By Jay Pasachoff: the Field Memorial Professor of Astronomy at Williams College and chair of the International Astronomical Union's Working Group on Solar Eclipses, Passachoff has seen 55 solar eclipses, as well as transits of Venus and Mercury.
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.
Image Credit: Jay M. Pasachoff, David Butts, Joseph Gangestad, and Owen Westbrook (Williams College Transit of Venus Team) with John Seiradakis and George Asimellis (Aristotle University of Thessaloniki, Greece); expedition run with Bryce Babcock (Williams College) and Glenn Schneider (University of Arizona)