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Old 16-12-2012, 15:19   #1
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Re-use almanacs?

I heard once that the almanacs repeat themselves every four years. So do you just need to buy 4 years worth and remember to cross out the year on the cover? ;-)
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Old 16-12-2012, 15:27   #2
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Ya suppose the astronomers at the Naval Observatories completely missed that factoid?

They may repeat in a very rough sense for the stars and sun. But not in any useful or accurate way. They do include a description of how to use the almanac in the following year, and describe the inaccuracy.
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Old 16-12-2012, 16:06   #3
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Re: Re-use almanacs?

Earth's magnetic field is continuously shifting. Past almanacs are useless.
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Old 16-12-2012, 16:55   #4
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Re: Re-use almanacs?

Part of almanac info can be re-used. You must apply corrections as given in the almanac.

You can re-use almanacs every 4 years but even when you apply the appropriate adjustments, this (I think) relates only to some part of the info in the almanac (I can only speak for the Sun data).

Look up 'Cook's Sextant Tables' for more relevant information. (Michael Cook, NOT Cpt. Cook, Vega Instruments, st albans, herts. AL1 UZ, England). Possibly no longer published.

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Old 16-12-2012, 17:22   #5
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Re: Re-use almanacs?

I've used the previous years' almanac, adding the corrections, but that's only good for Sun and Stars, no Moon and Planets. If you're seriously navigating by sextant, you want all the options open. If your sextant is only for "backup", chances are the math of an ordinary reduction will be tough enough without additional corrections to splice in. If you are a complete math genius (which I am not), Bowditch has a Perpetual Almanac from which you can extrapolate all the information necessary, further into the future than you could ever need. The reason most of us use almanacs and tables is that all the toughest math has been done for us by smarter heads (bless them!). I, too, have heard of a periodic repetition of some almanac info, but I think it's limited: precession, nutation, and tidal friction would preclude it's being identical in all respects.
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Old 16-12-2012, 17:26   #6
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Re: Re-use almanacs?

Why? A paperback commercial almanac is cheap as beans. A computer almanac, too, and it will take care of you if the end of the year finds you at sea without next year's edition. Yes, as other posters have said, you can "use" last year's almanac, with certain significant limitations and a considerable PITA component, but I say give it a miss. Just figure on getting a new one every year, with a computer version like ICE for backup.

www.celestaire.com is your friend.
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Old 16-12-2012, 18:44   #7
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Thanks for all the input. It Answered my question thoroughly! (Man I got a lot to learn)
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Old 17-12-2012, 02:21   #8
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Re: Re-use almanacs?

From Bowditch 2002
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