An hour into Finnair flight 603 from Helsinki there's a message from the flight deck. "Ladies and gentlemen," says the captain, "We have just crossed the Arctic Circle."

An hour later we land at Ivalo, 68.6 degrees north, and the world on this midwinter afternoon gives off an eerie sapphire glow. The ice-blue light emanates from everywhere and nowhere, saturating snow and sky. I take a hand from a glove and it, too, has turned blue from the light, or the cold, or both.