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    Re: Heliacal rising - definition?
    From: Frank Reed
    Date: 2011 Aug 17, 16:01 -0700

    There's an old, traditional rule for this which supposedly dates back to Ptolemy. See, for example, Leadbetter's "Compleat System of Astronomy" on Google books. By the old rule, stars of the first magnitude can be seen in morning twilight when the Sun is 12 degrees below the horizon (which clearly corresponds to our modern definition of "nautical twilight"). Stars of the sixth magnitude can be seen when the Sun is 17 degrees below the horizon (more or less the end of our modern "astronomical twilight"). Like the standardized times of sunrise and sunset, that's the end of the story. That is, there's no allowance to be made for extinction or height above sea level. Does anybody calculate heliacal rising anymore? Probably not literally and labeled as such, but there's plenty of astronomical software which will indicate non-visibility of stars in twilight using very similar rules. Come to think of it, I wrote something like that a few months ago... Of course, in modern software, the rules can be much more nuanced and realistic.


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