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Thanks I forwarded your response.
Guy

-----Original Message-----
From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf
Of Gary LaPook
Sent: Sunday, February 22, 2009 12:16 PM
To: NavList@fer3.com
Subject: [NavList 7390] Re: FW: [San Francisco Sailing] reusing old nautical
almanacs


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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