In a few hours, Venus will cross directly between Earth and the sun for the last time for another 105 years.
For earthlings on the East Coast, the so-called Venus transit will begin around 6 p.m. and last more than six hours. The planet will pass directly between the sun and earth in the form of a small black disc moving across the sun's face.
The transit is similar in many ways to a solar eclipse, when the moon passes directly between Earth and the sun, blocking its light. On May 16, parts of the western United States were treated to a rare annular solar eclipse--during which the moon blocks out all but the fiery outline of the sun so that a ring of fire appears in the night sky. Venus, known as Earth's twin planet because of its similar size and chemical makeup, is three times the size of the moon. But because it is so much farther away, it will appear as a small dot and move much more slowly across the sun than the moon does in solar eclipses.
If you choose to observe the transit, which ends around 12:50 a.m. EDT, health experts warn not to stare at the sun directly. Doing so can cause serious eye damage. NASA recommends using pinhole projectors or looking through a telescope with solar filters. If it's cloudy, or if you're unable to see the transit in the sky, NASA will also show the transit in a webcast.
The last transit of Venus happened in 2004, but the transit happens in twos, separated by long gaps. The next transits won't happen until 2117 and 2225. The last two occurred in December 1874 and December 1882. Captain James Cook, who mapped thousands of miles of uncharted territory in the Pacific Ocean and present-day Canada, was said to have seen the transit of Venus before that, in 1769, from a ship in the South Pacific.
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Seth Cline is a reporter at US News and World Report. You can contact him at firstname.lastname@example.org or follow him on Twitter.