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Re: Delta-T: was [NAV-L] Tycho Brahe Mars oppositions
From: Frank Reed CT
Date: 2004 Dec 3, 17:54 EST
From: Frank Reed CT
Date: 2004 Dec 3, 17:54 EST
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."
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.
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
[ ] Mystic, Connecticut
[X] Chicago, Illinois