Engines of Our Ingenuity

No. 682:
THE SPEED OF LIGHT

by John H. Lienhard

Click here for audio of Episode 682.

Today, we find we've known some things much longer than we thought. 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.

The speed of light is something we've figured out only in the last century, right? Before the likes of Einstein, we surely knew nothing. Well, once more, our forebears surprise us. It turns out we've known the speed of light since before the birth of Johann Sebastian Bach.

That knowledge came close on the heels of the invention of the first telescopes in the early 1600s. In 1644, Ole Roemer was born in Jutland, Denmark. He took up the new study of astronomy with the early greats of that field.

By 1675 Roemer was 31 and working in Paris with Picard. He was interested in the movement of Jupiter's nearest moon. He tracked it as it orbited in and out of Jupiter's shadow. It entered the shadow, then reemerged 42 hours, 28 minutes, and 35 seconds later. It moved with metronomic regularity.

All that fit the clockwork perfection we saw in God's firmament during the 17th century. In one hundred transits, Jupiter's moon could be relied on to emerge once more, right on schedule.

Six months -- 100 laps -- later, Roemer set his clock and focused his telescope on Jupiter. He waited. No moon! Minutes passed. No moon. Finally it danced out of the shadows a full 15 minutes late.

So Roemer considered what might've happened. Earth had swung hundreds of millions of miles away from Jupiter during the long winter months. Light had to travel that vast distance. It'd obviously taken the extra time to do so.

He put pencil to paper and concluded that light had to move 192,500 miles per second to lose just fifteen minutes. Not bad! Roemer was within three percent of the right value. And that was less than 70 years after we first had telescopes.

Just to be sure, he calculated when we'd get that 15 minutes back, as we moved back toward Jupiter. He was right again.

I came on all this reading a 120-year-old physics text. The author tells Roemer's story. Then he quotes more recent estimates of the speed of light. One is worse than Roemer's. And so your great-grandparents really had no better knowledge of the speed of light than we had when Isaac Newton was still young.

It's sobering to reflect on the knowledge of our forbears. We knew the diameter of earth within 15 percent 2000 years ago. We've done brain surgery and cataract operations even longer than that. And so, it seems, there was life before the computer -- before Einstein -- before the Industrial Revolution. I'm constantly amazed by how much the mind accomplished before it had modern technology.

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

(Theme music)


Tyndall, J., Light and Electricity: Notes of Two Courses of Lectures Before the Royal Institution of Great Britain. New York: D. Appleton and Co., 1883, pp. 19-20.

Routledge, R., Discoveries and Inventions of the Nineteenth Century. ca. 1890. (Reprinted by Bracken Books, New York, 1989.) pp. 298-301.

See also encyclopedia listings on Roemer. I am grateful to Ms. Jeffery Scoggins of Detering's Bookstore for alertly spotting the rare and offbeat Tyndall volume for me.


The Engines of Our Ingenuity is Copyright © 1988-1997 by John H. Lienhard.

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