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  #1  
Old December 18th, 2016, 05:26 PM
Southern Shark Southern Shark is offline
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Default Anyone ever used a sextant?

We'll have you? It seems like it would be good to have if global GPS went down.
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Old December 27th, 2016, 06:27 AM
Chief351 Chief351 is offline
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Default Re: Anyone ever used a sextant?

I've tinkered around with one. Keep in mind that the sextant by itself is of very limited use. To use it for fixing one's position you need a couple of extra "accessories". First, you need a set of astronomical tables that fix the geographical position of prominent stars at various times at Greenwich Observatory, then you need a very accurate clock to make sense of the tables. If you intend to use the sextant on land where you don't have a good horizon as you would on a boat in te middle of the ocean, you need a further device called an artificial horizon.

But then, when you've taken your accurate sights, and properly interpretated the tables, then done the math correctly, and assuming that your clock is really accurate......you will only be within a few miles of your real location. But that's good enough for a boat at sea, where you are able to pick out your destination by eyeball when miles away.

What a sextant does is very accurately measures angles between two oblects. You CAN, however, simpy turn the sextant on it's side and use it to measure the angles between two objects......say, between two mountain peaks or any two other prominent landmarks. Then, given an accurate map, you can then project those angles back on a map to show your position.

Not easy.
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Last edited by Chief351; December 27th, 2016 at 04:55 PM.
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Old December 27th, 2016, 10:45 AM
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ah1g ah1g is offline
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Default Re: Anyone ever used a sextant?

I have used one and have one of the plastic ones that is good enough for practice. To get a really accurate one you will pay about a grand for it. As said they are really good on the ocean not so much on land. And there is quite a bit of calculations you have to go thru to get a fix. I suppose now they have a app for that but since I have not messed with it in decades do not really know.
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Old December 27th, 2016, 04:44 PM
Chief351 Chief351 is offline
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Default Re: Anyone ever used a sextant?

Quote:
Originally Posted by ah1g View Post
I have used one and have one of the plastic ones that is good enough for practice. To get a really accurate one you will pay about a grand for it. As said they are really good on the ocean not so much on land. And there is quite a bit of calculations you have to go thru to get a fix. I suppose now they have a app for that but since I have not messed with it in decades do not really know.
But if you have something to run the app, just use the gps on the same device.

Actually, with a good set of contour maps and a boy scout compass you will fix your position more accurately with faster results than by using a sextant.

Since we're talking navigation here, a little thread drift would be OK, I suppose:

An amazing number of sailboat owners make it from the West coast to Hawaii by just sailing down the coast to a certain departure point - I forget from where - then just heading due West. Just by keeping track of their speed and distance (dead reckoning) they determine when they think they're getting close, then they switch to "navigation by contrail". They watch the sky and start following the contrails of the jetliners making their approach to Honolulu.

Seems crazy, but in days before the invention of the marine chronometer and the really accurate charts that resulted, just about all navigation was by sailing the latitude. The mariner would use a simple device known as a sun stick, and measure the height of the sun above the horizon, and correcting for the amount that the sun was known to peak higher or lower day by day, to stay on the same latitude line. The measurement was made simply by holding the stick vertically and seeing how far the sun climbed in comparison to what it did yesterday. Of course this meant staring directly at the sun for long periods of time, and many of these sailors went blind as a result. But by sailing the latitude you were sure to reach your destination eventually, unless you first ran out of drinking water or died of eating meat that had gone bad or died of scurvy.
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Last edited by Chief351; December 27th, 2016 at 05:06 PM.
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