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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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