Rare celestial sight beckons Tuesday as Venus crosses in front of sun

Odds are we’ll all be gone the next time this dance comes around.

The planets Earth and Venus waltz in their orbits at different tempos and at different angles. But every once in a while we can see Venus’ form clearly silhouetted against the brilliance of the sun.

One of those occasions comes Tuesday afternoon — and it won’t come again until 2117.

It is called a transit of Venus and, like an eclipse, it gives us a chance to witness the mechanics of the solar system. But it’s much rarer than a lunar or solar eclipse. There have only been seven transits of Venus since the telescope was invented.

“This is something that parents and grandparents should show their kids because they will never, ever in our lifetimes get to see this again,” said Dave Hudgins, a professor of astronomy at Rockhurst University and a spokesman for the Astronomical Society of Kansas City.

The catch is you can’t look at it directly without seriously damaging your eyes. But observatories in Kansas City, Louisburg, Kan., and Emporia, Kan., will offer free and safe public viewing through filtered telescopes. A NASA website also promises real-time views from several locations.

Venus is closer to the sun than Earth is, and there are times when it passes directly between us and the sun, allowing us to watch as it crosses. What you will see is a small black ball move slowly across the face of the sun. The transit will begin at 5:10 p.m. here and take more than six hours, so there’s no rush. It will still be going on when the sun sets.

Transits of Venus got astronomers very excited in past centuries.

Nicolaus Copernicus had realized that the planets orbit the sun and not Earth. Johannes Kepler had calculated when Venus (and Mercury) would pass directly between Earth and the sun. Then Edmond Halley had the idea of using a transit of Venus as a way of determining the distance between Earth and the sun.

“Every school kid learns that is 93 million miles,” said DeWayne Backhus, professor emeritus of physical science at Emporia State University.

But nobody knew that before astronomers and explorers were dispatched all over the globe to collect measurements of transits during the 18th century. Capt. James Cook saw one from Tahiti. Mason and Dixon, who would later survey the line that became the divide between the North and the South in the United States, were sent to Sumatra. They only made it to Cape Town, South Africa, but they measured a transit from there.

With a lot of mathematical calculations, the general size of our solar system became clear.

The distances have since been refined to greater accuracy. But transits still have appeal to scientists.

NASA’s orbiting Solar Dynamics Observatory will be watching Tuesday’s transit to help calibrate its instruments and to learn more about Venus’ atmosphere. Astronaut Don Pettit will be taking and uploading pictures of the transit from the international space station — because he can.

Transits of Venus actually come in pairs eight years apart, followed by a span of more than 100 years before they occur again. That’s because of the peculiarities of the orbits of Earth and Venus.

The last transit was in 2004, but it was almost over by sunrise in Kansas City and it was cloudy that day, so few people got a chance to see it.

Amateur astronomer and photographer Tom Martinez was one of them, and he’s eager for a second chance.

“It popped out from between the clouds for just a few seconds,” he said. “I just got a glimpse and that was it. So this time, I’m hoping for clear skies.”