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    Re: Henning Umland's javascript web app
    From: Frank Reed
    Date: 2018 Apr 11, 09:39 -0700

    Bill Lionheart, you asked:
    "It seems the version I have has a hard coded limit on the year as it does not have this year's delta T"

    Getting back to this original question, this is almost certainly not significant for you. Some NavList computation-enthusiasts get quite "agitated" about delta-T, but these worries are usually overblown. Modest variances in the value of delta-T, even up to 10 or 20 seconds, do not affect the positions of the stars or planets at any measurable level, and even the position of the Moon would be out by only 0.1-0.2 minutes of arc. And if delta-T is wrong by 60 seconds? What would happen then?? Turns out, not a whole lot! Even the Moon's position would be wrong by only about half a minute of arc. And such a large error in delta-T would not be a concern for decades even with a simple linear approximation to its values as in early versions of Umland's code.

    Why not just test the "web app" that you are using? I'm assuming this is a version of his code that you downloaded a while back, and you have already modified it replacing "trunc()" with "InRange()" or "UmlandTrunc()" or something similar. Also, if the code throws a warning for years greater than 2015. Just edit that to 2020. After the changes are made, run the app locally in your browser and try a date for next month, e.g. May 15, 2018 at 16:00:00 UT. Next, go to the USNO web app here: http://aa.usno.navy.mil/data/docs/celnavtable.php and enter the same date details. Then compare the results. You should find no problem, event at the level of a tenth of a minute of arc. You can then experiment with delta-T errors if you want. Edit Umland's code! Find the line that generates delta-T and change the constant starting value from 63.5 to 53.5. How much change do you see in the computed coordinates??

    There are numerous alternatives to Umland's old web app. This method of generating GHA and Dec coordinates reminds me of the "Spice Girls" ...because it's "so nineties". :) You can find better apps --that aren't twenty years old. Rather than specifying the tool first, why not describe your plan? What is your goal with computational tools that you would bring along on this voyage? And what is your "minimum hardware platform"? 

    Frank Reed
    Clockwork Mapping / ReedNavigation.com
    Conanicut Island USA

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