What's So Sexy About Venus' Transit?

Solar Eclipse. Lunar Eclipse. Venus Transit!

It’s a trifecta for celestial viewing that has taken place all in less than a month’s time. And it’s been an excellent boost for places like the or NASA/Ames’ Exploration Center.

We asked Chabot staff astronomer Ben Burress what’s the big deal.

 “You're part of history,” he said. "You don't want to miss it, it won't happen again in your lifetime." He waxed effusively about seeing the first part of the pair eight years ago—and that wasn’t even in person, it was via webcast from Turkey.

Scientifically, it was a huge advance, he said. This viewing is only the eighth in all of human history, because the telescope wasn't invented until 1608. In 1639 observation expeditions went out across the planet to triangulate to figure out how far away the sun was. "Once we got that ruler, we then knew the whole scale of the solar system." 

It’s not quite as impressive as the annular eclipse of the sun, the one that we just saw on May 20. It’s more like a dot, traveling along one side of the sun.

“It’s not just any dot,” Burress said. “This is the history of Venus. Capt. Cook saw it in the 1769 in Tahiti, that’s why there’s a place called Venus Fort in Maitavai Bay."

In the Bay Area, we are on the edge of the transit, Burress said. “We will see the beginning of it, 4-5 hours of it.” To see the end, look for the live webcast from the Big Island in Hawaii, at the observatory at Mauna Kea, at altitudes where the atmosphere is very clear and an array of giant telescopes are pointed toward the sky.

And there are phenomena to look for. The time to watch is the beginning and the end of Venus’ trek across the face of the sun, Burress said.

Here's how to sound like you know what you’re talking about:

• Tell people you’re going to be there at the beginning, because that’s where the action’s at.
• Start tossing around terms, like “the Black Drop Effect” and impress your friends, or 
• Ask fellow viewers if they can discern the aureole at the beginning or the end of Venus’ trek

The Black Drop Effect is a blurring caused by Earth's atmosphere, Burress said. Depending on atmospheric conditions and the telescope used, when Venus is close to the edge of the sun, it blurs, and looks like a teardrop. “It confounded scientists in the expeditions in 1761 and 1769 pair,” Burress said. “They needed to have very exact timing, down to the second. The black drop effect obscured that moment, making it difficult to tell when Venus touched the sun. By the next century 1874 and 1882 they were ready for it.”

As Venus "touches" the sun you can also look for a lighted rim of Venus on the side still extending into space, coming from Venus' atmosphere. It was the first observational evidence that Venus had an atmosphere, Burress said.  

, be safe. Do not look at the sun without appropriate solar viewing lenses. Because the Transit of Venus appears as a dot, this is an occasion where going to an observatory could be a better experience than viewing it alone or via a .

Safe viewing options include:

• Watch the transit at a planetarium or program by a university astronomy department. Because Venus will look quite tiny against the sun's vast surface, it will be best to watch this amazing event via professional projection on a large screen.
• Visit NASA's website for a live-streaming broadcast and enjoy a live chat with scientists, if you like.
• Make a simple "pinhole camera" using two sheets of paper: make a pinhole in the center of one sheet; then stand with your back to the sun, holding that sheet so that the sun shines through the pinhole onto the second piece of paper. You'll see an image of the transit of Venus projected on the second sheet.

The following devices will not protect your eyes: sunglasses, binoculars with filters, neutral density filters, or exposed photographic or radiographic film.

This is the last chance for anyone alive today to see the transit of Venus, since it won't happen again until 2117. The complete transit will take about six and a half hours.