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Old 05-11-2013, 22:03   #76
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Re: Celestial Navigation

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If you have a sextant , and you need to find the navigation stars , use your DR position , look up the stars azimuth and altitude , preset the sextant and the star will appear somewhere in the view finder

Dave
Ya...good luck doing that.
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Old 05-11-2013, 22:15   #77
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Re: Celestial Navigation

Not sure which model Davis the poster was referring to but I owned a Davis 25 for many years and found it to be accurate enough to navigate pre GPS days, in fact I really liked it, should have kept it.
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Old 05-11-2013, 22:16   #78
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I would not recommend the lowest cost Davis (lifeboat sextant ) to learn with. I used the lifeboat sextant and an Ebbco plastic sextant on several passages in the Pacific..... Good Luck ____Grant.
Good advice Grant... So I decide to ignore it sorta. I bought the cheap one for $20 on EBay. I'll buy a better one when I figure out what I'm doing and give the Davis to my grandson.
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Old 05-11-2013, 22:25   #79
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Re: Celestial Navigation

My crew member SkiprJohn and I are going to take shots going down the coast of Mexico. I'll let you know how we do.
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Old 06-11-2013, 09:30   #80
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Re: Celestial Navigation

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Dacust, High angle sights are very difficult. At 80 degrees or so, it is very hard to get any accuracy by bringing the sun down to the horizon. The only way to check your accuracy is to wait 24 hours and do it again. _____Grant.

In this case, there is an easy way to get a position using a sun sight.
For an altitude to be this high, the suns declination is going to be close to your latitude.
Take a couple of sights, say 10 minutes either side of meridional passage.
Convert the suns dec and GHA to lat and long, plot on the chart.
Reduce your sight and calculate the TZD. Then plot a position circle with radius equal to the TZD.
Make an allowance for distance run between sights, and you end up plotting two position circles which will cross in two places.
You can usually pick the correct cross as your position.
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Old 06-11-2013, 09:34   #81
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Re: Celestial Navigation

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Ya...good luck doing that.
What Dave said is the way I have always found a star.
Use the sight reduction tables for stars, work out the approximate LHA for civil twilight, then enter the tables to pick out the altitude and bearing of the star. Set the sextant to that altitude, point it in the right direction, and if the conditions are good, the star will appear in the telescope.
Of course, having a sextant with a star telescope helps.
If you wait until you can actually see the star in the sky and try and bring it down to the horizon, then your close to losing a decent horizon in the darkness.
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Old 06-11-2013, 10:08   #82
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Re: Celestial Navigation

Just some quick feedback. I downloaded the books of Mary Blewitt and Farley among others. Nav in a Tea Cup seems much friendlier with good illustrations. I downloaded Blewitt's book in Kindle form and I'm constantly flipping back and forth between text and illustrations, wish I had it in paper format Farley's PDF reads fine on a netbook screen. I'm not far enough to apprecaite them yet, but downloaded the excel spreadsheet and VB5 program by Farley also... he seems like a great resource for the newbie celestial navigator. Celestial Navigation

Good luck on your voyage and sightings Martin, keep us posted.
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Old 06-11-2013, 20:36   #83
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Re: Celestial Navigation

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Dacust, I am going to again open up an old argument on CR by recommending that you DONT learn to do a noon sight.
....
_____Grant.
I have heard that advice before and I think it has some merit. I think it's like the paper vs. electronic charts. I say the more the better. Why limit your sextant usage to just one time per day and only when there aren't any clouds at the wrong places at that one time?

-dan
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Old 06-11-2013, 22:16   #84
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Re: Celestial Navigation

Nigel, your experience may be different, but on my first passage using celestial, I usually did 3 shots of 3 different bodies every night for the duration of the trip. Never did any precalcs, and did it with a 35 dollar plastic sextant(Ebbco). Of course cloud cover sometimes cut it down or didnt allow any shots. This was my first long passage, and I went way into the overkill of navigation, but it was fun, and you can get very quick at working out sights when you do that many every night. A year later on a passage from the Tuamotus to Hawaii, I did a twilight round every 2 days until we got close to land. I wasnt nearly as nervous about my nav skills. I must add that I took the shots and called time to my wife who recorded the time and altitude, so it would be much slower to do it single handed, but you still should be able to get several bodies if it is a clear night. I never found the old adage of twilight coming quicker in the tropics to be true. _____Grant.
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Old 08-11-2013, 06:48   #85
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Re: Celestial Navigation

Hi Grant
One reason I like the pre calc way is that you have opportunity to find low magnitude stars. If your chosen bright stars are hidden behind cloud, they may be all the choice you have.
BTW, I use Plotting Sheet 5015 (published by British Admiralty) for star sights, very easy to work with.
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Old 09-11-2013, 15:06   #86
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Re: Celestial Navigation

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What Dave said is the way I have always found a star.
Use the sight reduction tables for stars, work out the approximate LHA for civil twilight, then enter the tables to pick out the altitude and bearing of the star. Set the sextant to that altitude, point it in the right direction, and if the conditions are good, the star will appear in the telescope.
Of course, having a sextant with a star telescope helps.
If you wait until you can actually see the star in the sky and try and bring it down to the horizon, then your close to losing a decent horizon in the darkness.
Yes...but are you talking about finding a star while on land or on the deck of a boat moving around. On a clear moonless night it's difficult to tell stars apart through the scope. By the time you think you have the star, the time has past.
I'm no "Joe Celestial" but noon shots are challenging enough.
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Old 09-11-2013, 17:35   #87
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Re: Celestial Navigation

Turn the cheap plastic sextant on its side, measure the angle between two objects, then measure the angle between one of the former objects and another object. Take the two angles and set them on a three arm protractor, line up the objects with the angles and the center is your position. Only works for piloting, but is really handy and is dead on. Also works for vertical objects if you know there height (distance off).
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Old 10-11-2013, 14:57   #88
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Re: Celestial Navigation

I was able to learn Celestial over last winter, see my earlier post on this: Self Taught Celestial

Enjoy the experience, it is a fun learn and knowledge of it will make you a better navigator.

I've been using a Casio solar atomic watch for timekeeping, accurate to within fractions of a second when you are in range of the synchronization signal.

The excellent book, "Emergency Navigation," by David Burch is also a great resource: http://www.amazon.com/Emergency-Navi...on+david+burch
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Old 10-11-2013, 15:56   #89
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Re: Celestial Navigation

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Yes...but are you talking about finding a star while on land or on the deck of a boat moving around. On a clear moonless night it's difficult to tell stars apart through the scope. By the time you think you have the star, the time has past.
I'm no "Joe Celestial" but noon shots are challenging enough.
If you are using a computer or navigational calculator for sight reduction, one can simply make a star observation, note the approximate azimuth and enter the time, altitude and azimuth into a star-finder program. This outputs the likely ID for the star, and then you carry on per usual.

My old HP-41 can do this, albeit kinda slowly, and the DOS based one (whose name I have forgotten just now) did it even better. I used this feature a lot when we were using celestial.

Cheers,

Jim
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Old 10-11-2013, 16:22   #90
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Ya...good luck doing that.
Works every time and used to be a requirement for RYA ocean master.

Dave
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