Ooops, the semidiameter value (below) should have been 16'(arc-mins)! -Luke
Luke Coletti wrote: > > 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
