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    Re: Nautical Almanacs
    From: Henry Halboth
    Date: 2015 May 27, 20:20 -0400

    Many, if not most, of us are familiar with the various websites available to facilitate astronomical/navigational calculations, many of which are excellent both in content and as time savers. However, the question arises "what do we do when the web is down, or there is a power failure, or electronic jamming is in progress, or our electronics have been fried in a lightning strike, etc., etc , ad infinitum."  In another time my emergency kit included my then 75 year old octant, the current Nautical Almanac, HO 208, my rated wrist watch, a Pilot Chart, notebook, and pencils - all done up in a backpack so as to allow maximum freedom of movement. All that I am saying is that in a foreseeable emergency the availability of a hard copy of the Nautical Almanac mght just make life a little bit easier - if your life does not depend on your navigation, the canned data is a great convenience and is highly recommended.  We have just gone through a lengthy discussion on emergency navigation and I thought this might be a useful addition thereto. 


    On Wed, May 27, 2015 at 2:26 PM, Frank Reed <NoReply_FrankReed@fer3.com> wrote:

    Just a reminder, nautical almanac-equivalent data has been available on my web site since 2004 here: http://reednavigation.com/lunars/nadata_v5.html. Sample output in the attached image.

    Most of these homebrew almanacs make the mistake of calculating the positions of the Sun, Moon, planets, and stars from long numerical algorithms. That is, they get the ecliptic longitude of the Moon, for example, from a series of a hundred or more cosine terms. But the future position of the Moon is already known from the JPL numerical integrations. Those tables of numerical integrations are, in fact, the basis of the values published in the official almanacs. And that's how any modern software should get the coordinates of the celestial bodies --by lookup in a table, not by long calculation. In my software, including the nautical almanac data generator above, the original JPL positions are used directly, though of course reduced in precision to a level appropriate for the maximum accuracy achievable with celestial navigation.

    Any of these homebrew almanac data sources, including mine, could be counted as superfluous so long as the USNO almanac engine remains available online at http://aa.usno.navy.mil/data/docs/celnavtable.php. It's the gold standard. And this is actually a data source that can be used by other software. You can "script it" and use its data in your own software. Sometime within the past three years, they (USNO staff) changed the interface so that it's possible to "script" the almanac engine using the URL alone (for tech-savvy readers, it's a "post" versus "get" change). The URL interface is not documented, but you can figure out a great deal of it by experimentation. Here's an interesting example that displays Sun data only: 
    http://aa.usno.navy.mil/cgi-bin/aa_flamenav.pl?ID=NAVLIST&calc= [...] &qqo=sun&yy0=1&xx0= [...] (click/follow this link ...copy and paste won't work). The "normal" URL (the address in your browser) generated by the initial data entry page inserts the argument "qqo=all" and you'll notice that here I have manually replaced that by "qqo=sun" (I have also changed the user ID as per their request). Experiment and see what happens.

    Frank Reed
    Conanicut Island USA

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