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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. 

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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.

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"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.

Contact III, also known as "egress interior," represents the last moment when Venus is still entirely contained on the sun's face, and it offers another chance to witness the black-drop effect. The last-in-a-lifetime show ends 18 minutes later with "Contact IV," or "egress exterior," when Venus finally moves off the solar disk.

Where and how to watch

As the times of these various events indicate, much of the world won't be able to observe the whole transit.

In most of North America, for example, the sun will set before the celestial festivities end, while much of Europe will witness only the last stages of the transit as the sun is coming up. Large portions of South America and Africa will miss out entirely.

However, some regions of the globe will be treated to the entire spectacle. These include eastern Asia, eastern Australia, New Zealand and the western Pacific, as well as Alaska, northern Canada and almost all of Greenland.

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