By Patrick Clinton
What makes stars twinkle?
--Michael Miller, Wheat Ridge, Colorado
You know when you dip a stick in the water, the way it looks bent? That's refraction, the tendency of light to change directions when it moves from one medium (such as air) to another (such as water). The same principle makes stars do the magical thing they do. As light travels from the star toward your eye, it passes through the turbulent upper atmosphere. There, rivers of air whiz around, constantly changing in temperature and density and therefore refracting light differently. As a result, your starlight gets jiggled through the chaos. Your eye registers the motion as twinkling. At the same time, interference patterns make the light grow rapidly brighter and darker, and the changes of refraction that cause twinkling also cause a steady stream of variations in the star's color. If there were no atmosphere, the stars would burn bright and steady: pretty, but not the poetic stuff that children's ditties are made of. You can think of twinkling as a kind of signal degradation, but hey, vive l'interference.
How long do bears hibernate? Don't they have to get up at all?
--Olga Yulikova, San Francisco, California
The "how long" part depends on the length and severity of the winter, since the purpose of hibernation is to reduce energy consumption during a season of scarce food and heavy demands on the body's internal heating system. Tennessee's black bears, for example, may hibernate for only a month or two, while the grizzlies in northern Alaska tend to conk out for seven months at a stretch.
A hibernating mammal's body temperature, heart rate, and energy consumption plummet. Here's the strange part, though: Small hibernators--mice and ground squirrels and the like--sleep deeper than their ursine cousins, but they're roused a lot more frequently. In fact, the little guys get up every four to six days to urinate, defecate, and eat food they've cached in their nests. Not bears. Bears curl up, get dormant in their dens, and that's it for months, according to the University of Iowa's Edgar Folk, a noted environmental physiologist who once worked with a team observing a hibernating bear via closed-circuit TV. "We watched him for a month. Not a movement." That means no food or bathroom breaks. "Bears recycle urea into protein, so they'll produce only about a glassful of urine in six months," says Folk.
It's just as well; you wouldn't want to run into a bear lumbering to the loo. But don't take this as carte blanche to go Goldilocksing. Bears torn from several months of slumber, says Folk, "are not nice."
I've always heard that on a cold night it's warmer to be naked in your sleeping bag than to wear a lot of clothes. This true?
--Don Aumann, Oakland, California
"I've heard that too," says Major Dave Rutledge, commandant of the U.S. Army's Northern Warfare Training Center at Fort Greely, Alaska. "It's the mitten-glove idea, that too many clothes can inhibit the circulation of warm air and therefore heat efficiency." In fact, an excess of clothing can use up the sleeping bag's dead-air space and even compress some of the insulating material, making you a cold camper. That said about too much modesty, it's not a good idea to snooze in the altogether either. Your usual human helping of sweat, dirt, and oils soak into the bag's fill and can rob you of insulating ability. Nighttime winter attire for Rutledge's charges is polypropylene long underwear, a hat, and socks. The hat will insulate as well as keep your hair oil off the bag. And the socks--well, they're for when you get up in the night, because menacing fur and oil and all, you're still no bear.
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