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    Re: Almanac data
    From: Peter Hakel
    Date: 2009 May 30, 17:11 -0700

    Thank you for your response, you raised an important point.  There indeed are different expressions for DeltaT depending on the era. So far I focused on today and near future.  I will put your suggestion on my to-do list for the next batch of improvements.

    Peter Hakel

    From: "frankreed@HistoricalAtlas.com" <frankreed@HistoricalAtlas.com>
    To: NavList@fer3.com
    Sent: Saturday, May 30, 2009 4:08:05 PM
    Subject: [NavList 8463] Re: Almanac data

    Peter H., you wrote:
    "Our recently expanded suite of Navigation Spreadsheets can now calculate almanac data and process lunar distance sights.  A free sample (Moon GP, SD, HP, and phase) can be downloaded "

    I tried out some dates and noticed that the Moon's position is right on the money for 2009, but it's a few tenths of a minute of arc off c.1950 and a couple of minutes of arc out if I go back two centuries. Those are fairly big error for lunars. It appears that you're using a simple expression for delta-T that's quadratic in time. That's fine for current dates within a decade or two of the present (considering that a few seconds difference in time is not important for lunars), but it's no good for historical lunars. Folks who experiment with lunars usually expect the tools to work correctly with historical observations, or at least the software should provide some indication when the inputs are outside acceptable limits for the code. Maybe you should set up a little "lookup table" for delta-T. That's what the code on my web site does (lunar distances data as well as common "Nautical Almanac" ephemerides here: www.HistoricalAtlas.com/lunars).

    By the way, I tried your advice from an earlier message and ran your spreadsheet on the "pocket" version of Excel on my cell phone. And yes indeed, it works great!


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