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
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
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