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    Re: A NEW Online Nautical Almanac
    From: John Brenneise
    Date: 2004 Jul 5, 09:24 -0700
    Do any of these online almanacs take into consideration the switch from the julian calendar to the gregorian calendar?  The date of the switch varies from one geopolitical region to another...
     
    John
    ----- Original Message -----
    From: Frank Reed
    Sent: Sunday, July 04, 2004 9:48 PM
    Subject: A NEW Online Nautical Almanac

    I've created a new web tool which can provide detailed, accurate online 'Nautical Almanac' data. It uses the same engine as the lunars prediction page which I've described recently on the list. It includes all (? have I missed anything ?) of the information in Reis's Online Nautical Almanac except the "v" and "d" interpolation figures, though not in the same format. It's valid over a much longer time period, and I believe that the accuracy is excellent. As relatively new software, there are likely to be bugs popping up. Let me know when they bug you!

    Access the online almanac via the link on my lunars page here:
    http://www.HistoricalAtlas.com/lunars
    or directly here:
    http://www.HistoricalAtlas.com/lunars/nadata_v5.html

    On this page, you can select the object that interests you (including navigational stars will generate a rather long list) at the top. Next choose any date from 1750 to 2050. For most purposes, you would select "Gr. Mean Time" as the time basis for the almanac. If you're interested in historical almanac data, you might want to use Greenwich Apparent Time. The prefered modern angle format is degrees and tenths of minutes (dd mm.m) but you can also get data in degrees, minutes, and seconds (dd mm ss) or in decimal degrees (dd.dddd).

    If you select the option "Visible at DR only" and enter your latitude and longitude, you will get almanac data only when the objects are in the sky from your location and visible (the Sun has to be below 5 degrees for the stars to show up in the table (don't complain that Venus is visible in daylight sometimes... you can always DE-select this option if you're interested in daytime navigation sights).

    There is also an option to display SHA instead of GHA. This was really just something for me while I was experimenting and it may disappear or change later.

    This web tool is doing relatively little calculation. The positions of the Sun, Moon, and planets are stored in a ten megabyte database to the nearest arcsecond. These positions are originally from the JPL numerical integrations, so they are intrinsically very good. Note that this approach sidesteps the issue of coding algorithms to generate the positions. The Moon positional data here is considerably more accurate than the almanac data found in most online tools. The positions of the stars are precessed, nutated, and advanced for proper motion using the usual algorithms (see Meeus "Astronomical Algorithms", for example) and should be accurate to a fraction of an arcsecond. The "delta-T" correction for dynamical time (ephemeris time) is tabulated internally at ten year intervals based on the list in Meeus and interpolated for years between those dates.

    When calculating positions for "Greenwich Apparent Time", the software iterates until the GHA of the Sun is exactly equal to the Greenwich time. So if it's noon at Greenwich, the Sun's GHA is zero degrees exactly.

    NOTE ALSO: I've added a "Greenwich Apparent Time" option to the lunars prediction page here: http://www.HistoricalAtlas.com/lunars/lunars_pre_v5.html
    Using this option, you can generate data which can be compared directly with the old (pre 1834) Nautical Almanacs. Also, if you have old lunars sights recorded with Local Apparent Time, it is a much simpler matter now to reduce those. You don't have to locate an old N.A. or convert times to mean time. I've got a couple of pages from old almanacs on my web site here: http://www.HistoricalAtlas.com/lunars/na.html

    It's interesting to see how the distances on those old pages compare with modern calculations. I get an average error for the August 1843 lunars of about 0.24 minutes of arc (that is, the published data has an average absolute error of about that amount compared with the calculations generated by my web site tool). This sounds about right since Lecky commented that published lunars (before c.1880) were inaccurate to an extent of about 6 to 8 minutes in the longitude and multiplying that 0.24 by the usual factor of 30 yields 7.2 --right in the range Lecky described.

    Frank R
    [ ] Mystic, Connecticut
    [X] Chicago, Illinois
       
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