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
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
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
A. D. Aczel, A Measurement Whose Time Has Come.
New York Times, Science Times, September 9,
2003, pg. D3.