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    Re: Sunset, sunrise, civil & nautical twilight
    From: Dan Allen
    Date: 2002 Jan 30, 2:21 PM

    The canonical angles are
    Sun rise/set: 0 degrees
    Civil twilight: 6 degrees below
    Nautical twilight: 12 degrees below
    Astronomical twilight: 18 degrees below
    The definitions have to do with the amount of light visible.  The first one is 
    self explanatory (but due to refraction the sunrise
    or sunset is actually at 50 minutes beneath the horizon...)
    Civil twilight is the time when cars are supposed to have their headlights on 
    by.  One can no longer see details of things well at
    this time.  People common say at this time that it is dark outside.
    Nautical twilight is when the horizon at sea is no longer visible (but there 
    still may be some light in the sky).  That is, if one
    was out to sea the line of the horizon cannot be made out any more, and thus 
    sextant observations without artificial horizons could
    no longer be taken.
    Astronomical twilight is when it is as dark as it is ever going to get.  There 
    is no more light in the sky from the sun.  (The moon
    may light things up though!)
    These definitions hold true in any latitude at any time of year.  However as 
    you observed the conditions do not allow some of these
    events to occur at certain combinations of times and places, i.e., in the 
    summer in the Arctic none of these occur at all.
    Specificially, on the summer solstice (near June 20th) any place north of 66 
    degrees north latitude will not have the sun set, let
    alone enjoy any twilights: the sun is above the horizon all night long.
    If you try and do the math to solve for such things you will end up taking the 
    arccos of 2 or something like that -- which is not
    defined for real numbers.
    Dan Allen
    -----Original Message-----
    From  Navigation Mailing List
    Sent: Wednesday, January 30, 2002 1:04 PM
    Subject: Sunset, sunrise, civil & nautical twilight
    A definition I've read for these is:
    Sunset occurs right when the top of the sun disappears over the horizon.  
    Civil twilight occurs when the sun is 6 degrees below the
    horizon, and nautical twilight occurs at 12 degrees.
    Sunset might make sense but the 6 and 12 degree notion doesn't seem right to me.
    The closer to a pole you are, the longer twilight lasts (yes or no?) in which 
    case the degrees don't work.
    Would someone be willing to clarify this for me?
    mail2web - Check your email from the web at
    http://mail2web.com/ .

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