Once-in-a-century Transit of Venus will show off planet moving across Sun

It’s now or never for astronomers of all stripes to watch the planet Venus crawl for about six hours across the face of the sun.

That’s something that won’t happen again for another 105 years.

It’s called a “Transit of Venus” and it occurs Tuesday in the Western Hemisphere and Wednesday in the Eastern Hemisphere. 

Sky and Telescope magazine’s Kelly Beatty is waiting for the transit. He’ll watch on Point Venus on Western Hemisphere island of Tahiti, one of the best seats on Earth to see this rare celestial event.

See a map of the Transit of Venus.

“The Tahitians have gone all out for this,” he said.

This transit is so rare because of the relationship that Earth and Venus have to each other and the sun. It repeats only rarely, Beatty said. Though it was seen eight years ago across parts of the world, the next transit won’t be for decades.

“This is literally my last chance ever to see this,” he said.

In North America, you can watch a partial Venus Transit starting at about 6 p.m. Eastern time Tuesday and lasting until sunset.

Unlike a solar eclipse, where the moon completely obscures the sun, Venus isn’t big enough or close enough to Earth to completely block out the sun’s light. Instead, Beatty said, it’s a little black dot about 3 percent of the size of the sun that will slowly move in front of the sun.

“Some people have eyesight good enough to see that,” Beatty said. “But of course we don’t want you looking directly at the sun. You need some kind of projection.”

According to Sky and Telescope magazine, there are a few ways to safely look at Venus as it moves across the sun.

“You can view through special ‘eclipse shades’ (not regular sunglasses) or a dark rectangular arc-welder’s glass,” the magazine wrote. “Or, you can set up your telescope or even tripod-mounted binoculars to project the Sun’s image onto a white card or other flat surface. Solar filters are also made to fit over the front of your telescope.”

Venus will appear to be moving across the sun very, very slowly. All told, it will last more than six hours, but because of sunset and such, it won’t be visibly for that long in most places. In fact, in certain parts of Africa and South America, it won’t be visible at all because the transit will begin after the sun sets.