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    Re: DeltaT fits
    From: Frank Reed
    Date: 2010 Apr 5, 05:05 -0700

    Peter, you wrote:
    "Interpolation on a table starts with a search, which is a task of higher complexity than what I am trying to confine myself to."

    We're getting into coding details here, so I don't want to bore the group with this, but consider: you could make one "line" of data for each calendar day. For one arcsecond accuracy, you can probably cram all the data for the ecliptic latitudes and longitudes of the navigational planets, Sun, and Moon into 500 bytes. Maybe more, maybe less. This would give the daily positions for the outer planets, the hourly positions for the Moon, and positions at other intervals for Mars and Venus. Now since each data line has a fixed length, the search algorithm is nothing more than an array index. After that you're doing simple linear interpolation on the actual positions. That's LIGHTNING fast. For some of the celestial objects, the data stored this way may even be SMALLER than the long algorithms that you're currently using. So your application will be dramatically faster, and, if you're lucky, a little smaller.

    People have prejudices when it comes to the way things are calculated. I guarantee you that some potential customers for your applications read about it and think "that's just a spreadsheet. it can't be any good." That's because they've never been exposed to the awesome power of the dark side of the force! Er... I mean... that's because they're unfamiliar with the amazing computational and presentational versatility of Excel. Similarly, most of us assume that an equation is a better way to get astronomical ephemeris data. Even if it's just a fit. For example, there are "equations" for the equation of time. They're just approximations and they have their place as such (I use them when necessary), but they're exceedingly popular even in circumstances where they shouldn't be used because we have these little prejudices. If someone asks me for a more accurate way of calculating the equation of time, and I say, "just subtract the Sun's true GHA from the GHA of the Mean Sun and convert to time units," they're often disappointed DESPITE the fact that this is the real answer to the question. They seem to feel almost cheated because I haven't given them an equation similar to the one that they already know. While it is an equation, EqT=GHASun-GHAmeanSun just doesn't satisfy that desire.

    -FER


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