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    Re: Definition of the twilight intervals
    From: Robert H. van Gent
    Date: 2015 Aug 17, 08:38 +0000

    Hi Brad,
    Thanks for your detailed response and for mentioning Dutton's text - I do not have it 
    but I will look for a copy.
    Dutton's definitions agree with those which I also find in the (older) astronomical literature.
    However, modern editions of Bowditch's "The American Practical Navigator" do not seem 
    to be consistent with the definition of astronomical twilight.
    In the 2002 online bicentennial edition (the latest to which I have easy access), 
    astronomical twilight is defined as the interval between -0 degr 50 arcmin 
    and -18 degr (table 1516 on p. 227) while the Glossary of Marine Navigation 
    in the same publication defines it as the interval between -12 degr and -18 
    degr (p. 725).
    Whom should one contact to point out this inconsistency?
    Rob van Gent
    -----Original Message-----
    From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Brad Morris
    Sent: maandag 17 augustus 2015 7:47
    To: Gent, R.H. van (Rob)
    Subject: [NavList] Re: Definition of the twilight intervals
    Hello Rob
    I think they are discussing this incorrectly.   The Civil Twilight time
    figure given in the nautical almanac is an instant in time defined by the sun's orb 
    being a set number of degrees below the observable horizon.
    There is no "interval" because the sun is at that angular displacement for just one 
    instant.  Similarly for nautical twighlight and astronomical twilight given 
    in the Nautical Almanac, the times given are for the sun at a particular 
    displacement below the horizon.
    Now what are those displacements?
    -6°,-12° and -18°, for Civil, Nautical and Astronomical respectively.
    But Dutton "Navigation and Piloting" says that the period, which ends at each of 
    those given instants, also has a start.  Article 2810 (in my copy, yours may 
    vary) defines the angular start of that period for ALL THREE as the same 
    value, to wit, the upper limb of the sun is at 0°.
    So per Dutton, we have
    Civil Twilight Period 0° to -6°
    Nautical Twilight Period 0° to -12°
    Astronomical Twilight Period 0° to -18°
    Dutton was "the" book used to teach at the US Naval Academy, when celestial was king. 
     So if you want to know if the periods are overlapping or not, the definitive 
    answer is that the periods do indeed overlap.
    On Aug 16, 2015 10:32 AM, "Robert H. van Gent" < 
    NoReply_RobertH.vanGent@fer3.com> wrote:
    > Hi,
    > There is a discussion at the moment at 
    > https://en.wikipedia.org/wiki/Talk:Twilight#Article_contradicts_itself
    > _on_twilight_definitions
    > on the question whether civil, nautical and astronomical twilight are 
    sequential (i.e. non-overlapping) intervals as many internet sources claim or 
    that they are overlapping intervals (as is claimed in modern editions of 
    Bowditch's *The American Practical Navigator*.
    > Any thoughts on this matter?
    > Rob van Gent ("Astrolynx")
    > View and reply to this message
    >  Gent-aug-2015-g32467>
    View and reply to this message: 

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