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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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  #1  
Old 10-24-2009
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Beating the dead horse of CN

Howdy all!

Before I get started on my questions, I assure you, I have read all the threads here and on other sites regarding Celestial Navigation and have learned a ton so far. My questions are for the serious sextant users.

As a light scout in the Army, Land Navigation is my bread and butter. When I train my soldiers, I enforce the compass, map and protractor as primary means of navigation and to use the GPS to confirm their plots. I teach hasty, primitive means of finding cardinal directions and I am a big believer in knowing how to navigate using the most primitive of resources before I rely on GPS. When I attend ASA 101-108 courses after I retire, I want to go in with a fairly good grasp of CN.

Recently I have purchased an Astra IIIB sextant, nautical almanac (altitude correction tables) and a few books on how to use them. Since I am landlocked for the next year, I can only practice land navigation (artificial horizon) with the sextant. So far, best accuracy is on the order of 1 mile.
So, getting to the point of this post, being a simple minded knuckle dragger, I will start with my issues in finding latitude by double meridian altitude, using an artificial horizon.

To paraphrase some of what I have read and understood, "A meridian altitude of the sun's lower limb ('noon shot'), when an artificial horizon is used, this type of observation may be called a double meridian altitude, because the angle measured by the sextant is twice what it would have been if a sea horizon had been used."
This basically comes down to a ton of math.... "keep in mind, by "height of the sun" I mean height of the sun above the horizon." J. Gottfred.

"Height of sun times 2 (artificial horizon), less index correction gives corrected height of the sun. Divide by 2 to get height of the sun, less refraction of the air correction gives height of sun corrected for refraction. Adding semi-diameter of sun for current date gives height of the sun as observed. Angle between the zenith and horizon, less the height of sun, gives zenith distance.
Subtract south declination of sun for final latitude." J. Gottfred.

2 questions:

Am I just slow, or doing this totally wrong; is it supposed to take several hours to find latitude this way?

Is finding latitude by double altitudes, using an upper limb observation of the sun, which takes about the same amount of time to take readings, an easier or better method?

I will get into finding latitude by double altitudes and longitude from GT, local time, magnetic variation in another post, my brain is kinda fried at the moment. I look forward to your replies to these 2 questions.

Take Care!
Ben
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Old 10-24-2009
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Over my head

When I was 25 miles deep in the SE Asia wargames jungle, I would have given anything for a gps. It was a lot of guesstimation and 'Willie Peter up 200' from the arty guys to get a reference point. A sextant would not have helped w tripple canopy...so I'm not going to be very helpful to someone at your level. But I would think just moving a little...5 minutes and then plotting the second point would do what you are asking. I am interested in seeing the real answer and find stellar and sextant navigation fascinating.
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Old 10-24-2009
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I'm fairly new to CN so please take these answers with a grain of salt but, for what it's worth, here goes...

By "artificial horizon" I take it you mean that you're shooting the reflection of the sun in a pool of water or oil or some such? In which case, yes, the sextant angle will be double the true angle. However the Astra IIIB can also take a bubble level which, although I've never used one, I understood to be equivalent to using the horizon and so you just use the sextant angle as measured.

If you're using the pool of oil method, don't forget that what you're observing as the reflection of the "lower limb" will actually be the upper limb (and vice versa). In other words, the reflection of the sun will be inverted.

Whichever technique you're using, it shouldn't make much difference whether you observe the upper or lower limb. Basically, it's the same, you just need to read off the correct column in the almanac. Usually you'll observe the lower limb simply because it's easier/more accurate to line up it with the horizon but sometimes it may be obscured (cloud, haze, etc) so the upper limb is used.

As to whether it should take hours... don't know. Depends on how quick you are with numbers and whether you use a calculator. I understand that in the old days before convenient calculators, they would figure on many hours to do the reduction so maybe you're doing really well. I always use a calculator (I figure there's no point frying my brain doing something a couple of dollars worth of silicon can do quicker and more accurately) and it still takes a significant length of time although nowhere near two hours.
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Old 10-24-2009
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You are allowed to use a calculator when doing celestial. It shouldn't take hours though, even if you're figuring by hand. A mile off isn't bad - you use your plotted position to give you an idea of where you are, and then take bearings from what's around you to make it more precise. Practicing on land will make taking a sight from a moving deck later much easier.
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Old 10-24-2009
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Thanks for the responses.

I think this process takes me so long is because the use of the sextant is new to me (arrived in the mail 4 days ago) and when I make my observations, I am double and triple checking that I am not forgetting a piece of the equation. Not to mention, as I stated earlier, I am a simple minded knuckle dragger.

At the moment I am not using a calculator, can't find one that isn't being used and the PX here doesn't have any; funny how it's the little things that kill us. For my artificial horizon, I am using a leveled, highly polished plate of "glass" used in advanced military optics. It's good to know the motor pool Chief.

Since there is no real manual that came with the sextant, I am relying heavily on books such as, "The Sextant Handbook" by Bruce Bauer, "Celestial Navigation For Yachtsmen" by Mary Blewitt and "The Complete On-Board Celestial Navigator, 2007-2011 Edition: Everything But the Sextant" by George Bennett and racking the brains of some aviators when I get the chance.

Again, thanks for the replies and the reassurance. I will post again shortly with a few more questions after I mess around with my star finder kit.

Take Care!
Ben
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Old 10-25-2009
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Work your way through Blewitt in particular and you'll get there. Also make use of the many versions of "shortcut" calculation sheets (just add and subtract from almanac inputs/Table 5, etc.). You have the advantage of knowing the lat/lon of where you are already, and so can see immediately how far off you are, and why.
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Old 10-25-2009
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You might also try to get a copy of the Navy's Bible: Bowditch. He taught the entire crew on some of his ships how to navigate with a sextant - 200 years ago. It's a standard issue text at Annapolis.
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Old 10-25-2009
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Bowditch is freely available as a pdf - either chapter by chapter or the entire book - here Maritime Safety Information

I found it a very worthwhile reference... and thanks to all you american taxpayers for making it available...
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Old 10-26-2009
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I have a copy of The American Practical Navigator Bowditch, Pub. No. 9, 2002 on order, so I estimate it will reach me in Iraq in about 2 months.

Thanks for the link Sun, good to have a soft copy I can get into right now.

Until I fully digest all this information, I will hold off on my questions about Longitude from GT, Local Time, Magnetic Variation. It's been quite a few years since I took a trigonometry course, so calculating this is making my head feel like one of the victims from the 1981 movie "Scanners". I figure the calculator I ordered will arrive the same time Bowditch does so, until then I will continue to self teach with the aid of the books I have and pencil the math.

Just out of curiosity, how long would any of you say it took for you to become proficient with the sextant and all of its uses?

Thanks for continuing the responses, Take Care!

Ben
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Old 10-27-2009
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Hi Sandflea,
As a learner, if you get under 10nm off and consistent, you are ok. As proficient, under 2-3nm is acceptable because you are within eyeball navigation.

Have a think about this, but at exactly noon, the sun is due N of you so your lattitude is what you read off the sextant!
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