A once-in-a-lifetime journey

Ben Franklin watched it. Sousa named a march after it. Crowds jammed the sidewalks of New York to see it in 1882. But nobody alive today has ever seen the silhouette of Venus crawl across the face of the sun.

On Tuesday, that sky show -- astronomers call it a transit of Venus -- will return for the first time in 122 years, visible from much of Earth. Thousands of schools and hundreds of museums have set up programs, and tours to good viewing sites have been booked. Even people who don't want to leave their homes will be able to follow a live Webcast from Greece.

All this to watch a black dot inch across the lower part of the sun.

Only the tail end of the event will be visible in Missouri, and there are no plans to hold a viewing at Southeast Missouri State University's observatory, said physics professor Dr. Michael Cobb.

In the 18th and 19th centuries, the transit of Venus let scientists probe what one contemporary called "the noblest problem in astronomy": determining the distance from Earth to the sun. That in turn could be used to figure the distances to all the planets.

Though that problem was eventually solved with precision by other means, the Venus transits produced the first relatively solid estimates.