On June 5, astronomers will witness an astronomical event that has been observed in the modern era only six times previously, a passage or transit of the planet Venus across the face of the sun. The next time such an event will occur is 2117, so researchers are gearing up to devote as many resources as possible to view the transit from both Earth and space. Among other things, scientists hope observations of the transit will inform efforts to detect extrasolar planets -- those around distant stars -- a feat that is achieved in part by observing the dimming of a star as a planet passes between it and the Earth.
Venus' orbit around the sun lies in a slightly different plane than the Earth's, so transits occur only rarely and in pairs. The cycle of transits is 243 years long, with intervals of 8, 105.5, 8 and 120.5 years. The last transit was in 2004 and was also heavily observed, but the upcoming transit will be the first in which astronomers also have a satellite, the Venus Express, in orbit around the planet.
The first known transit of Venus was predicted for Dec. 7, 1631 by the German astronomer Johannes Kepler, but he died before the transit occurred. The next one, eight years later, was observed by only two individuals in England, Jeremiah Horrocks and William Crabtree. By the next transit in 1761, early scientists had realized that observations of it could help establish the size of the solar system. Expeditions, including one by Capt. James Cook, were sent around the world to monitor the event. By precisely timing the beginning and end of the transit from many locations on Earth, astronomers hoped to use simple triangulation to calculate the distances to Venus and the sun. But the so-called black-drop effect, in which a black spot appeared between the planet and the sun prior to their merging, made precise measurements virtually impossible, and calculations provided only very rough estimates. In the transits of 1761 and 1769, however, astronomers noticed a halo of light around the planet, indicating that Venus has an atmosphere.