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There are two "yes" answers to this question. The first "yes" answer is 
for use of a Nautical Almanac for the following  year. The last section 
of the Explanation Section of each Nautical Almanac describes how to do 
this. Basically it entails using a constant offset in time  5 hours and 
48 minutes, and  a small constant correction factor. This works for the 
sun and GHA Aires but not for the moon or planets.

The second "yes" is more general and allows for the use of a Nautical 
Almanac perpetually. There is a four year cycle to almanacs, think of 
the first as the leap year and then the other years as leap year plus 
one, leap year plus two and leap year plus three. For whatever year you 
are in in this four year cycle you can use any almanac for any other 
year that is for the same cycle. For example for 2009, which is leap 
year plus one, you can use 2005, 2001, 1997 etc. which provides 
reasonable accuracy if the almanac is not too old. There is a slight 
change every four year cycle and you can determine the amount of this 
slight change and apply it as a constant correction factor. Just compare 
the values for the same date in the current almanac with the old almanac 
and determine the difference and then apply it as a constant. Borrow a 
friend's current almanac and make the comparison with your old almanac. 
Another good place to get the data to compare is the Navy website, 
http://aa.usno.navy.mil/data/docs/celnavtable.php. The correction factor 
for each four cycle varies slightly throughout the year so compare for 
the month for which you are interested. The values vary +/- about one 
quarter of minute for GHA Sun, and about +/- three quarters of a minute 
for declination Sun.  The correction for Aires is a constant 1.84' for 
each four year cycle.

Looking at the data for today at noon GMT and comparing it to the same 
time and date in 2005 you find the constant difference for the Sun's GHA 
is 0.3' and declination of 0.8' ,small enough changes that they could be 
ignored for most practical navigation. Even though the correction for 
Aires is a constant 1.84' for each four year cycle today's comparison 
shows a 2.2' change but this is due to the insertion of a leap second 
last December.

This periodicity of the almanacs is used in the perpetual almanac in 
Bowditch which provides a method to calculate the correction factors if 
you don't have two almanacs to compare. It also provides correction for 
the slight movements of the stars if great accuracy is needed.

gl

gl


Guy Schwartz wrote:
>
> An interesting questions posed on a local bay area sailing group.
>
> I will relay the answer back to Max
>
> Guy
>
>  
>
> *From:* SanFranciscoSailing@yahoogroups.com 
> [mailto:SanFranciscoSailing@yahoogroups.com] *On Behalf Of *maxjosefspring
> *Sent:* Sunday, February 22, 2009 12:13 AM
> *To:* SanFranciscoSailing@yahoogroups.com
> *Subject:* [San Francisco Sailing] reusing old nautical almanacs
>
>  
>
> I wonder whether it would be possible to re-use an out-dated nautical
> almanac for celnav purpose by determining the "error" through an
> accurate measurement.
> It appears to me that might work fine for the sun and stars, but
> probably not for the moon and the planets.
> Has someone maybe even done this?
> -Max
>
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