Twice every century, about eight years apart, the planet Venus passes between Earth and the sun. And the next such transit is this week, on June 5th.

Visible to the protected eye, Venus will appear to be a traveling "beauty spot" across the surface of the sun.

It is exactly the kind of event that fuels the scientific curiosity and enthusiasm of an astronomer like Adam Block, the manager of public observing programs at the Mount Lemmon Sky Center.

The passage of the planet Venus across the sun is expected to begin at 3 p.m. local time, but it may take a few minutes for the planet to be visible to the casual observer. Those who wish to see the event without a telescope may do so, but only if they look through solar-shielded eye protection, like the cardboard "Eclipse Shades" the Sky Center is making available through outlets like the Flandrau Science Center on the University of Arizona campus.

Interested viewers can also follow the planet's progress online, via live streaming video made available on the Sky Center's website.

Block says that although the transit of Venus may be a subtler event than a solar eclipse, its scientific and historical importance are greater than its aesthetics.

"We're going to be seeing another planet go in front of the sun and that's important in terms of humanity's understanding of how big the universe is," says Block. "By seeing Venus go in front of the sun we've had an opportunity to know how far away the sun is, and that gives us a sense of how big our universe is."

The event is rare in terms of a human life span, occurring about every 105 years. It also always comes in pairs, so what sky-watchers will see on June 5th is actually the second half of an event that first occurred in 2004.

As Block says, unless medical science makes a major breakthrough in longevity, the June 5th appearance of Venus back-lit by the sun will be the last chance for most of us to ever see this portion of our solar system's ongoing ballet.