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Hi,

As far as I know civil twilight was first introduced by the Berlin astronomer 
Johann Elert Bode (1747-1826) in the first edition of his _Astronomisches 
Jahrbuch oder Ephemeriden_, published annually from 1774 onwards (see the 
edition for 1774, second part, p. 22). Bode named it 'gemeinen Dämmerung' or 
'bürgerlichen Dämmerung' and defined it as the moment when the twilight arc 
(the earth's shadow on the atmosphere) passed through the zenith, thus 
conveniently marking the time when artificial lighting was needed for indoor 
activities. Bode adopted Lambert's value of nearly 6.5 degrees - the 
currently used value of 6 degrees (exactly) dates from the late 19th century. 

Nautical twilight was first introduced in 1936 by Leslie John Comrie (1893-1950), 
Superintendent of H.M. Nautical Almanac Office from 1930 until 1936, as a 
convenient means of dividing the interval between the end/begin of civil and 
astronomical twilight in two nearly equal parts. Regular tabulations of the 
times of nautical twilight first appeared in the 1937 edition of _The 
Nautical Almanac and Astronomical Ephemeris_ and its abridged version for navigators.

The Wikipedia claim that the concept of nautical twilight already dates from the 
18th century is unfounded. 

Rob van Gent

-----Original Message-----
From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Frank Reed
Sent: maandag 22 september 2014 8:16
To: Gent, R.H. van (Rob)
Subject: [NavList] Re: History of Twilight

Continuing... As I noted in the previous message, astronomical twilight came first and has 
been defined by 18° below the horizon "since antiquity" or at least for some 
centuries. Civil twilight, specifying the Sun about 6° below the horizon 
dates back a little over 200 years, and the choice of exactly 6° as a 
standard apparently dates from Kimball's article in 1916. What about nautical 
twilight?? I can't find any evidence of the use of the expression "nautical 
twilight" and its modern definition before about 1940. Its introduction 
around then would make sense since this was when twilight star sights were 
becoming much more popular. I would not be surprised if the new definition of 
nautical twilight was based on an established rule of thumb that simply 
"split the difference" between the limit of civil twilight and the limit of 
astronomical twilight. Brown's Nautical Almanac in 1936, which I have here on 
my bookshelf, mentions only civil twilight and cites its importance for 
navigation since that is when first magnitude stars first become visible for 
sights (or are last visible in the morning).
Steve: the file is, in fact, "attached", but you're signed up for the plain text 
version of messages, so you have to copy and past the link into your browser. 
You can also modify your NavList options and receive emails in "html" format. 
That includes clickable links to attached files. You could also just  [LINK: 
http://fer3.com/arc] visit the message boards.
Sean: yes, those acronyms (military folk do love those acronyms!) can be found in 
publications in the early 1950s and probably date from the second world war. 
The suggestion in the wikipedia article that attacks in the 18th century were 
timed to coincide with nautical twilight are completely unsourced and 
therefore probably just someone's homebrew theory ...or a simple error. It is 
much more likely that dawn attacks were timed to coincide with the beginning 
of civil twilight --in the traditional sense meaning the time period when you 
can see what you're doing-- which happens to be well-matched by the modern definition.
-FER

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