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    Re: Definition of the twilight intervals
    From: Paul Hirose
    Date: 2015 Aug 17, 12:14 -0700

    The Explanatory Supplement to the Astronomical Almanac (1992) says,
    
    "9.341 Twilight. This is caused by the scattering of sunlight from the
    upper levels of the Earth's atmosphere. It begins at sunset (ends at
    sunrise) and is conventionally taken to end (or begin) when the center
    of the Sun reaches an altitude of -18°."
    
    The next paragraph has the altitude limits for civil and nautical twilight.
    
    So the twilight periods overlap, according to the Explanatory
    Supplement. However, I don't have the latest edition, which was
    published recently after years of preparation. Someone please verify the
    definition is the same.
    
    

       
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