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Old 15-09-2014, 09:18   #1
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Beginner to Celestial navigation

Hi,

For no other reason than to preserve my last few remaining grey cells I have decided to study Celestial navigation and have joined this forum in the hope of getting some help.

I have realised celestial nav has evolved over hundreds of years from the magic nut to whatever is the latest development and it is important not to be confused with the different methods available. I happened to have a copy of a 1981 Macmillans almanac which apparently has the required tables and adequate explanation to work out a sight. Unfortunately the explanations were not sufficiently straightforward for me to understand!.

Undaunted I turned to ebay and purchased a copy of Mary Blewitts Celestial Navigation for Yachtsmen. I have made much better progress with this book and now have a reasonable understanding of GHA LHA SHA Hs Ha Ho HP declination the intercept method etc etc however there are a number of things in the book I am still confused about.

For example. There is an explanation that when a sun sight is taken it is usually the lower limb and a correction SD needs to be allowed for (generally around 16'). However in the worked examples in the book for sun sights no allowance is made for SD?

I look forward to your reply.

Thanks

Mike
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Old 15-09-2014, 09:50   #2
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Re: Beginner to Celestial navigation

Welcome to the forum Mike.

I don't know the book you refer to, but I believe many on here do have this book.

You are correct that when reducing the observed altitude of the sun, that a correction for Semi Diameter has to be applied. While 16' is about average, for a more accurate value, the exact SD value will be found in the daily pages of the current years Nautical Almanac.

No idea why the worked examples in the book have omitted this, hopefully someone else with the book can jump in with an explanation.
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Old 15-09-2014, 10:11   #3
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Re: Beginner to Celestial navigation

Mike,

I too wanted to give this whole sextant thing a whirl, so I bought one of those cheap plastic "practice" sextants to play with. It came with a small booklet that explained the basics of a noon sighting. Now, whenever I'm out there at noon and the sun and horizon are visible, I break out the sextant and try my hand at a sighting. Pretty simple stuff. With just a little practice I've been able to nail down lat/lon within 20 or so miles. With more practice on various correction factors, and with a better sextant, I'm confident I can probably get this down to a couple miles. If nothing else, it impresses my crew!

I haven't even gotten in to other types of navigation using stars, moon, other times, almanac/tables etc. Figure I'll master the basic noon sighting first before I overload my feable mind with that other stuff.

Good luck!
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Old 16-09-2014, 04:57   #4
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Re: Beginner to Celestial navigation

If you read page 27, the last line in Sextant Altitude to Observed Altitude states,"This last correction includes both refraction and semi-diameter, so we have:"

Then she shows the calculations.

Hope that helps.
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Old 16-09-2014, 08:23   #5
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Re: Beginner to Celestial navigation

TESTING
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Old 16-09-2014, 08:27   #6
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Re: Beginner to Celestial navigation

The reason for the test above is I have just tried to post twice but the post would not go.

I should explain I do not have a computer at home due to no signal. I am in the library now and the broadband is pretty poor here also. Perhaps this is why my post did not go. Hope I have better luck now.

Here goes.

Mike
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Old 16-09-2014, 08:38   #7
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Re: Beginner to Celestial navigation

Success!

Thanks for replies.

I do not have my book to hand to check page 27 but have noted the wording and will check tonight. From memory my book is the 10th edition, not sure if any of the editions carry ammendments?

And now to the next confusion.

As I said I have 1981 Macmillans and now the Mary Blewitt book. While studying corrections to apparent altitude for the moon last night I expected the almanac & the book to give identical corrections. The information is presented differently but I am sure I have understood and have ensured the same corrections are included in both cases. However with an assumed App Alt of 40 degrees macmillans gives a correction of 55.1 min and the book 53.7 min. Neither of these include corrections for HP.

Look forward to more replies.

Mike
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Old 16-09-2014, 08:40   #8
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Re: Beginner to Celestial navigation

I should have mentioned the App Alt is for the Lower Limb.

