Darkest Day

By Bob Berman|Sunday, December 01, 1996
RELATED TAGS: SOLAR SYSTEM
April may be the cruelest month, but December certainly is the darkest--the darkest, in fact, for everyone north of the equator. On December 21, at 9:05 a.m. eastern time, Earth’s northern axis points farthest away from the sun, and the sun’s path across our sky is its lowest of the year. That occasion is the winter solstice.

You’d think the solstice would hold few surprises. Think again. When the media announce that December 21 has the longest night, people generally expect to get their earliest sunset or latest sunrise, or both. They get neither: the earliest sunset happens half a month before the solstice, and the latest sunrise doesn’t occur until January. At the solstice, it’s merely the sun’s low position that makes for our shortest day.

Another myth is that the solstice brings a kind of permanent night to everyone north of the Arctic Circle’s latitude of 66.5 degrees. This would be the case were it not for our atmosphere, which, when the sun is at the horizon, bends the image of the sun upward by the sun’s own diameter. On the solstice, the sun is actually always visible anywhere south of latitude 67.

And how about those esoteric tours to where it’s midnight at noon? How far north must you trek to find a place with 24 hours of night? Farther than you’d imagine. Even 500 miles beyond the Arctic Circle, atmospheric refraction keeps the day bright enough to read by. At 800 miles north of the circle, you’d experience nautical twilight, with a distant horizon still visible in the darkness. To experience total noonday night on the winter solstice, you’d have to place yourself within 400 miles of the exact North Pole.

No stargazer is nutty enough to do that. After all, even with 24 hours of nonstop sky gazing, the North Pole offers 50 percent fewer stars than are visible during a single 12-hour night in the United States. (That’s because no new stars ever rise there; the same ones keep going round and round.) So let’s cancel this unpleasant solstitial thought experiment in favor of the positive offerings of the long December nights of 1996.

This month’s highlight: The Geminid meteor shower on December 13. Unlike last year or next year, the moon will present a harmless crescent that sets by 8:30 p.m., allowing excellent observing conditions for this rich but odd shower.

The Geminid display is reliably lavish. It offers at least as many shooting stars (60 per hour) as its more famous summer cousin, the Perseid shower. And the Geminids’ exceptionally slow speed--just 17 miles per second, half the velocity of the Perseids--lets them appear as discrete falling objects instead of blurry streaks. One more plus: Unlike the summer’s late-night Perseids, the Geminid shower reaches its greatest intensity before midnight. Since the meteors of this past August were entertaining but not exceptional, the frosty Geminids may well be the Shower of the Year. Not an undeserved honor. The only reason, it seems, that this distinctive display is relatively unknown is that few sensible people willingly remain motionless under a chilly December sky.

Eleven days later we get our brightest night--on Christmas Eve-- with a full moon that is also the year’s highest. While a full moon will fall on a specific date (like your birthday) every 30 years on average, 1996 has the only Christmas Eve occurrence in this half century. So while it will subdue the background stars and foreground meteors, we can perhaps sacrifice useful astronomy that one night to provide Christmas carolers with a celestial spotlight.
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