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    Re: Heliacal rising - definition?
    From: Marcel Tschudin
    Date: 2011 Aug 18, 02:19 +0300
    A long time ago I was trying to approach this sort of problem. For this I was searching in the literature trying to find measured brightness densities of the sky and its change during twilight. This search did unfortunately not reveal much. It further would require the necessary difference in brightness for a point source to become visible either for the "normal" human eye or with some aid. Frank you indicate some software. How do they approach the problem? Are there now more (measured) data available on this?


    On Thu, Aug 18, 2011 at 2:01 AM, Frank Reed <FrankReed@historicalatlas.com> wrote:

    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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