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    Re: History of Twilight
    From: Frank Reed
    Date: 2014 Sep 21, 23:13 -0700

    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 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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