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Old 08-09-2006, 05:28   #1
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Question Celestial Navigation Help Needed

Hello all, I am new to the Cruisers Forum.

I am trying to teach myself Celestial Navigation for that SOMEDAY cruise.

I have read several books, Celestial Navigation for Yachtsman, and the Complete Onboard Celestial Navigator. BUT, obviously my thick skull will not let the information into the brain matter department.

Yesterday I took a sun sight using a ARTIFICAL HORIZON and got the following readings:

Sun Fix LL
lat 39.52N lon 75.08W (using my GPS)
7 sept 2006
Time 13 22 29 UTC
Local 09 22 29
Sextant Reading 61 deg 28'
Observed reading 30 deg 44' (above halved)
No Index error
No dip
Altitude correction +14 deg 40'

For a corrected sight of 44 deg 48'

My sight reduction tells me I am about 800 miles from where I am

What the heck am I doing wrong?????????????????

Thanks in advance and Smooth Sailing

Bill
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Old 08-09-2006, 05:50   #2
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With an error this big you could be using the wrong time zone. All times should be in GMT. Perhaps you did the conversion from your local time zone to GMT incorrectly - its an easy error to make.

Hope this helps
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Old 08-09-2006, 05:51   #3
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I think I just found my own problem, I added 14 degrees 4 min instead of 14.4 minutes. DUH. Man thsi forum is good!!!!!
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Old 11-09-2006, 01:46   #4
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I would be a little suspicious of a zero index error unless you have a very expensive sextant. and you indicate 0 dip, was the sextant on the waterline?
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Old 11-09-2006, 07:40   #5
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What artificial horizon?

Note that if your artificial horizon is a reflector type, such as the little gray plastic Davis unit that breaks down into it's own little box, and not a bubble horizon attached to your Sextant you have to use 1/2 the reading on the Sextant after you have applied your index error adjustment as the Sextant is looking at the angle between the Sun and its reflection and you need the angle between the sun and a line tangent to the earth's surface. Further, it is extremely unlikely that your Sextant has no index error. One minute of error (time or angle) is equal to about 17.29 statute miles or 15 nautical miles on the earth's "surface". Your .4 hours verses 4 minutes error would only account for about 300 miles of your 800 mile error (20 minutes x 15 miles) so I suspect the error is in your horizon reading or your time keeping.

Celestial navigation really isn't too hard tho' a lot of texts make it seem so. The hard part is getting a good site which takes practice and then more practice. An excellent simple text is Mary Blewitt's Celestial Navigation for the Yachtsman.

Cheers,

s/v HyLyte
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Old 11-09-2006, 19:20   #6
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Index Error zero

I have adjusted the index error to zero, and I am using a Davis artificial horizon. I am halving the reading, applying no dip, no index error, and adding 16' for semi diameter. When doing a sight reduction using the celnav program my LOP's are coming in within .1-.2 nm. I made a big math error when I added the 14 degress for a total corection instead of 14'.

I am sure that sights taken on the water will be nowhere near as precise.

The artificial horizon sure makes practicing sights more pleasureable, now I just need to learn how to use the plotting sheets.
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Old 12-09-2006, 01:18   #7
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Plotting Sheets

There are 3 different plotting sheets available to download at http://www.mobilegeographics.com/biblio/navbooks.php

I much prefer plota
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Old 12-09-2006, 05:37   #8
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Question Thanks for the tip on the plotting sheets

Now a question on Nautical almanac and pub 249.

Do I need a new nautical almanac each year?

Do I also need pub 249 and does it need to be replaced yearly?

Currently I am reducing my sights by computer, but I want to do them by hand also.

Thanks,

Bill
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Old 12-09-2006, 06:14   #9
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The official almanacs are annual, however, other organisations do produce a 1year almanac with corrections for up to 5 years.

Not sure what you mean by pub 249. If this is the Air Navigation tables, then they are available as 3 volumes. vol 1 has a limited shelf life of abt 5 years, whereas vol 2 and 3 are lifetime.

All 3 volumes can be downloaded for free from http://www.nga.mil/portal/site/marit...c24fd73927a759
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Old 12-09-2006, 12:20   #10
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Question Thanks Talbot

Thanks for the info, If I am understanding correctly, I do need to buy a Nautical Almanac each year, or do the air tables have ALL the information I need?
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Old 12-09-2006, 13:22   #11
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It is posible to buy a nautical almanac that has a longer validity, but will add in additional corrections to the calculations, so better to stick with the official one year validity version. It is also possible to download daily pages of the almanac from http://www.tecepe.com.br/scripts/AlmanacPagesISAPI.isa
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Old 12-09-2006, 13:53   #12
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Aloha Loose Ends,
Good to hear you are practicing and sorting things out. Whatever Talbot has been telling you is the absolute truth and practice makes perfect (or within a few miles).
Just a few months ago a friend and I reviewed our noon shot skills and we got pretty close. At the time the sun was North of us. Now it is South again and would require a 140mile round trip to get a good noon sight. The artificial horizon thingy is pretty nice to practice with.
I've also been able to tune out index error with my sextant. Makes it handy but it is good to check it again frequently.
Good luck!
Kind Regards, JohnL
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Old 13-09-2006, 00:25   #13
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Hi all,

I have a IIIB and am able to get rid of most IE with of tweaking.

Anyway, is there a source for almanac information online? MSI provides sight reduction tables in PDF form - is there a similar almanac provider online? i'd like to have a compilation of all of these on my computer.

Thanks,
Aaron N.
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Old 13-09-2006, 00:26   #14
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Hi all,

I have a IIIB and am able to tune out most IE as well.

Is there a source for downloading the entire year's almanac information in .pdf or some similar form? MSI provides sight reduction tables; somewhere for the almanac info?

Thanks!
Aaron N.
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Old 13-09-2006, 06:15   #15
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Skiprjohn, I zero out my index error,

before each practice session, it is amaizing how just putting the sextant in and taking it out of the box can knock the adjustment out. My job requires that I travel within about 180 mile radius of my home, so I am trying to take a couple of sun shots everyday from where ever I am. Currently I am using my GPS for Lat/Lon coordinates to do my reductions. When I feel I can be confident in my sights, I hope to then use my location sort of like a DR position and see if I can plot my lat/lon with a sight and a plotting sheet (then use the GPS to verify my actual location).

That is my plan.......I guess I will see how it goes.

This is going to be a very long learning process. The most helpful thing would be a book with nothing but Celestial Navigation problems with the answers given so that one could see if the sight / plot was worked out correctly.

I need to make sure I have all the tools, proper books etc that I need.

Bill
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