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Old 20-10-2013, 09:12   #31
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Re: Celestial Navigation

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Originally Posted by highseas View Post
On a small boat mistakes are easily made using sextant,mostly due to fatigue.Poor visibility also reduces its usefulness.With GPS ,far less likely.If you had to choose one or the other?
While there are many decisions in life that are one of the other (what clothes to wear today, which car to buy (unless you are Bill Gates or Jay Leno), which woman to marry (unless you live in Saudi Arabia or Utah)), navigation methods are not an either/or choice. While the OP did not indicate he was looking for a backup method in his first post he did in post #7.
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Old 20-10-2013, 14:09   #32
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Yep, that is an awesome skill to be able to actually get a longitude from a noon siting.
It sure is. Start 10 min before local noon, grab 3 heights along with times, grab max height at noon for lat, note the times when the sun passes down thru the 1st 3 heights, plot the 6 times on graph paper for exact time of local noon, apply the equation of time and 1 correction and you have long. Much quicker than a morning sight. Even an 11 y.o. can do it!
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Old 20-10-2013, 14:41   #33
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Re: Celestial Navigation

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I want to learn about the sextant as a back up source, when your GPS or other electronic don't work, or when a lightening fries all your electronic, so that I can get back to shore.
Remember to take a mechanical clock!
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Old 20-10-2013, 14:54   #34
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"The lo-tech Navigator" by Tony Crowly and "Emergency Navigation" by David Burgh are great books for the curious navigators.

You can find your position without the time, just with sextant, almanac, pencil and paper. Or build your own sextant. Great books for a long passage!
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Old 20-10-2013, 15:07   #35
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"Celestial Navigation in the GPS Age" by John Karl is another good one.
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Old 20-10-2013, 15:25   #36
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Re: Celestial Navigation

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Originally Posted by Jimbo485 View Post
It sure is. Start 10 min before local noon, grab 3 heights along with times, grab max height at noon for lat, note the times when the sun passes down thru the 1st 3 heights, plot the 6 times on graph paper for exact time of local noon, apply the equation of time and 1 correction and you have long. Much quicker than a morning sight. Even an 11 y.o. can do it!
I haven't heard about this method. All you have done is track the sun. Can you give me a place to check this out. Usually it takes several objects together with reference points.
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Old 20-10-2013, 16:07   #37
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Re: Celestial Navigation

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I haven't heard about this method. All you have done is track the sun. Can you give me a place to check this out. Usually it takes several objects together with reference points.

This link will take you to the instruction manual for a Davis sextant. Tucked away in the manual is a description of a method to find longitude at meridional passage (you'll need to know GMT)

EDIT Sorry link not working, try this one

http://www.davisnet.com/product_docu...0_IM_00011.PDF
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Old 20-10-2013, 16:38   #38
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Re: Celestial Navigation

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Originally Posted by nigel1 View Post
This link will take you to the instruction manual for a Davis sextant. Tucked away in the manual is a description of a method to find longitude at meridional passage (you'll need to know GMT)

EDIT Sorry link not working, try this one

www.davisnet.com/product_documents/marine/manuals/00011-220_IM_00011.PDF
Okay, thanks. I'll check it out.
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Old 20-10-2013, 16:45   #39
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Re: Celestial Navigation

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Originally Posted by thomm225 View Post
I haven't heard about this method. All you have done is track the sun. Can you give me a place to check this out. Usually it takes several objects together with reference points.
If you can work out how long after mid day at Greenwich it is till mid day at your location, you can work out your longitude. The earth rotates 360 degrees per 24 hours, so one degree every 4 minutes.

You need to know the time at Greenwich though.
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Old 20-10-2013, 16:50   #40
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Re: Celestial Navigation

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Originally Posted by thomm225 View Post
I haven't heard about this method. All you have done is track the sun. Can you give me a place to check this out. Usually it takes several objects together with reference points.
The method involves getting your longitude from the meridian passage of a heavenly body, in this case, the sun. It's very easy and it works!

You don't even need to "track" the sun. One good method is to sight the sun as it's still rising a few minutes before local apparent noon (LAN). When you feel you've got a very good sight -- before LAN -- note the sextant reading and the GMT. No sextant corrections are needed.

Then, go on to take the noon sight for latitude, just as you normally do.

Afterwards, reset your sextant to the same reading as before noon, and wait until the sun just kisses the horizon. Note that time, add the earlier time, and divide by two. That time is LAN. You can enter the Nautical Almanac to find the sun's GHA at that exact time, and that is your longitude.

Some years ago I developed an easy form for doing both meridian transit sights and noon sights. I'd be happy to provide the form and explanation to anyone interested. Just email me: bill at wdsg dot com

Bill
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Old 20-10-2013, 17:18   #41
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Re: Celestial Navigation

This discussion takes me back to a time when I sat for my Mate's ticket (equivalent in tonnage and sea time to a 100 ton US license) in Vancouver in the early 60's.
We all trouped down to English Bay to take our first noon sight one May.
After doing the calculations, looking up the tables and figuring our position, I had us smack dap in downtown Calgary! Very humbling but a great learning experience! Phil
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Old 20-10-2013, 18:11   #42
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Re: Celestial Navigation

I used to keep an old cartoon in my copy of Bowditch, of Dennis the Menace holding up a calculator, with the caption, "Who needs brains when you've got batteries?". It reminded me that Murphy's Law hasn't been repealed. I still have two sextants. It takes, at least, a couple hundred sights to become relatively proficient at shooting a sight on a rolling deck. Add nasty weather, fatigue and fear, and you've got a powerful incentive for learning what to do when the fit hits the shan.
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Old 21-10-2013, 00:02   #43
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On passages my son and I compete with the sextant. Only 1 person can get the lat each day, but we compete against each other onboard to see who can get the closest long. by each having a couple of minutes with the sextant before and after local noon, using the method described above.

Backgammon is good, too...
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Old 21-10-2013, 01:23   #44
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Re: Celestial Navigation

On land or sea, I have the sun daily, except for really bad days when it is totally invisible,

I have the moon most nights, after a few days in the same direction, you can pick out the bigger stars in the sky that are in certain positions, and you use them to go by,

If your going from A to B, Its quite easy, You know where North, South, East and West are,

You want to go west, The sun or moon are behind you, Or a particular star or two, to keep you in a straight line,
If they all fail, You have this bright shiny compass to do it for you,

You know where you started from, You know where your going, Who really cares if your a couple of days early or a couple of days late, Your in a sailboat and cruising, Where does the time factor come into it,

Unless your trying to find a very small island in the middle of the Pacific, You Have GPS, And that means looking at a little small screen for about 10 seconds, And the GPS already has your position within a few feet on the screen already, You havent touched a thing,

The rest of the countrys, You cant miss them if your sailing towards them,

America, Its West or East,
Mexico, Its East or West,

Australia, Its North,South, West, Or East, You know where you started from, Just go back the same way,

I have never been lost, In Australian Desert or Bush, Or on sea,

Sextants look good in the Museums I have visited, Never had a need for one tho,
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Old 21-10-2013, 04:31   #45
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Re: Celestial Navigation

This is a free star chart that you can download. I use on the boat a lot since I'm usually far away from the city light pollution to locate various stars, planets, and constellations..

It is really good and even includes man made satellite orbits.

Just click on your Operating System (OS) to download

Stellarium
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