NavList:

There's no mystery about the difference in time period for sunrise based on latitude and declination. All you have to do is look at the rate of altitude change which varies with the change in azimuth around the time of sunrise which is amplified by the change in refraction at low altitudes. Looking at the simplest case and ignoring refraction, on the equinox , September 22, 2016 the sun rose on the equator at 0555 on azimuth 090 and one hour later the azimuth is still 090, the sun went straight up. The rate of change in HC (not Hs) was 15-00.2' /hour. At 45 degrees south the sun also rose at at 0555 at azimuth 089 but one hour later the azimuth had changed 10 degrees to 079 and the rate of altitude change of Hc was 10-33.0' only 2/3rds the rate on the equator. Doing the same comparison on the solstice, December 22nd, at the equator the sun rose on azimuth 113 and that changed to 114 after one hour and the rate of Hc change is 13-44.0' per hour. At 45 south it rose on azimuth 124 which had changed to 114 after one hour and the rate of Hc change was only 9-15.5' per hour.  So, no mystery.  gl

Gary. That’s another interesting way of looking at it.  I’m sure your figures are correct.  They tie in with what I pointed out in my Figure 2 except I was talking about the length of the twilight zone.  However, what I was actually trying to do was to explain using Figures 1 and 3 ‘why’ that happened.  DaveP