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    Re: History of Twilight
    From: Steve E. Bryant
    Date: 2014 Sep 21, 23:39 -0500

    Thank you Frank. The attachment you intended did not make it along with the 
    email; but I do appreciate your narrative 
    Sent from my iPad
    > On Sep 21, 2014, at 7:21 PM, Frank Reed  wrote:
    > It turns out that the 18° magic number for the altitude of the Sun below the 
    horizon as the definition of astronomical twilight has been standard "since 
    antiquity". Also of interest, the times of twilight seem to have been of 
    greater concern to meteorologists (such as they were a century ago) than to 
    astronomers or navigators. It appears that astronomical twilight was defined 
    very early, civil twilight more recently (c.1800 but not settled even in 
    1916), and nautical twilight some time after that --when? The definitive 
    analysis of twilight and the history of the concept, up until then, was 
    written by H. H. Kimball in 1916 and published in the "Monthly Weather 
    Review". I am attaching a copy of the paper. Kimball also discussed the 
    varying arbitrary definitions of sunrise and sunset that were in use in the 
    nineteenth century. He notes that older German sources defined sunrise and 
    sunset as the instant when the geocentric sun has an altitude of zero degrees 
    (H_c=0° in navigator's terms). There's lots of interesting material in the 
    article. For example, he notes that the brightness of twilight when the Sun 
    is about 8° 40' below the horizon is comparable to a sky with a Full Moon (at 
    the zenith, but at any moderately high altitude would be equivalent). This, 
    incidentally, confirms something that many navigators know but it's not often 
    stated plainly in the textbooks: a Full Moon night is comparable to nautical 
    > By the way, the crepuscular arch described in Kimball's paper is the shadow 
    of the Earth on the atmosphere, which apparently figured in some early 
    definitions of civil twilight. There was a  [LINK: 
    recent article on the Sky & Telescope web site describing it.
    > -FER
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    > Attached File: http://fer3.com/arc/imgx/On-twilight.pdf
    > View and reply to this message: 
    > http://fer3.com/arc/m2.aspx/History-Twilight-FrankReed-sep-2014-g28683

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