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    Re: twilight computer
    From: Gary LaPook
    Date: 2009 May 31, 02:18 -0700
    Many years ago I modified my E-6B to make it into a twilight computer for polar flight and I am attaching images of it.

    Image 5938 shows the E-6B with a photo copy of the polar region attached to the slide. Image -39 shows a closeup.  -40 and -42 shows the slide positioned to place the north pole under the grommet.-41 show a pink dot made at the 70º north circle intersection with the Greenwich meridian representing the location of the sunrise-sunset line for the sun at declination  20º south by way of illustration. The sun will not rise at all within 20º of the north pole and this is how the twilight calculator works.

    -42 shows the slide repositioned so that the horizontal line is under the pink dot. We would then draw a line by tracing this horizontal line to put in the sunrise-sunset line. We place another line parallel to the first 6º north of the first line to represent nautical twilight. On -43 I placed a piece of pink highlighter tape in this position and its width is about 6º so it represents the twilight band.

    -44 shows the slide again positioned with the pole under the grommet and the pink tape shows the twilight band at noon GMT. South of the pink tape the sun is up, north of the tape it is dark.

    -45 shows the face rotated 30º clockwise  (index at 330º) showing the twilight band at 1400 Z and -46 shows the same at 1600Z.

    I am also attaching the pages from H.O. 216 showing this type of twilight computer.


    Robert Eno wrote:
    I have been looking for one of these for 20 years. No luck so far. I know
    the man who helped to developed it and had the pleasure of handling the
    original prototype in 1990.
    The twilight computer is discussed in a publication called: "Arctic Air
    Navigation" by Cmdr. K.R. Greenaway.
    Attached, are some photos from the book.
    ----- Original Message -----
    From: <waldendand@YAHOO.COM>
    To: <NavList@fer3.com>
    Sent: Friday, May 29, 2009 4:51 PM
    Subject: [NavList 8445] twilight computer
    Try google patent for this twilight computer:
    Patent number: 2633295
    Problem is in northern latitudes it is possible that a whole flight may be
    in twilight, when neither sun nor stars can be observed.  Good to plan
    around this.
    I've been looking for one of these.  Anyone got one?

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