A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2020 Dec 24, 20:53 -0800
David, you wrote:
"E.g. do you count the time that the Sun is just below the horizon?"
I think you can assume that this is based on the standard definition of sunlit hours in the modern world, which is the time between the standard, listed time of sunrise and the corresponding time of sunset. So the Sun's upper limb is on "the" horizon (tricky details in that "the").
"Do you count the Sun’s intensity; i.e. the angle it strikes the ground (it’s not icy at the Poles and blazing at the Equator for nothing)?"
No. As near as I can tell, this calculation does not describe any measure of energy on the ground. It's designed to show something specific in the "whaddaya know" category, not necessarily scientifically important. It's clever trivia.
And you wrote:
"If you count daylight as any time it’s not actually dark, the Polar Regions probably do win."
OK. Why? I'm not asking for any rock-solid logic or computation here. I'm just wondering why this makes sense to you on a gut level.
"However, I would have expected the colour bands to be the same for both the Southern and Northern Hemispheres. Otherwise, it’s not fair on the penguins is it."
Pengins live in Ecuador! :)
But ignoring our fine feathered friends... it's not hard to see why there's a North/South asymmetry by simplifying a bit. Consider observers exactly at the poles and use a simple (and traditional) definition of sunrise and sunset, namely the instant when the Sun's unrefracted center is on the true horizon which means 90° from the zenith. This is where the definitions of the equinoxes come into play. At the north pole, the Sun is above the horizon in 2021 from Mar 20 until Sep 22. That's 51% of the year. So per day, that's 51% of 24 hours, which implies 12h14m of daylight per day up there. The 49% at the south pole is equivalent to 11h46m. Of course this happens because the Earth slows down on the side of its orbit centered on aphelion which occurs in early January so the total length of Fall plus Winter in the northern hemisphere is longer than Spring plus Summer. Note that the Sun is bigger in angular diameter during that shorter stretch so the implication for insolation deserves a little scrutiny. Does it cancel out? Does it cancel out and then some??
So what's up with the bump around the Arctic and Antarctic Circles? I'm not sure why those are the local maxima, but I suspect it's dependent on refraction at the horizon.