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    Re: The Star of Bethlehem and Navigation
    From: Frank Reed
    Date: 2009 Jan 3, 16:44 -0800

    Marcel, you wrote:
    "I'm wondering whether the special event in the sky could have been in
    relation with this "conjunction" and the moon, like e.g. the moon
    occulting two or three planets at the same time."
    I've never heard of such an event in that time period. Given that people have 
    been trying to come up with plausible conjunctions and occultations to 
    somehow explain the Star of B since at least the time of Kepler, I suspect 
    they've probably found the genuinely interesting cases. But try it for 
    yourself. It's certainly possible that something's been missed! I recommend 
    Aldo Vitagliano's "Solex" software for this sort of thing: 
    http://chemistry.unina.it/~alvitagl/solex/. It has a primitive user 
    interface, but it's very fast, the accuracy is extremely high, and it can 
    show almost any set of circumstances.
    "I imagine that such an event is fairly rare and could have been a reason for 
    those dealing with Astronomy/Astrology for travelling to a distant place 
    where they could see it."
    Any such event would have been visible along a long path (a "line of position" 
    in fact). Even if they had the predictive ability to determine that an event 
    would not be visible from their own longitude and latitude (and this is 
    unlikely), they could not have determined a single location just by the 
    requirements of visibility.
    "After what I learned here, this should be possible to verify for the
    location. If I understood it correct, the inaccuracy of +/- 2 degrees
    seems only to refer to the local hour angle "Tau" and not to the
    declination, i.e. that the calculated declinations would actually be
    fairly exact."
    Rather than saying calculated declinations, it would be fair to say that the 
    "geocentric positions (right ascensions and declinations)" would be fairly 
    exact. Fairly exact for the date in question means that the geocentric RAs 
    and Decs would be generally accurate to the nearest arcsecond for the planets 
    and accurate to +/-5 arcseconds for the Moon. For some true stars, the 
    positions may also be as inaccurate as the position of the Moon. And since 
    the uncertainty in local time affects only longitude (on the Earth), you 
    could also say that any maximum or minimum latitude calculations would be 
    fairly exact. 
    By the way, the introductory web page shows an example of an ancient 
    astronomical observation (the most ancient?) that puts a strong constraint on 
    delta-T: the solar eclipse of March 5, 1223 BC seen at Ugarit in Syria. If we 
    have records stating that an eclipse was seen from a specific place, all we 
    need to do is change delta-T by trial and error until the eclipse path 
    crosses the observer's location. The catch in this case is that the tablets 
    recording the eclipse aren't dated. So you need some other information to 
    narrow the date to the nearest century or two.
    Navigation List archive: www.fer3.com/arc
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