Also see Mark Anderson's explanation of why the transit is a critical time for learning to study the atmospheres of other planets.
On the afternoon of June 5 in the United States, a small black dot will crawl across the blazing face of the sun in one of the rarest celestial events: a transit of Venus, when our sister planet passes between us and our star. It won’t happen again for 105 years.
That dot is our closest planetary neighbor and, in many ways, Earth’s near-twin. Venus is 7,521 miles in diameter, just 5 percent smaller. It has an iron core and a thick, heat-trapping atmosphere. It orbits at about three-quarters the distance between Earth and the sun. If astronomers spotted such a planet orbiting another star, they would conclude it was an ideal place to search for life.
