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Old 10-06-2010, 03:38   #16
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<snip>
5) Not trying to confuse the issue, but what you, the navigator, are trying to do is simply measure an angle of a specific object (like the Sun) above your horizon, and if you do that and know the exact time you made that measurement, everything else you're going to is MATH.....and that will give you a Line of Position (LOP)......

If you have a "idea" of where you might be (you are keeping a log, right?), you can use that position (or your current estimated position derived from a DR plot), and plot this against your celestail LOP.....(or even better LOP's from a morning, noon, and afternoon sights...)

And for basic celestial navigation, that's it......
Thanks for a concise and meaningful answer. I know that I am known as one of the gadget-guys and a lot of our "resistance" is the "you gotta be Galileo to even begin to understand this stuff."

FWIW - I think a lot of us intuitively know it "ain't that hard"

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{I'd not recommend it, but, you actually can navigate the "Captain Ron" way......and you'll eventually find land.....("....besides, if you get lost, you just pull in somewhere and ask directions...") }
ASK DIRECTIONS!!!!!!!!!!!!!!!!!!!!!!

Where the hell is that sextant thingie????
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Old 10-06-2010, 04:49   #17
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...and you look just soooo cool and intelligent perched up on deck using one...
Chris
Now I got to get one.
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Old 10-06-2010, 08:41   #18
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If my 12 yr old daughter can do it, even you can do it. Just get online and start searching. lots of free info out there, some really complex, some not so much. Even if you're off a few miles, you should be able to see land from that distance. LoL
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Old 10-06-2010, 08:47   #19
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Thank you all, and thanks to John and Bill in particular for your insights. It has certainly bled away some of the trepidation, at least with respect to the sun and moon sights. I've always enjoyed developing new skills and I suspect this one will bring much satisfaction, as well as being a system-redundant backup.

FWIW, I was surprised at the strength of the emotions that were triggered with this issue (not just here, but when I raised the question at the marina a few days ago!). I thought it was going to come to blows yesterday, with the GPS-kung-fu facing off against the Sextant-karate right there on the dock... Most of us sail, I'd venture, not because it is the simplest or fastest or most reliable means of travelling from A to B, but because there's beauty in it, there's poetry in it, pleasure to he had being able to work with the wind and water versus firing up an engine. To me a sextant falls into the same category - sure, with multiple GPS back-ups on board and sufficient precautions, carrying a sextant probably can't be rationally justified *purely* on the basis of its back-up value. But there's something immediate about it, something not hidden away in a black box or floating unseen in orbit, something that feels connected to the world... Anyone that owns Ashley's book of knots probably already understands what I'm ineptly trying to describe here...

Anyway - many thanks! Now I just need to find a place to put the dang thing...
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Old 10-06-2010, 17:19   #20
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Aloha,
You might just type in Celestial Navigation in the search engine next to my signature and see what kinds of response you get.
For all the electronic only navigators, the sunspots predicted this year might cause some flip flops in your GPS. Its happened before and I'm not certain it won't happen again. A cheapy Davis plastic sextant and a current Nautical Almanac will get you within 15 miles of your true position in a noon sight.
If you decide to just sail for Africa and you sight land, which way do you go to find the port you want? North or South?
kind regards,
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Old 10-06-2010, 18:44   #21
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Simple description of determining position lines?

Is there e.g. a web page giving bare bones description of how I get from taking a sight to plotting the position line.

I have Mary's astro book with lots of pictures showing the world is round, and long descriptions of the calculation process. Seems to me there is a simpler way to lay out the process.

P.S. 'What next' might include learning about the major stars. Stellarium (Stellarium) is freeware that helps you identify stars & planets, and see how they move in the sky.
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Old 10-06-2010, 19:41   #22
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Is there e.g. a web page giving bare bones description of how I get from taking a sight to plotting the position line.

I have Mary's astro book with lots of pictures showing the world is round, and long descriptions of the calculation process. Seems to me there is a simpler way to lay out the process.
....
I used to teach celestial navigation. Send me an email and I'll send you a PDF with some elementary forms and description of how to find your position at noon.

Email is bill at wdsg dot com.

Bill
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Old 12-06-2010, 01:09   #23
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So here is the basics of Celestial as I understand them

1) From the almanac you can determine where on earth any celestial body (sun, moon, stars, planets) is directly overhead at any given second of time.

2) Using the sextant you could measure the angle between your zenith (directly overhead) and the body. For every minute of arc between your zenith and the body you are that many nautical miles away from the body's position on the earth (this is the part I have never seen written down or explained anywhere by anyone and as best I can tell why so many people don't get celestial, they just know the steps and get an answer.) In practice you can't find your zenith, but you can usually find the horizon and 90deg - Horizon angle = zenith angle so one extra math step gets you the info you want. In practice you also have to make several corrections to your sextant reading for various errors.

3) Knowing your range from a known position you can plot a circle of position (CoP). Plot several circles of position and you get a fix, just like with other types of navigation.

4) The plotting is where it all gets tricky. On any reasonable sized map the pencil line you draw for the CoP is going to be 10's or 100nm wide on the chart. Additionally since the CoP will be 1,000's nm in diameter, perhaps 10,000
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Old 12-06-2010, 07:23   #24
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I am searching for a used sextant in working condition. E-bay and Craigs list were unsuccessful. I can't always afford the best but appreciate quality design and construction. Dave
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Old 12-06-2010, 07:41   #25
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The best doesn't always cost the most.

Quality sextants include the Plaths, Simex, Tamaya, Aires, Freiburger, and others.

IMHO, the Plath sextants (either C. Plath or Cassens & Plath) are the best marine sextants ever made. I have two of them...have had others, but if I were looking for a sextant now it would surely be a Plath. I like those from the 60's and 70's.

Right now there are several Plaths for sale on eBay. Search "sextant plath".

The 1968 Plath in Maryland looks good to me; others maybe.

Bill
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Old 12-06-2010, 07:45   #26
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To ensure that a used sextant is indeed in working condition, you will need to get it calibrated and certified. That may cost more than the savings from buying it used.
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Old 14-06-2010, 16:05   #27
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To ensure that a used sextant is indeed in working condition, you will need to get it calibrated and certified. That may cost more than the savings from buying it used.
Um, can't you calibrate it yourself?? I vaguely remember there being procedures to adjust the mirrors and calibrate the sextant?
Tom
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Old 15-06-2010, 12:02   #28
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Sextant calibration

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Originally Posted by teejayevans View Post
Um, can't you calibrate it yourself?? I vaguely remember there being procedures to adjust the mirrors and calibrate the sextant?
Tom
The index error can be user adjusted to a minimum, set the sextant to 0-0'0" and adjust the mirror till the horizon lines up with itself or almost does so.

The hard part is figuring out the variable error thru the range of the sextant, sort of like a deviation card for the compass.

With a scientific calculator, 100'-200' measuring tape and a big room/field you could lay out a number of angles very accurately and check yourself. I think the hard part would be determining the point of convergence for each angle so you would know exactly where your sextant needed to be placed. for plastic sextants I would redo this check at several different ambiant temps.
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Old 15-06-2010, 13:06   #29
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Assuming you have a quality sextant (Plath, Freiburger, etc.) and that the sextant hasn't been abused -- either by dropping it or improper cleaning of the index teeth -- you don't have to worry about calibration across the arc. With Plath's, e.g., the calibration error is shown on a card and is usually zero "This sextant is free of errors for practical use."

Other calibration can be done by the user, following simple directions found in most any book on sextants. These are mostly checking verticality of mirrors and setting the zero point on the micrometer, IIRC. Done in a few minutes.

Bill
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