On June 5 in the Americas and June 6 in the rest of the world, people will be able to see one of the rarest predictable events in astronomy: a solar transit of the planet Venus. Over a six-hour period the disk of Venus will be silhouetted against the sun. Seeing it safely requires a special eye-protection filter, available for a dollar or so—alternately, a telescope or binoculars can safely project an image onto a wall or sheet of paper. But if you miss it, your next chance won't come until the year 2117.
Every century or so, the relative orbital motions of Earth and Venus bring them into perfect alignment with the sun, producing a pair of transits separated by eight years. Only six transits have been observed in history: in 1639; 1761 and 1769; 1874 and 1882; and 2004. Observing them was once the "noblest problem in astronomy" (as an English Astronomer Royal put it), because until the 20th century it was the only way to determine the distance from Earth to the sun. Hundreds of expeditions went as far north and as far south as possible to make giant triangles with Venus and thereby maximize the precision of the measurement. The most famous was probably Captain Cook's voyage to Tahiti in 1769.
Transits have lost none of their capacity to surprise astronomers. At the time of the 2004 event, nobody alive had seen one. I remember watching it on images streaming back from NASA spacecraft. When Venus was about halfway onto the sun's edge, the planet’s atmosphere became visible, bending sunlight toward us. This unexpected phenomenon has led us to plan worldwide observations of Venus's atmosphere during the upcoming event. We should be able to detect a dimming of the sun by a tenth of a percent as Venus blocks that much of the sun's disk. Our view should help researchers interpreting Venus's atmosphere with the European Space Agency's Venus Express spacecraft. In addition, it provides ground truth for the Kepler spacecraft's observations of exoplanetary transits around other stars.
I personally will observe from the University of Hawaii's solar telescope on Haleakala volcano on Maui, supported by the Committee for Research and Exploration of the National Geographic Society. I hope to incorporate results from the space-based observations in time to include them in my address to the American Astronomical Society in Alaska the following week. And we hope that people in 2117, as the next transit of Venus approaches, look back at the scientists of 2012 and say we carried on with skill the tradition of Captain Cook and the Astronomers Royal.
View a slideshow of past transits, as well as a map of when and where the upcoming transit will be visible.