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    Re: Definition of the twilight intervals
    From: Bob Goethe
    Date: 2015 Aug 17, 12:41 -0700

    Only a fool would seek to contradict Dutton's on matters of navigation. 

    That said, I was out before dawn this morning taking sights of stars in Orion and in its neighborhood:  primarily Aldebaran, Alnilam, and Betelgeuse.  I used a app for my Android phone which, when I enter the coordinates of my location, gave me the times of Astronomical/Nautical/Civil Twilight. 

    I observed the horizon clearly enough to take star sights within a minute or so of the projected beginning of nautical twilight.  When Alnilam disappeared, I was still able to take sights of Aldebaran and Betelgeuse...being brighter as they are.

    I continued taking sights of these two stars as the dawn continued to progress.  It came to be that finally, I looked up from the notebook where I had just recorded my sextant data, and 100% of the stars had disappeared in the brightening sky.  I looked at my watch, and it read precisely the same time, to the same minute, as the predicted beginning of civil twilight from my app.

    I don't know what frame of reference Dutton has in mind as it talks about huge overlaps between the various types of twilight.  But from my point of view as an observer with a sextant in his hand, the beginning of nautical twilight is a more-or-less discrete moment, and it comes to a well-defined end right at the time when civil twilight begins.  If there is an overlap, there is not *much* of an overlap.

         I can imagine that if I had had a clear view of the horizon over
         water, looking to the west rather than to the east, with some 
         bright stars there...I might have found that the stars hung 
         on a little past the beginning of civil twilight.  But looking at 
         stars with azimuths ~35° from where the sun would rise, 
         nautical twilight ended as decisively as if one turned on a 
         light switch.

    When using a phone app or a web site like www.heavens-above.com, the predicted value for the beginning of nautical twilight is of enormous value in planning when you want to be in place to start taking star sights.  And the predicted value for the beginning of civil twilight was accurate enough today that if it was NOT an unequivocal end to nautical twilight, it was certainly an end to my observational period for stars.

    Now, if you have a planet - Venus, for instance - bright in the dawn sky, then it will continue to be visible throughout civil twilight.  In fact, if I am not moving around and am focused on my task, I can track with Venus after sunrise.  It can be visible in the sky after the trees in the yard are showing distinct shadows in the morning sun.  Or course, if you take your eye off Venus for more than a moment or two after dawn, it is all-but-impossible to find.

    Perhaps Dutton was thinking in terms of "when can you take sights of a planet with a magnitude of -4.5".  Using *that* as a frame of reference, I can well appreciate that one might think of nautical twilight and civil twilight overlapping.  I have too little use for astronomical twilight to even think about how it fits into Dutton's scheme.

    Bob Goethe

       
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