Engines of Our Ingenuity

No. 1837:
DISTANCES IN SPACE

by John H. Lienhard

Today, how far is the moon? The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

An interesting note in this week's New York Times, Science Times section — it's about measuring distance in space. Consider the problem: all the familiar measures of distance are related to our bodies. A yard or a meter is roughly the length of an arm. The inch is the length of a finger digit. A mile or a kilometer is about as far as we can make out a distant human figure.

Distances in space are vastly larger than we are, so they become abstract. You and I can speak of a light year, but I can't form any concept of so large a distance. Light travels 186,000 miles in a single second. That's already incomprehensibly large. In a year, light travels six trillion miles and that is further beyond our comprehension than the national debt.

When we use the light year as a measure of interstellar distance, we are reciting numbers that are far beyond anything we can wrap our minds around. And then we reflect upon the fact that, the far fringes of the universe are fifteen billion light years away.

Another measure that we hear occasionally is the parsec. That one is less straightforward. Imagine that you're traveling far away from the solar system, and looking back at Earth. You'd be one parsec away when the diameter of Earth's orbit took up one second of an arc in your visual field. The diameter, not of Earth, but of its orbit about the sun — and only one second of one minute of one degree. A parsec is a little over three light years; it has nothing to do with times or rates.

For a more user-friendly measure of large distances, try the A.U. — the Astronomical Unit. One A.U. is the mean distance from Earth to the Sun — about ninety million miles. Science writer Amir Aczel points out that Mars, which hovers over our houses this summer of 2003, is now within four tenths of an A.U. Distant Pluto is thirty-nine A.U. — off in the cold fringes of the solar system.

We need, says Aczel, a measure of vastness that makes more sense than any of these. He suggests a new measure, the jet year. That's the distance you'd fly in a jet in one year's time. At six hundred miles an hour, that would come to a little over five million miles. Mars is presently about six jet years away from us.

I'm intrigued by Aczel's suggestion. Does it bring large distances (if you'll pardon a bit of hyperbole) down to Earth? I wonder. I know what four jet hours feels like. I enter the cabin fresh and rested. I get off in what, at first, looks like the same place, but now I am cramped and achy. I have difficulty associating any distance at all with a jet year.

Azcel points out that career jet pilots will fly more than the distance to Mars and I wonder how many frequent flyer miles a jet year would earn me. I'd better content myself with noting that, if I were laid end-to-end 209-million times, I would reach the moon.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)


A. D. Aczel, A Measurement Whose Time Has Come. New York Times, Science Times, September 9, 2003, pg. D3.
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The Engines of Our Ingenuity is Copyright © 1988-2003 by John H. Lienhard.