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    Re: Delta-T: was [NAV-L] Tycho Brahe Mars oppositions
    From: Fred Hebard
    Date: 2004 Dec 3, 20:54 -0500

    On Dec 3, 2004, at 5:54 PM, Frank Reed wrote:
    > Alex E wrote:
    > "The algorithm of such calculations is contained in the book
    > of Meeus "Astronomical Algorithms". I doubt that these calculations
    > can be easily done by hand, but they can be done with a computer
    > and I suppose that Frank Reed's program can do this."
    > I've got software that implements those algorithms, but that's not
    > necessarily the best way to do it these days, and it's not what my
    > online almanac tools are doing. The JPL DE406 ephemeris data is
    > publicly available and exceptionaly accurate. With some coding
    > trickery it can be converted into a database about ten megabytes in
    > size. From there, it's just a "lookup" and an interpolation to get the
    > positions for any date and time (in the range of validity).
    > But to George's point, the Solar System has some moderate chaotic
    > behavior on long time scales, and that implies that the computational
    > cost increases exponentially. That makes it deterministic but in a
    > useless manner. It's worth comparing this with the weather. Weather
    > prediction is the textbook (or coffee-table book) case of chaotic
    > dynamics in a system where prediction might be very helpful. The
    > computational cost of a weather forecast increases exponentially,
    > literally doubling for every day or two that you add to the forecast.
    > So we can read a forecast in the newspaper for three days in the
    > future and it may have some validity, but we will probably never have
    > a weather forecast that can reliably predict whether it will be
    > raining thirty days from today. The computational cost of suc a
    > forecast would be astronomical. Some authors describing this chaotic
    > behavior get a little carried away and write in despair that the
    > future is hopelessly unpredictable. And yet, with only trivial
    > computational cost, I can practically guarantee you that there will be
    > one day in July of 2005 when the high temperature in Mystic,
    > Connecticut will be within five degrees of 85 degrees Fahrenheit.
    > Chaos excludes some aspects of predictability... but not all.
    Shoot Frank, you don't even need to calculate that!  But I am curious
    how you might.
    Are the JPB ephemerides similar to the data in the Nautical Almanac,
    i.e. tabulated declination and right ascension over time?

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