Transit of Venus: What to expect (+video)

Venus will cross the face of the sun for the last time until 2117. Here's what it will look like.?

By Mike Wall,?SPACE.com / June 5, 2012

Paul Hyndman captured this stunning view of Venus crossing the face of the sun in hydrogen-alpha light on the morning of June 8, 2004 from Roxbury, Connecticut. He used an Astro-Physics 105-millimeter Traveler telescope fitted with a Coronado Solarmax90/T-Max and 30-mm blocking filter, a TeleVue 2X Powermate lens, and an SBIG STL-11000M CCD camera.

Paul Hyndman

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Today's historic Venus transit is a marathon event lasting nearly seven hours, but skywatchers who don't have that kind of time can break it down into a handful of key milestones.

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Venus treks across the sun's face from Earth's perspective today (June 5; June 6 in much of the Eastern Hemisphere), marking the last such?Venus transit?until 2117. Few people alive today will be around to see the next transit, which makes the rare celestial sight a premier event in the astronomical and skywatching communities.

The Venus-sun show will begin around 6 p.m. EDT (2200 GMT) and end at roughly 12:50 a.m. EDT (0450 GMT) Wednesday, with the exact timing varying by a few minutes from point to point around the globe.

First contact

Before you even attempt to observe the transit of Venus, a warning:?NEVER?stare at the sun through binoculars or?small telescopes?or with the unaided eye without the proper safety equipment. Doing so can result in serious and permanent eye damage, including blindness.

Astronomers use special solar filters on telescopes to view the sun safely, while No. 14 welder's glass and eclipse glasses can be used to observe the sun directly. [How to Safely Photograph the Venus Transit]

With that warning stated, here's a look at the first major stage of the?transit of Venus.

The transit officially commences when the leading edge of?Venus?first touches the solar disk, an event astronomers call "Contact I" or "ingress exterior." This milestone occurs at 6:03 p.m. EDT (2203 GMT) for observers in eastern North America, while skywatchers on the other side of the continent will see it a few minutes later, at 3:06 p.m. PDT.

Second contact and beyond

Next up is "Contact II," or "ingress interior" ? the moment when Venus moves fully onto the sun's face. This will happen 18 minutes after Contact I. [Venus Transit of 2004: 51 Amazing Photos]

If you're viewing the transit through a good telescope, you may see a dark teardrop form, briefly joining Venus' trailing edge and the solar disk just before Contact II. This so-called "black-drop effect" bedeviled efforts in 1761 and 1769 to?measure the Earth-sun distance?by precisely timing Venus transits from many spots around the globe.

Scientists once thought the black-drop effect was caused primarily by Venus' thick atmosphere, or by viewing through Earth's ample air. But astronomers also observed it in images of a Mercury transit snapped by a NASA spacecraft in 1999. Mercury has an extremely tenuous atmosphere, so the prevailing wisdom had to go.

"Our analysis showed that two effects could fully explain the black drop as seen from space: the inherent blurriness of the image caused by the finite size of the telescope, and an extreme dimming of the sun?s surface just inside its apparent outer edge," Jay Pasachoff of Williams College, who helped analyze the 1999 Mercury transit pictures, wrote last month in the journal Nature.

After Contact II, Venus continues its long, slow and slanting trek across the sun's face. The next major milestone comes at roughly 6:25 p.m. PDT (0125 GMT on Wednesday; switching to Pacific time now, as the sun will have set in eastern North America), when Venus reaches the exact center of its transit path ? a point known as "Greatest Transit."

Earth's so-called sister planet will keep traveling across the solar disk for another three hours or so. The beginning of the end for the transit comes at about 9:30 p.m. PDT (0430 GMT Wednesday) with "Contact III," when Venus' leading edge touches the boundary of the solar disk.

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