I didn't catch it on the first viewing, either; it was only thinking about the way the world was set up -- the fact that they travel from pole to pole -- that made me start wondering if the seasons are realistic in their world, and then I noticed the relative brightness of the nights at (or near) the pole compared to elsewhere in the show.

It's really a fantastic show. I've had several people recommend it to me, but had never sat down and watched it before. Once we started, I really couldn't stop; it mashed down a lot of my narrative buttons.