Charles,
The location of the geometric Sun at Sunrise/Sunset is -(50/60)degs.
This is based on a mean atmospheric refraction value of -34'(arc-mins)
and a semidiameter for the solar disk of 12'(arc-mins). Sunrise occurs
when the limb of the Sun not the center (which most calculations are
made for) breaks the local horizon.
Also in this vein, I'd like to second Daniel Wenger's example of using
a locally oriented globe to judge the time to sunrise, sunset, twilight,
not to mention the actual time. It's a very intuitive sundial type.
-Luke
Charles Gann wrote:
> I've always been confused about when the three twilights begin and end. My
> best guess is that the order is:
>
> (1) Astronomical twilight begins (altitude = -18 deg)
> (2) Nautical twilight begins (altitude = -12 deg)
> (3) Civil twilight begins (altitude = -6 deg)
> (4) Sunrise (altitude = 0 deg)
> (5) Sunset (altitude = 0 deg)
> (6) Civil twilight ends (altitude = -6 deg)
> (7) Nautical twilight ends (altitude = -12 deg)
> (8) Astronomical twilight ends (altitude = -18 deg)
>
> If this is not right, would someone please set me straight and "end" my
> confusion. (My "Confusion twilight" "began" quite some time ago :)
>
> Charlles
