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Old 13-08-2011, 04:15   #1
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Join Date: Jul 2011
Location: N 6442' E 02114
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Bowditch Long-Term Almanac / Free Download

here is a link to a surveying site with download of the good old Bowditch long term almanac together with a shortstory (two links) and a copy of the Bowditch long term almanac as PDF, great to print a few of these and have around, it is just a few pages and works to within 2.2' at worst and likely less - Long Term Almanac, or, what to do when you can't use your total station and GPS

the short-story is actually a nice tutorial on terrestrial astronavigation with pretty in depth discussions between the old surveyor and his helper on precision, formulas...

If you like astronavigation you know that the two hardest things to carry along are sextant and Nautical Almanac.
Dr. Geoffrey Kolbe's Long term almamac 2000-2050 is easy carry and has sight reduction table included, the Bowditch version is an even easier carry -5 pages- true no sight reduction table and bit more math involved but what do you expect from 5 pages.

in my poinion the perfect compliment to this almanac is the Sight Reduction Tables by Henning Umland, especially the smaller one.

I admit the larger one allows great precission even enough for for lunar distance calculations but the size is factor in most grab bags.
the smaller one is usually within 1' or so as far as I have found and together with the almenac you would be able to see everything inside your "circle of theorethical precission", let us say 3,5'.
as most of us know in practice on a small vessel it would be wise to add 2-3' of "wobble" (both mathemathical and practical).

Oh and as for lugging sextant, check out the Bris-sextant, fits in a film can together with calibration sheet and instructions.

if one wanted to go Spartan i gues Bowditch-longterm+short-tables+bris-sextant+wristwatch should all fit in persons pockets, 10 printed sheets (if printed both sides) and a filmcan is not that much and would still allow navigation to within range where coastal piloting can take over.

I have actually reduced both the bowditch almenac and the table to 44% when pi printed them then carefully trimmed the edges and had them laminated and spiral-bound, hard part was getting the firm to carefully get the punched holes through plastic only with few mm margin to avoid water to "bleed in", it is what I use for my grab bag.

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Old 14-08-2011, 11:29   #2
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Re: Bowditch Long-Term Almanac - Free Download

Thanks for the link, every navigator should have one, and know how to use it. I have seen these things called Perpetual Almanacs. Such a thing may exist for the sun, but the correct term is Long Term vs Perpetual. While this is a great aid to navigation, bear in mind that it is for the sun and stars only, as it should be. To accurately calculate the apparent GP of the moon requires an observatory and a team of scientists with a main-frame computer, according to Dr Dogett of the US Naval Observatory in the 1980s. Things may have changed by now, but I still dont think there is any way to tabulate (or calculate on a stand alone machine) the GP and corrections of the moon long term. And the same applies to the planets, to a certain extent.

This is unfortunate since the moon is the best celestial body for navigation, and the planets are the second best, because the brighest bodies will give you the best accuracy. The exception to this is the sun, which of course is the brighest body, but when it is visible, normally nothing else is visible. So where you can get the most accurate sight from the sun, you can only get one LOP, unless you do a running fix or a meridian passage. These techniques should be in the navigators trick bag, but they are not all inclusive. The most accurate fix is to do a round of sights on the moon, planets, and stars. During evening twilight, the planets will show up first, so of course you will want to shoot these first, and during dawn twilight the planets will be the last ones standing so you can afford the time to shoot them 2 or 3 times. Of course when the moon is visible, you would want to shoot that a couple of times. After getting 6 or 8 sights, you only need to compute and plot 3 or 4. Then if the cocked hat is too big, you can compute and plot a couple more.

What is the point of all this rambling? Well, what I am trying to say is that the long term almanac is great, but you should also have a hard copy of the Nautical Almanac for the current year.

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Old 14-08-2011, 12:46   #3
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Location: North Channel of Lake Huron
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Thank you for a fascinating link and read.
Once I get back home,I'm dragging out my old sextant and re-reading my old Celestial Navigation course notes.
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Old 16-08-2011, 05:06   #4
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Re: Bowditch Long-Term Almanac - Free Download

If my cruising was bit further south than north Atlantic close to Arctic circle and bay of bothnia slightly south and east of that I would certainly agree, still, I have current nautical almanac, but not in my grab bag.
As for moon and planets, uhm, agree to some extent, but as I am at so high latitude I have no access to stars and planets from may to september which leaves moon and sun...
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