New vocabulary became necessary in the past few years, however, as hazy light started lingering on the southern horizon deep into the months when the sky normally would contain nothing but stars.
Residents of Nunavut struggle for words when asked to describe the phenomenon.
"It's like dawn," said Marty Kuluguqtuq, a municipal worker in Grise Fiord. "It's like a glimmer on the horizon."
Philip Manik, a board member for the Hunters and Trappers Association in Resolute Bay, 1,700 kilometres south of the geographic North Pole, said the light has grown steadily brighter over the past decade. "Right now, the horizon is rainbow-coloured. It covers almost the whole horizon, even at the darkest time of year."
At first, scientists were skeptical about the natives' reports of unnatural brightness in winter. Climate data showed the Arctic getting warmer, and the rules of physics suggested that such low-density air would make the winter darkness even more profound.
But the sliver of light at the edge of the sky grew brighter each year, and locals kept calling the weather station in Resolute Bay for some kind of explanation.
"A lot of Inuit were reporting this across the North, which contradicted my original thinking," said Wayne Davidson, a fluent speaker of Inuktitut and the town's Environment Canada station operator for nearly two decades.
"When I figured it out, I thought, Oouf! What a goof I was!"
Mr. Davidson guessed the strange twilight is a mirage, a little-known effect of climate change in which thermal inversions carry light from the South into the dark North. He tested the theory by floating a weather balloon into the sky in Resolute Bay on a November day more than a week after the winter night began, and the sky showed an unnatural glow.
The balloon recorded the temperature near the ground at -27. Climbing to 285 metres, it warmed up to -15, and at 657 metres it reached -12. "That's a very steep inversion, one of the steepest I've ever measured."
With help from the Dutch physicist Siebren van der Werf, Mr. Davidson developed a detailed explanation: The sharp difference between the cold air on the ground and the warmer air above makes a prism that reflects light hundreds of kilometres away.
Mariners have recorded such tricks of the light in their logbooks for centuries. One type of thermal inversion was even named Fata Morgana for its tendency to create illusions of castles that resemble the crystal palace of King Arthur's legendary sister.
But only the modern trend toward global warming could have blanketed the North with enough warm air to create a widespread and sustained brightening of the Arctic nights, Mr. Davidson said.
"It's very strange, and it only started happening in the last 10 years, which coincided with the massive warming of the planet."
Mr. Davidson coined a term for the phenomenon: "Y V Ulluq Q," combining the initials of well-known researchers in the field with the Inuit word that comes closest to describing bright twilight.
Northern residents call it something else: "It's a bad sign," Mr. Manik said. "Any climate change will surely affect the hunters and the animals."
Snow used to fall on Resolute Bay in late August, Mr. Manik said, but now it arrives in September or October. "There is a change. Definitely a change. The ice conditions have changed a lot. The climate is different every year. It doesn't get as cold as before. It's not good."
