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2203Re: DX-Atlas Twilight Setting

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  • bill_w4zv
    Oct 31, 2010
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      --- In dxatlas_group@yahoogroups.com, "db4iw" <db4iw@...> wrote:
      >
      > I wonder which setting for the twilight zone (civil, nautical, astronomical) would be the best for judging possible grey line conditions for MW and for the lower SW bands. Is there any suggestion or rule of thumb?

      The short answer: it makes no difference. You should be interested in the actual time the sun is on the horizon, and you'll see this is constant for all 3 choices in DX Atlas. The 3 choices are actually showing the times after the sun sinks below the horizon by different amounts:

      Civil: 6 degrees below horizon
      Nautical: 12 degrees below
      Astronomical: 18 degrees below

      http://en.wikipedia.org/wiki/Twilight

      I've been using actual sunrise/sunset times (not twilight) for 30 years and originally calculated them based on the formula in ON4UN's first 80m Dx-ing book (which had an error I discovered and corrected in a note to John).

      If you use the actual sunrise/sunset times, you'll learn to judge the best time for propagation based on experience. For example, I learned that the best long path propagation on 80m usually comes with ~40 minutes of common darkness and the peak is usually half-way between (e.g. 20 minutes past my sunset and 20 minutes before DX sunrise). On 160m it's a bit more:

      http://lists.contesting.com/archives//html/Topband/2002-02/msg00132.html

      For short path propagation there's sometimes a very pronounced peak right at sunrise/sunset, or the best propagation may not begin until *AFTER* sunrise on the DX end (usually extending 15-20 minutes past his sunrise). Learning this is simply a matter of experience but you should always be looking at actual sunrise/sunset times so you can make accurate relative comparisons.

      BTW actual sunrise/sunset times are those displayed by DX Atlas when you press Ctrl-D when a target location is highlighted (i.e. none of the "twilight" times).

      73, Bill W4ZV (DXCC: 347 on 80m, 331 on 160m)
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