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This many | The transit of Venus

The numbers behind an extremely rare, much anticipated wonder.
 (Photograph: Alan Tough)
Photograph: Alan ToughVenus (the silhouetted black dot) passes in front of the sun, as seen through some clouds in 2004. The transit of Venus on June 5, 2012, will the last such occurrence until the 22nd century.
Families with telescopes at Adler Planetarium
This stained-glass window in the English village Much Hoole commemorates Jeremiah Horrocks and the first observed transit of Venus.
By Web Behrens |

You don’t have to be a stargazer to notice Venus: Sooner or later, everyone catches sight of our closest planetary neighbor because it’s the brightest “star” in the night sky. Typically dazzled by its brilliance, the world will soon experience an irresistible Bizarro version when Venus appears as a black dot crossing the face of the sun.

Called a “transit” by astronomers, it’s like a solar eclipse, but with Venus in place of our moon and much smaller in appearance. A rare event of enormous historical and scientific significance that occurs on an irregular and complex 243-year cycle, the phenomenon happens next on June 5—the last chance to witness it this century.

“It’s an elegant, very slow-moving spectacle,” says Indiana astronomer Chuck Bueter, founder of, who observed the previous transit in 2004. “When you actually see it, you grasp the three-dimensional aspect: There’s Venus, a planet the size of Earth, and in the background, this enormous sun. The sun becomes huge, and all of the sudden you get it. It’s sublime.”

On Transit Day, June 5, the Adler Planetarium hosts a free viewing party from 4–9pm, including kid-friendly activities. On the other side of Lake Michigan, Indiana events include a viewing on the Notre Dame campus and at Warren Dunes State Park, where people can watch until the sun sets over the lake.

1639 Year of the first known observation of a transit, predicted and observed by young Jeremiah Horrocks, a 20-year-old English mathematician.

1761 Year of the next visible transit. This time (and again in 1769), the world’s scientists rallied for the rare event: Hundreds of astronomers, mostly from Europe, traveled the globe for months—braving wartime naval attacks, floods, forest fires, superstitious townspeople and extreme environments—to time the transit in multiple locations, from Siberia to South Africa.

1883 This year's transit became such a widespread cultural phenomenon, the cover of Harper's Weekly depicted 19th-century children gazing at it through smoked glass.

8 years between transits, which occur in pairs, but then don’t happen again for more than a century.

93 million miles from Earth to the sun—a distance 19th-century scientists calculated with near-accuracy thanks to transit data.

1/32 The diameter of Venus compared to the sun from Earth’s perspective (as seen here in the 2004 transit).

5:04pm Chicago time when the silhouette of Venus will first break the outer edge of the sun on June 5.

65% likelihood, based on the past 30 years’ weather patterns, that Chicago will see clear skies on any day in June.

105 years until the next transit of Venus in 2117.

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