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May 2, 2004
Outside Magazine
Outside magazine, April 1998

By Elizabeth Royte

How do monarch butterflies stay on course while migrating between Mexico and the United States?
— J. Espy, Lima, Ohio

Like most migrating species, butterflies use internal "compasses" to monitor the sun's position and maintain a northerly direction during their annual 800-mile migration, which wraps up this month. But there's more: Monarchs home in on the chemical odor of billions of fallen butterfly scales at lepidoptera-friendly rest stops en route and use traces of magnetite in their wings to detect magnetic charges in the rocks. Superb pilotage notwithstanding, the flight does have inherent dangers for the one-ounce gossamers. Winds of up to 50 miles per hour dash some errant fliers to the ground. Still more die of exhaustion and starvation during the four-week winged marathon. "But you've got to give their tiny brains credit," gushes Thomas C. Emmel, professor of entomology and zoology at University of Florida-Gainesville. "It's a remarkable journey."

What's a moonbow, and where might you see one?
— Van Taylor, Orlando, Florida

The little-known moonbow is a ghostly white arc that appears when the light of a full or almost full Moon is refracted in a waterfall's mist. But whereas rainbows require only a smattering of raindrops and a few rays of sun, moonbows are decidedly more finicky: The falls must be in front of you and the Moon at 42 degrees in the sky behind you; the water must produce a heavy mist; and the river's gorge must be wide enough to let in plenty of moonlight. Which helps explain why Kentucky's Cumberland Falls and southern Africa's Victoria Falls are the only places known to yield moonbows on a regular basis. Niagara Falls used to have one until air pollution and city lights obscured it. All that's left now is an artificial "neonbow," compliments of the reflections of nearby No Vacancy signs.

What causes knuckles to crack? Does it damage anything?
— Allen McBride, Munford, Alabama

The sound of recreational knuckle cracking — the kind you do, say, while waiting in line at the grocery store — remains one of our great anatomical mysteries. But here's what experts have hypothesized: Bending your pinky backward sends synovial fluid, which lubricates joints, sloshing from one side of the knuckle to the other. "Studies suggest the popping noise is that of a gas rushing in to fill the space," explains Dr. Steven Green, of New York's Hospital for Joint Diseases. Mildly repulsive, yes, but surprisingly harmless, as the fluid quickly drains back into position. "Knuckle cracking leads to nothing," declares Green, refuting age-old warnings of arthritis and swollen joints. "Except maybe nasty looks from the person beside you."

What kind of trees absorb the most carbon dioxide?
— Aaron Agontar, Coconut Creek, Florida

All green plants absorb carbon dioxide from the atmosphere through their leaves during photosynthesis, but the amount fluctuates greatly. Carbon dioxide absorption falls off during the winter when deciduous trees lose their leaves and skyrockets when buds re-emerge in early spring. Older trees plagued by deteriorating cells, as well as those rooted in colder climes, grow slowly and thus absorb less carbon dioxide, consuming an average of about five pounds a year, while the fastest-sprouting trees-young evergreens with a year-round growing season-inhale the most. How much? Rainforest trees such as members of the genus Leucaena, which typically grow from seed to 20-foot-tall saplings in their first year, absorb about 50 pounds annually-roughly the amount of exhaust your car spews out in an hour. Sighs Sandra Brown, a University of Illinois forest ecologist and EPA researcher, "It's not what you'd call an encouraging ratio."

For much of April, the constellation Leo will be visible directly overhead in the night sky, with the bright star Regulus at its base. The Moon will be a few degrees southeast of Regulus on the seventh and full four days later-the first full Moon since the vernal equinox. In the predawn hours of the 22d, the Lyrid meteor shower will peak directly overhead in the constellation Lyra, with an average of 15 shooting stars per hour. Just before sunrise on the 23d, the waning crescent Moon will be visible 1.5 degrees east of Jupiter and Venus, the two brightest planets in the sky. On this same day in parts of the Southern Hemisphere, the Moon will block both planets in what will be the most spectacular double occultation since 1601. — Jean Quashnock

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