If the sun shines bright and warm on your skin today, count yourself more fortunate than the people in Alaska’s four arctic national parks. They’ll get a few hours of ambient light if the sky is clear, but there’ll be no warming sun.

Alaska has four National Park System units situated north of the Arctic Circle (66.5 degrees North), to whit: Gates of the Arctic National Park & Preserve, Kobuk Valley National Park,
Cape Krusenstern National Monument, and Noatak National Preserve. These arctic national parks have much in common besides physical factors like large size, beauty, and harshly cold and snowy winters. They also have some interesting solar effects rooted in the fact that they are on the poleward side of the Arctic Circle. This insures that all four will have some summer days when the sun never sets below the horizon and some winter days when there is no direct sunlight. From the former you get “Land of the Midnight Sun.” From the latter you get “Land of Twilight.”

Though astronomers and nitpickers are technically correct to quibble about the exact timing of the shortest day (see the Postscript), today – December 21 -- is generally accepted to be the shortest day of this year in the Northern Hemisphere. This is, in any event, the day when the noonday sun is directly overhead (90 degrees above the horizon) at the Tropic of Capricorn, which lies at about 23 1/2 degrees South latitude.

Because Earth’s axis is tilted nearly 23 1/2 degrees from the plane of the its orbit around the sun, and because the Northern Hemisphere is tilted away from the sun at this time of year, the sun’s tangent rays fall at nearly 66 1/2 degrees North latitude. This is the same as saying that the Circle of Illumination intersects the Arctic Circle.

The portent of this is quite clear. No place north of the Arctic Circle can receive direct sunlight during the winter solstice. That’s because one full rotation of the earth on its axis keeps arctic places within the unlit half of the globe.

Arctic locations do get ambient light from natural sources, including starlight, moonlight, and dim sunlight that is refracted and scattered from below the horizon. Thus, for example, Noatak National Preserve, which lies entirely north of the Arctic Circle, will be blessed today with several hours of twilight and about 1 1/2 hours when the refraction of sunlight makes it appear that the sun is actually above the horizon.

There is a positive side to the winter solstice. Beginning today there is a reversal of the shortening of days and lengthening of nights that began with the summer solstice last June. For the next six months, the days will grow longer in the Northern Hemisphere.

Postscript: If you mention the term “winter solstice” in the same sentence as December 21 while in the presence of a scientist or someone from the Southern Hemisphere, prepare to get an earful. The latter will remind you that it’s summer in the Southern Hemisphere, not winter, so the appropriate term to use is December solstice. The former will remind you that, even though the December solstice is an astronomical event that occurs at a precise instant -- that is, the instant that the noon sun is directly overhead at the Tropic of Capricorn and the Circle of Illumination intersects the Arctic Circle -- Earth's orbit is elliptical, not circular, and Earth itself is an oblate spheroid, not a perfect sphere. This means that, if the shortest day is to be defined as the least time elapsing between sunrise and sunset, it's impossible for all places on Earth to experience the shortest day of the year on the same date. The difference can be up to several weeks.