I know it's only trivia compared to the technical discussion above, but I do smile at the approach of those pax who take the "Santa Claus" one-day flights to Rovamiemi, Finland, in December. Depart UK about 0800 local, 3 hour flight to Finland. Dark on arrival.
"It's dark. When does it get light ?".
"It doesn't".
"What, not at all ?".
"Well, now, what date is it ?".
"December 21. So ?".
"And whereabouts are you ?"
"On the Arctic Circle. So ?".
"Then it will be dark all day".
"But we've brought the children all this way ...."