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The Straight Dope

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On every commercial airliner, the little "card in the seat pocket in front of you" warns you not to use AM or FM receivers because they might interfere with navigation equipment. How so? I could understand not operating a transmitter, since any aircraft has a large number of radio receivers, but why would a simple portable radio be a problem? --Curious in Chicago

You probably have the idea that radio receivers are passive, vegetablelike devices incapable of making trouble for anybody, and at one time, I suppose, you would have been right. But technology has made great strides over the years. Today a receiver can wreak every bit as much environmental havoc in its way as a transmitter--and not just in the form of boom boxes on the subway, either.

Most modern receivers use something called a "local oscillator," which is sort of an internal transmitter. The oscillator generates signal A, which is mixed with the somewhat raw (if entertaining) incoming signal B to produce nice, easy-to-work-with signal C. There's usually some sort of shielding around the oscillator, but it's not always real effective and sometimes errant signals leak out to make life difficult for other radio equipment nearby. If the other equipment happens to be an aircraft navigation device, somebody could wind up digging furrows with a $25 million plow. So do your bit for air safety and bring a tape player instead.

Interesting digression department: Radios aren't the only gizmos spewing electrical graffiti into the ozone--so do lots of other things, such as electric typewriters. Typewriter static isn't a threat to airplanes, but it did get the Pentagon to thinking, jeez, what if the Russkies figure out how to pick up that stuff and translate it? So the National Security Agency set up a program called Tempest to test "information processing equipment" to see if it gives off "compromising emanations," the better to spy-proof government offices. If you want to do the same for your office, Uncle Sam can send you its official list of Tempest-tested products. Who knows, it could be the status symbol of the 90s.

Why does the same side of the moon always face the earth? It seems like quite a coincidence that it should rotate so perfectly in sync with us. Is there any slippage, so that parts of the dark side of the moon are slowly being revealed to us? --Trigby Perea, Northridge, California

You're right to be suspicious about this, spud, but let's clear up one thing first: there is no "dark side of the moon," the popular expression notwithstanding. All of the moon is illuminated at some point during the month-long lunar day; it's just that we can't see when it's high noon on the back side.

As for the moon's rotation, you're right in thinking the timing is a little too neat to be coincidental. It was different once upon a time, though. Experts think that billions of years ago the moon was much closer to the earth than it is now and rotated much faster, so that over time the entire lunar surface could be seen from earth.

But "tidal friction" slowed the moon down. The earth's gravity caused the side of the moon closest to us to bulge outward, just as the moon's gravity causes our oceans to bulge and create tides. The continual deformation of the lunar crust as it rotated relative to the earth acted as an interplanetary brake, and eventually the moon slowed so that the same side always faced toward the earth. (The moon also got farther away.) The result is called "captured" or "synchronous" rotation, and it's common throughout the solar system.

Eccentricities in the lunar orbit and whatnot periodically do bring some of the moon's back side into view, a process called "libration." In all, about 59 percent of the moon's surface is visible at some point. But you'd better look while there's still time--the moon continues to recede from us. Admittedly this occurs at the rate of just four inches per month, so it's not as though you have to rush out right this minute. The pull of the moon has also had a braking effect on the earth, causing our rotation to slow (and thus our day to lengthen) at the rate of one second per hundred thousand years. Good news, at least in the long term, for those who complain there's never enough time in the day.

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

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