Registered: 06/02/06
Posts: 5357
Loc: SOCAL
Both precision and accuracy are needed. "Without accuracy, what's the point." The operators manuals for the Davis sextants have instructions for adjusting the mirrors and essentially reducing alignment errors to near zero. With errors removed (implying it is accurate), precision is the question and limitations of the sextant design will determine that attribute. The Davis Mark 3 can be read to 2 minutes of arc which is a built-in precision limit; the Mark 25 can be read to 2/10's (0.2) of a minute of arc (12 seconds of arc if my math is right). I'm aware of one high end sextant that is guaranteed to 9 seconds of arc. If they are adjusted properly to remove alignment errors, they should be accurate too. A lot more money buys you a tad bit more precision.

But what is the requirement for a highly precise sextant when due to motion of the boat (combined with the steadiness and eye of the navigator) the best you can expect is +/- 5 minutes of arc? You can read minutes worth of accuracy to a precision of 12 (or 9) seconds -- so what.

That may be one reason the Davis Mark 3 is actually a pretty good choice. Its price, simplicity and ease of adjustment make it a pretty good fit for a sailboat which is low to the water and constantly moving. OTOH, if you are on a largish vessel where the height of the seas is not as significant, and where pitch and roll are much more reduced, a higher end sextant may be worthwhile. In that case you are probably a salaried navigator who has been working at sea for years and your sextant is one tool in your toolbox.


FWIW, my opinion...

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#280539 - 05/02/16 08:44 PM Re: Found an interesting new thing to learn... [Re: Russ]
AKSAR Offline
Veteran

Registered: 08/31/11
Posts: 1233
Loc: Alaska
Originally Posted By: Russ
But what is the requirement for a highly precise sextant when due to motion of the boat (combined with the steadiness and eye of the navigator) the best you can expect is +/- 5 minutes of arc? You can read minutes worth of accuracy to a precision of 12 (or 9) seconds -- so what.

I haven't personally tried working with a sextant, but I've long been interested in various aspects of navigation. Note that 1 minute of Latitude translates to 1 nautical mile of position, so +/- 5 minutes means you are at best within 5 nautical miles north or south of your true position. This agrees with what I've been told by experienced navigators. What they've said is that on a large ship (relatively stable), a good hand with a sextant might get within 1 nautical mile of their true position. On a small boat (a much more "lively" platform), you will do considerably worse.

I think one of the most amazing feats of small boat navigation ever done was by Frank Worsley on Shackelton's 1914 expedition. He successfully navigated the James M. Caird, a 22 foot lifeboat, 800 miles across Drake Passage and the South Atlantic to South Georgia Island.
Quote:
Worsley was faced with the task of navigating the Southern Ocean to South Georgia. There was no margin for error as the James Caird would sail into the South Atlantic if he missed the island; this would mean almost certain death for those in the lifeboat but also those remaining on Elephant Island. Fortuitously, the weather was fine on the day of departure from the island and this allowed Worsley to obtain a sun sighting to ensure that his chronometer was rated.
----------------snip-----------
For most of the voyage, the weather proved to be stormy and so overcast Worsley was unable to take more than a few sightings with his sextant. He described one sighting as "...cuddling the mast with one arm and swinging fore and aft round the mast, sextant and all..." and he would "...catch the sun when the boat leaped her highest on the crest of a sea...". At times the sea conditions were so rough he was braced by the other crew members when taking his sightings. On occasion, the temperature was bitter and each man would spend one minute shifts chipping away ice that coated the top surfaces of the James Caird, affecting its buoyancy. The heavy seas meant there was considerable risk that a man could go overboard.

After two weeks, Worsley began to worry about the lack of sightings and advised Shackleton he could not calculate their position to less than 10 miles (16 km) accuracy. As a result, Shackleton opted to aim for the western side of South Georgia which meant, given the prevailing winds, that if they missed their target they would be carried onto the east coast of the island. The following day, they began to observe drifting seaweed and seabirds circulating overhead, indicating the presence of land ahead. On 8 May, through mists and squalls, the crew sighted South Georgia's Cape Demidov, precisely in line with the course calculated by Worsley. He saw a "...towering black crag, with a lacework of snow around its flanks. One glimpse, and it was hidden again. We looked at each other with cheerful, foolish grins."
For a fascinating biography of Worsley, which explains how he became the amazing seaman he was, see: Thomson, John (2000). Shackleton's Captain: A Biography of Frank Worsley. Christchurch, New Zealand: Hazard Press Publishers. ISBN 1-877161-40-3.
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"Toto, I've a feeling we're not in Kansas any more."
-Dorothy, in The Wizard of Oz

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