In this photo released by the European Space Agency on Wednesday, Nov. 28, 2007, the mosaic of the Venusian surface, obtained thanks to radar images from NASA's Magellan spacecraft, is seen.
American astronomer Jay M. Pasachoff says it will be a once-in-a-lifetime experience for millions of people around the globe, and won't happen again until 2117.
Venus usually passes below or above the sun, Pasachoff said in this month's Physics World.
He believes observations of Venus' transit will help astronomers who are looking for extrasolar planets orbiting stars other than the sun.
"In other words, by looking up close at transits in our solar system, we may be able to see subtle effects that can help exoplanet hunters explain what they are seeing when they view distant suns," Pasachoff wrote.
Researchers will also hope to catch a glimpse of Venus using the Hubble Space Telescope.
Passachoff works at Williams College in Williamstown, Mass.