Where is the Endless Arctic Night?

Now that the tourists have flown south, and the last reporters have left the arctic whaling grounds, we can discuss an old northern myth just among ourselves. We don't have to tell anyone else.

All right, here it is: Barrow doesn't endure two months of total darkness every winter. In fact, our northernmost town never experiences 24 hours of total darkness at any time of year.

You can see why this news shouldn't get around.

It may not be so easy to see why it isn't a preposterous lie. Before the deluge of black photographs (or maybe videotapes) start arriving from zip code 99723, let me explain what astronomers mean when they discuss the days and nights of high-latitude locations.

First, a place has to lie above 66 degrees 33 minutes north (or south) latitude before the sun should be either above or below the horizon for a full 24 hours at some time during the year. That's the latitude of the Arctic (or Antarctic) Circle. However, the atmosphere plays tricks with light. Because refraction apparently lifts the sun a little more than its diameter when it is lying on the horizon, in summer the midnight sun is visible at 66 degrees. In winter, someone would have to go beyond 67 degrees by a bit not to see the sun at all on the winter solstice.

Barrow, at close to 71 degrees north latitude, certainly meets that qualification. But now we enter the twilight zone--literally.

Civil twilight is roughly defined as the time after sunset when enough light remains so that most outdoor activities can be continued. It's formally limited to the period when the sun is less than 6 degrees beneath the horizon.

Then there's nautical twilight, which ends when it becomes too hard for a sailor to pick out the line between sea and sky. That occurs when the sun drops down to 12 degrees below the horizon. Technically, the final stage, astronomical twilight, ends only when the upper rim of the sun drops to 18 degrees below the horizon.

In Barrow, the disc of the sun doesn't climb above the horizon for roughly a month on each side of the winter solstice. However, at noon on the winter solstice the sun's upper rim is only about 4´ degrees below the horizon, and that's bright twilight as far as astronomers are concerned. Even at Thule in northern Greenland, at about 77 degrees north latitude, conditions only approach the limit of nautical twilight. There's a weak glow in the southern sky to illuminate Christmas Day's noon.

To find true night at noon on winter solstice in the high latitudes, a traveler would have to stand within about 5´ degrees of the pole. But we don't have to tell our tourists about that. We can let them go on thinking that Alaskans bravely endure the Arctic night. As Barrow residents can testify, winter days are dark enough in the real twilight zone.