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    Re: determination of longitude and the prime vertical
    From: Frank Reed CT
    Date: 2007 Apr 15, 04:15 EDT
    George H, you wrote:
    "If Polaris is indeed that far out, can any predictions from that site be
    trusted, for precise work? I wonder if others have comments about that."
    That was one of the reasons I put together my own online Nautical Almanac (at the address below). You may recall that this is not the first time problems have cropped up in the star data on that site. A couple of years ago, you helped Omar Reis fix the problem with proper motion (of all the stars), which, in fact, he had not included at all. For historical analysis, this was a very big problem. I can't tell what's wrong with his Polaris position, but yes, it's wrong. It seems to be a pattern: many software developers have built tools for calculating the positions of celestial bodies for navigation. They usually get the planets and Sun and Moon right. But they frequently have problems with the navigational stars. I suppose this is because the testing procedure is very different. For stars you have to look at very small errors accumulating over years, and you have too look at the positions during different seasons of the year. With the planets, using any of a variety of methods, once you have it right for every January 1st for five years in a row, you're probably all set. Because of the way Solar System positions evolve, it's just not very likely that an error could emerge that would not show up on those five test dates.
    42.0N 87.7W, or 41.4N 72.1W.

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