by John H. Lienhard

Click here for audio of Episode 1642.

Today, a comet approaches. 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.

H alley's Comet appeared in the sky when Mark Twain was born in 1835. The comet moves in a seventy-five or seventy-six-year orbit, and, as it neared Earth once again, Twain said,

I came in with Halley's Comet... It is coming again ... and I expect to go out with it... The Almighty has said, no doubt: 'Now here are these two unaccountable freaks; they came in together, they must go out together.'

Sure enough, he died on April 21, 1910, just as the comet made its next pass within sight of Earth. And we hear echoes of Shakespeare:

When beggars die, then are no comets seen:

The heavens themselves blaze forth the death of princes.

But the orbit of Halley's comet specifically mocks the human lifespan. I find a strange song in an 1858 book of French songs: "The Comet of 1832". Since that was three years before Halley's Comet, the song-writer may've made a mistake. Or he may be referring to a different, or even to a fictional, comet. The song begins,

God is sending a comet against us

We shall not escape this great impact

I feel our planet crumbling already...

And it ends,

Go quickly to confession you timorous souls

Let us be done with it, the world is old enough

... The world is old enough.

Halley's Comet was named after Edmund Halley, who realized that several earlier comets were one and the same. Then he calculated its narrow elliptical orbit. The comet rides in from the outer fringe of the solar system, makes a tight turn around the sun (inside the orbit of Mercury), and then sling-shots back out, far beyond Neptune. Halley predicted the comet's return in 1758, but he didn't live to see it. Many older comets turned out to be previous sightings of this one. It was seen just as the Normans set out across the English Channel to conquer England.

The nucleus of Halley's comet (like that of any comet) is a ball of frozen gases and other matter -- only about six miles in diameter. As it nears the sun, the material warms and sublimates into gas. The result is a great tail of ionized gas -- maybe a hundred million miles in length. As it passes about the sun, the solar wind sweeps the tail outward, so it points away from the sun. It doesn't stream out behind the comet as we'd expect.

In fact, as we look at comets, we sometimes view them along the line of their tails, even though they aren't moving toward us. When that happens we see unlikely, and sometimes ominous, shapes -- multiple tails or question marks in the sky.

The strange brilliance of comets has always touched us with awe. As a teacher, I especially like the way the great biologist Carl Linnaeus used the metaphor when he said of his students,

... the true discoverers are among them,

as comets among the stars.

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

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