Every hundred years or so, the Earth, Venus and the sun queue up in a relatively straight line -- in an event called the transit of Venus -- so that observers on Earth can watch our less-than-habitable sister planet drift across the face of the sun. It takes place in pairs set eight years apart (the last one took place on June 8, 2004; the next will come June 6 this year), but they roll around only once a century or so. In his new book, Nick Lomb, curator of astronomy at Australia's Sydney Observatory, gives the rundown on the event. With the invention of the telescope in the 1600s, the transit of Venus became a hot ticket for astrophiles, who often went to great lengths to check it out. How great? Well, Captain James Cook, for one, sailed across the globe to Tahiti to view it in 1769. By providing a third point of reference, the transit of Venus made it possible for astronomers to measure the distance from Earth to the sun, which unlocked a lot of other data, including the mass of the sun and the other planets. Lacking modern solar filters, observers had to watch the transit by positioning a telescope to project the sun's image onto a piece of paper in a darkened room. There's better equipment available these days. But if you miss Venus's appearance in June, you'll have to wait a while -- until 2117 -- to catch the transit again.