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Re: Definition of the twilight intervals
From: Lu Abel
Date: 2015 Aug 17, 14:59 +0000
The understanding that Civil and Nautical twilight started when the sun just disappeared (ie, 0 degrees) until the sun was -6 and -12 degrees, respectively, below the horizon was my understanding also.

The United States Power Squadrons also teaches this in their celestial courses, likely because they also think of Dutton as the bible of navigation.

To: luabel{at}ymail.com
Sent: Sunday, August 16, 2015 10:47 PM
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

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")

```

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