Today, we chase a mini-eclipse. 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.

Every century or so, Venus makes two passes between us and the Sun -- about ten years apart. Unlike our Moon, which blots out the Sun, Venus is far away. Watch it through a dark glass and you see only a small black spot moving across the sun's disc. Venus blots out nothing, but that crossing, or transit, of the Sun gives the information we need to calculate the size of the solar system. Eighteenth-century astronomers saw a huge scientific payoff in observing and measuring their two transits.

Now think about solar eclipses -- those infrequent transits of the Moon across the face of the Sun. People will travel far to see one. Transits of Venus are even rarer, and observing the two that occurred in the 18th century was terribly important to astronomers.

So a young French astronomer named Jean Chappe set out to measure the 1761 transit of Venus, which would be visible in June from Tobolsk in central Siberia. Chappe left Paris with an expedition to Tobolsk in November the year before. Astronomy professor Donald Fernie describes the trip: The first eight days on the wretched roads from Paris to Strasburg left all the glass in Chappe's instruments smashed and his carriages ruined. Resupplied, he caught a riverboat down the Danube from Ulm to Vienna. From there, cross-country to Warsaw where he arrived on January 22nd.

In Warsaw, his group switched to horse-drawn sleighs. They made it to Moscow on March 17th. Now they had to cross the Ural Mountains while the weather stayed cold. Tobolsk lay in a vast marshland that'd be impassable once the snows melted. Chappe arrived in mid-April. It'd taken him five months to get there.

He set up his small observatory on a nearby mountaintop while spring flooding savaged the region that year. He was threatened by locals who thought they were being punished for letting a foreigner mess with the Sun. But the day came, and Chappe made measurements that served astronomers for the next century. It had, in fact, been well worth the trouble. But trouble it had been.

The second transit that century occurred in 1769. Chappe died of disease on his journey to Baja California to measure that one. Captain Cook's astronomer viewed it from Tahiti. And another French astronomer went all the way to the Philippines in a nightmarish four-year voyage, only to be foiled by cloud cover.

In all, 18th-century astronomers made eight observations which fixed the distance from Earth to the sun until Venus made two more transits in 1874 and 1882. We now wait for the next transit on June 7th, 2004. There'll be no wild horse-drawn journeys across the tundra this time. We'll view this one from a more remote place than Tobolsk. For now our telescopes ride in space. No more dreams dashed by random cloud cover. No more wolves or shipwrecks. But there will be new adventures, new dangers and -- new surprises.

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

(Theme music)

Fernie, J. D., Transits, Travels and Tribulations, III. American Scientist, March-April 1998, pp. 123-126.

Since I did this program early in 1998, a new book on the subject has been completed: Sellers, D., The Transit of Venus. Magavelda Press, 2001.

For more on observations of the transit of Venus, see the Wikipedia article.