Mike
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Old 16-09-2014, 10:28   #9
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Re: Beginner to Celestial navigation

Mike,

Most navigators avoid moon sights like the plague, too many corrections and too many vagaries.
However, if needs must, the following is the order of correction to be used for the most accuracy

Sextant Alt
Index Error___________
Obs Alt
Dip ___________
App Alt
Semi Dia (plus correction for augmentation)
Parallax _____________ (Horizontal parallax x Cos. App Alt)
True Alt


Sorry, you will have to excuse the lack of alignment here.
The above is the most accurate in theory, in practice it is useless as you are talking couple of tenths of a minute against the quick and dirty method

In the above example, the Semi Diameter is taken from the daily pages of the almanac, and then corrected for augmentation (the effect that the moons diameter appears to change with altitude)



Most navigation tables contain a table of the moons Total correction
In Norries Table, the arguments for total correction are apparent altitude against HP.
In the Nautical Almanac, it uses the same arguments, but a different layout to the tables.

The different methods all give slightly different results
.
However, in all the cases of total corrections that I have seen, the arguments are Apparent Alt against HP.
I would check the books again and see if they were using different value's of HP.
With total corrections of 55.1 and 53.7, it looks like total correction method was used, but the HP was different
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Old 16-09-2014, 12:39   #10
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Re: Beginner to Celestial navigation

William F. Buckley, (like him or not) has a great chapter in his book "Airborne". Fun to read and to the point.
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Old 18-09-2014, 08:13   #11
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Re: Beginner to Celestial navigation

Thanks for further replies.

Everything noted.

Frank F was spot on. Everything was clear enough in the book as he indicated. I will pay more attention next time!

Mike
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Old 18-09-2014, 09:47   #12
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Re: Beginner to Celestial navigation

Mike, you seem to have a good start at Celestial, but (with due respect to Nigel) dont discount the moon sight. For about a third of the month you can cross a sun line with a moon line during the day, and get a fix rather than just a sun lop which will have to be advanced a couple of hours later to get an approximate fix. When you have learned to do a sun line (not just a noon sight) you build your knowledge from there to stars, planets and moon. I dont know your age, but daytime moon sights are easier than trying to find a hazy horizon without very good eye sight. I used a form taken from Robert Kitteredges book "10 easy steps to celestial navigation" that could be used for any celestial body, and it worked very well. I did many thousands of miles with plastic sextants , but found that the LIFEBOAT SEXTANT was useless for star,planets,moon(at dusk), due to lack of a telescope. An Ebco plastic sextant with a 4 power scope worked fine. I need to break out the books to get the cobwebs out of my brain, but I loved doing celestial. Just my 2 cents worth. ______Grant.
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Old 19-09-2014, 03:53   #13
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Re: Beginner to Celestial navigation

Good point about the day light fixes with the moon, it's the night time one's with a supposedly good horizon which causes problems.

For the day time, do not forget about Venus, that visible some of the time, but the sight usually needs pre-working in order to set the sextant alt first.

Have fun
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Old 19-09-2014, 07:57   #14
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Re: Beginner to Celestial navigation

Thanks again.

The book does explain moon sights used to be difficult but have been made easier with the modern tables.

Nigel. Ref your instructions for taking a moon sight. It looks familiar to me except for the bits in brackets.

(plus correction for augmentation)

(Horizontal Parallax x cos. App Alt)

These are not familiar?

It may be they are covered in the Blewitt book and I have missed them!

Would appreciate an explanation.

Mike
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Old 19-09-2014, 09:22   #15
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Re: Beginner to Celestial navigation

I have considered starting a thread called CELESTIAL FOR THE AGING EYESIGHT. Sun, Moon, and Venus would be the only bodies used. I would have to hit the books, so that I didnt put my foot in my mouth about details, but it might be of interest to those of us in the over 60 set. Maybe just a passing thought. ________Grant.
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