People living in the North get used to sympathetic remarks from southerners about the dark northern winter.

The sympathy, most will admit, is not entirely justified. After all, most Yukon communities get several hours of sunlight a day, even at the darkest time of year. Even Old Crow, the only Yukon community north of the Arctic Circle, experiences less darkness than most southerners, and some northerners, realize.

"Literature often suggests that it is dark at all points on or north of the Arctic Circle during the day of the winter solstice," says Carleton University geographer Chris Burn.

In fact, much of the area north of the Arctic Circle enjoys long periods of twilight bright enough for most daytime activities, even at the darkest time of the year. You have to go a lot further north to find true Polar Night, says Burn.

Polar Night is not just the period when the sun stays below the horizon, he says. It's the period of winter, lasting more than 24 hours, when there is no twilight at all.

And because of the way the sun's light interacts with the atmosphere, the earth experiences a great deal of twilight.

When the sun is near the horizon, its light enters the atmosphere at a low angle. The atmosphere bends the light around the curve of the earth, so that an observer on earth can actually see light from slightly below a straight line to the horizon.

Sunset and sunrise occur when the top of the sun appears above, or disappears below, the horizon. At that point, the centre of the sun is just under a degree below the horizon, and most of that extra bit of day is due to the bending of the sun's rays by the atmosphere.

"In the arctic winter, when the sun remains at low levels for a relatively long time, small changes in atmospheric conditions can cause large changes in the length of the day or the duration of twilight, due to bending of the sun's rays" says Burn. "People wintering in the High Arctic have even reported seeing the sun several weeks before it 'should' have risen."

"Of course, these considerations only hold for clear days, because clouds may veil the sun, and, in the Yukon, mountainous terrain also interferes with the passage of the sun's rays. People living in Keno, for example, have the sun in winter blocked by hills to the south, while at Mayo the sun can be seen every day of the year, clouds permitting."

There are three distinct definitions of twilight, related to how far the sun is below the horizon, he says.

Civil twilight occurs while the centre of the sun is between 50 minutes and 6 degrees below the horizon. Transport Canada allows planes to fly by Visual Flight Rules during civil twilight, and street lights aren't usually turned on until the end of civil twilight.

"In the evening, darkness forces us to stop normal outdoor activities towards the end of civil twilight," says Burn, "but even then it's not pitch black."

Nautical twilight lasts even longer, until the centre of the sun drops 12 degrees below the horizon. At the end of nautical twilight, it's usually impossible to make out the horizon, Burn says.

Finally, there's astronomical twilight, which lasts until the sun is 18 degrees below the horizon.

"In the Arctic there is generally no trace of the twilight glow in the southern sky when the sun is this far below the horizon," Burn says.

Just as there are three definitions of twilight, there are three definitions of Polar Night. Civil Polar Night is a continuous period of winter darkness, lasting more than 24 hours, with no trace of civil twilight. Nautical Polar Night is a similar period without nautical twilight, and Astronomical Polar Night is a prolonged period of darkness without astronomical twilight.

The southern limit of Civil Polar Night is latitude 72 degrees 33 minutes, Burn says. That's well north of any Yukon community. In fact, only a handful of communities in the Northwest Territories experience Civil Polar Night.

Nautical Polar Night is even more unusual. The only Canadian locations that experience it are government stations at Alert and Eureka in the Northwest Territories.

"Astronomical Polar Night is not experienced at any point on the Earth's land surface in the northern winter," says Burn.

And as for the myth that Polar Night begins at the Arctic Circle -- because of the bending of the sun's rays by the atmosphere, someone standing almost a degree north of the Arctic Circle might still see the sun rise and set briefly on the shortest day of the year.

For more information, contact Dr. Chris Burn at the Department of Geography, Carleton University, Ottawa K1S 5B6.