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The Straight Dope 

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Two questions. First, does it bug you when people write you questions on bar napkins, like I am? Second, as we leave a stoplight in our cars and look at the wheels of the car next to us, we notice that when the RPMs reach a certain point we get the optical illusion of reverse rotation. What causes this? --J.R. Newman, Washington, D.C.

After 17 years of mystery mail, J.R., believe me, the only letters that bug me are the ones that smell, bubble, or tick. I'll tell you one thing, though--if you notice the reverse-rotation phenomenon looking out your window, you're a lot more pixilated than I thought. Reverse rotation usually only shows up when you're watching a movie.

Basically what you're seeing is a strobe (i.e., stop-action) effect. A movie camera operates at 24 frames per second. If a wheel is turning at some multiple of 24 revolutions per second, the spokes will be in the exact same position every time the shutter opens. Ergo, the wheel will appear to be motionless (though possibly blurred) when the movie is played back. If the wheel now begins to slow down slightly, it doesn't get a chance to rotate all the way around to its original position before the shutter opens again. Therefore it appears to be rotating backward.

Don't get it? Then try the following demonstration, courtesy of Rainbows, Curve Balls & Other Wonders of the Natural World Explained by science writer Ira Flatow. Get a hand eggbeater. While rotating the handle, look at a TV picture through the eggbeater's blades. (For best results, point the eggbeater at the TV and look through the blades the long way.) When you get to 30 revolutions per second, the speed at which a TV picture flashes, the blades will appear to be motionless. Slow down slightly and the blades appear to rotate backward. Fun for the whole family. Incidentally, this is one of the few times you'll see reverse rotation outside the movies. People tell me they've noticed it on other occasions in real life as well, but they can never recall the circumstances. I'll concede it's a possibility, but absent some mechanism for flickering or blinking, it's damned unlikely. If you want to dispute the issue, therefore, be prepared to dish up some facts.

Who decides on the names of hurricanes? They go over different countries, so if the U.S. names a hurricane, can Cuba give it a different name? This is something everyone should know. --Carrie Seros, Silver Spring, Maryland

Call it meteorological imperialism, but the U.S. pretty much calls the shots in the hurricane department. American weather agencies began assigning girls' names to major tropical storms in 1953, apparently having gotten the idea from military forecasters. Later the assigning of names for Atlantic hurricanes was turned over to the World Meteorological Organization, a UN agency, theoretically making it an international responsibility. (Naming of hurricanes in the eastern Pacific is handled through a bilateral agreement with Mexico.) Evenhanded as all this sounds, there's no doubt Uncle Sam still runs the show. When the U.S., prodded by feminists, proposed that boys' names alternate with girls' starting in 1979, a committee of the WMO accepted without a whimper. To keep the Central and South American countries happy, a few Spanish names (e.g., Ernesto, Rafael) are sprinkled amongst the Anglos.

In case you were wondering, lists of names for hurricanes (and, in the western Pacific, typhoons) are established well in advance. For the Atlantic there are four alphabetical lists, which rotate so the same list is used every fourth year. Names of blockbuster storms (such as Hugo, no doubt) are retired for a while. While this is all very rational and scientific, I still prefer the system used in Australia around the turn of the century--they used to name hurricanes after politicians. Think of the possibilities. President Bush obviously isn't scaring anybody, but Hurricane Bush--we'd have 'em on their knees in a week.

Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.


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