Transit of Venus Today

BU has set up stations for viewing the transit of Venus, beginning at 6 this evening. Photo by Mark Mathosian

Early this evening a dark dot will sail slowly across the face of the sun. The dot will be small, but the occasion monumental—this is the last chance in any of our lifetimes to witness the transit of Venus, the fleeting span when our neighboring planet passes directly between the Earth and the sun.

To educate the BU community and the public about the event, and offer people a chance to view it safely, the College of Arts & Sciences department of astronomy is setting up viewing posts around campus, as well as providing a series of short lectures about not just the science, but the colorful history of a phenomenon that has captured humans’ imaginations since it was first observed in 1639.

Venus transits occur in eight-year pairs followed by spans of at least a century—only seven have occurred since the invention of the telescope. The last transit, in 2004, was visible at dawn, but it didn’t get as much attention, says John T. Clarke, a CAS professor of astronomy and director of the Center for Space Physics, because people had another chance to see one eight years later. The next transit won’t occur until 2117.

Clarke says that legendary British explorer Captain James Cook set off in 1768, on the first of his three voyages of exploration to the Pacific, to observe and record the transit of Venus. According to the National Aeronautics and Space Administration (NASA) website, when Cook, then a lieutenant in the Royal Navy, started for Tahiti, he “might as well have been going to the Moon or Mars. He would have to steer across thousands of miles of open ocean, with nothing like GPS or even a good wristwatch to keep time for navigation, to find a speck of land only 20 miles across.” But it was worth it to observe the transit, which, Cook and others hoped, could solve a puzzle consuming astronomers of the day. They knew that at least six planets orbited the sun (Uranus, Neptune, and former planet Pluto hadn’t yet been discovered), as well as the spacing of those planets. But the absolute distances between them, in miles, were unknown, and Venus was the key that could lead to a solution. At that time, comparative observations of the transit, called parallax measurements, were the only existing way to determine the scale of the solar system, Clarke says, and the method, along with all our modern day technology, is still used today.

It’s dangerous to look directly at the sun, of course, but BU’s astronomy department is marking the celestial event with information and activities enabling people to peer through reflecting telescopes to safely view the transit, which in the northeastern United States will begin around 6 p.m. and will be visible for about two hours. There are other ways to safely view the transit: on a projection screen, for example, or by looking through disposable “eclipse shades.” For those who haven’t hung on to their shades from the last solar eclipse, the best bet is to view the transit through the small telescopes that will be set up around the Charles River Campus at four strategic viewing posts—those with unobscured views of the sinking sun—scouted out by astronomy students during the last week. “You’ll actually be looking at a reflection of the sun” as Venus makes its way across, says Clarke.

“We’ll have about 30 undergraduates and graduate students at stations up and down Comm Ave, and on the roof of the College of Arts & Sciences building,” he says. Those visiting CAS will also learn about the lore and science of the transit. “Our presenters will explain how you measure parallax, talk about the history of the event, and answer questions like, why was Cook sent to observe this?” This presentation runs from 5:45 to about 7 p.m.

Viewing stations will be set up on the rooftop of the College of Arts & Sciences, 725 Commonwealth Ave., on a grassy knoll along Comm Ave near the Mass Pike overpass across from University Road, in front of the barbershop at the corner of St. Mary’s Street and Comm Ave, and on the College of Communication lawn, 640 Comm Ave. If the weather is overcast, the astronomers will present an image of the transit broadcast from Hawaii or elsewhere in CAS Room 522.

The notion of using observations of the transit of Venus to determine the distance from Earth to the sun was proposed in the early 18th century by English astronomer Edmond Halley (of comet fame), who died in 1742, more than two decades before the 1769 transit that brought Cook to the South Pacific. But astronomers heeded his suggested method of comparing calculations of the time it took Venus to cross the sun’s surface. These days the transit is also used as a reference to help astronomers measure the tiny dips in stellar brightness of distant stars similar to our sun, which could indicate the existence of planets in transit. Data from NASA’s Kepler mission suggest there may be more than 50 billion planets in the Milky Way.

For nonastronomers, the transit is more of a spectacle to capture the imagination, spawning a string of websites in scores of languages. And yes, there’s an app for that—the free Transit of Venus phone app will allow people to send their observations of the 2012 transit of Venus to a global experiment to measure anew the size of the solar system, an effort spearheaded by the nonprofit Astronomers Without Borders.

A map and more information on the BU transit of Venus event, which is free and open to the public, is available online.