"We are now on the eve of the second transit of a pair, after which there will be no other 'til the twenty-first century of our era has dawned upon the Earth, and the June flowers are blooming in 2004."

--U.S. Naval Observatory astronomer James Harkness, just before observing the 1882 transit of Venus

On June 8, sky watchers will be treated to a celestial event that no one alive today has ever seen. For the first time in 122 years, earthlings will witness the silhouette of Venus moving across the face of the sun. During this miniature version of a solar eclipse, Venus appears as a black dot against the roiling solar surface. Unlike the moon, which looms so large on the sky that it covers the entire sun during a solar eclipse, Venus during a transit masks only one-thousandth the area of the sun's surface and blocks a mere 0.1 percent of its light.

As seen from Earth, a transit of Venus occurs just twice every century or so, in pairs spaced 8 years apart. Each transit lasts about 6 hours. For the June 8 event, observers in Europe and Asia will be best placed. In the eastern United States, about 1.5 hours of the transit, from sunrise until roughly 7:25 a.m. Eastern daylight time, should be visible. Observers should not look directly at the sun, taking the same precautions as they would when viewing a solar eclipse.

To record these rare events in the 1700s and 1800s, Captain James Cook and other explorers traveled the world. Transits provided the only means at that time for accurately measuring the scale of the solar system (see box p. 248). Now, some astronomers are planning to observe the June 8 transit to learn about solar systems far beyond our own.

"Transits are the one way we can now study key properties of extrasolar planets," says Sara S. Seager of the Carnegie Institution of Washington (D.C.).

Sun-watching space observatories such as SOHO (Solar and Heliospheric Observatory) and TRACE (Transition Region and Coronal Explorer), which orbit above Earth's turbulent atmosphere, should have ringside seats for the transit.

OVERCOMING OBSTACLES Characterizing extrasolar planets in distant solar systems hasn't been easy. Against the glare of the star it orbits, each orb is too faint to be seen outright. So far, astronomers have detected extrasolar planets only indirectly, almost exclusively through the gravitational tug they exert on their parent stars.

Scientists have an opportunity to learn much more about a distant planet when it passes between Earth and its star. The duration of the transit and the amount by which starlight is dimmed by the planet's passage provide the only way astronomers can now determine the mass, size, and orbital inclination of these unseen bodies.

Among the more than 125 known extrasolar planets, astronomers have observed transits for just two. These transits have involved giant, Jupiter-mass bodies that closely orbit their parent stars and therefore block more starlight than smaller planets do.

By analyzing the specific wavelengths of starlight absorbed during a transit, astronomers have also made the first discovery of an extrasolar planet's atmosphere. Using the Hubble Space Telescope to monitor the periodic dimming of the star HD209455, already known to have a massive planet closely orbiting it, scientists deduced that the planet has a bloated atmosphere that contains hydrogen, sodium, carbon, and oxygen (SN: 2/14/04, p. 109).

Therein lies the grand opportunity of Venus' June 8 transit: The planet's passage across the sun will provide the only local counterpart to a distant, atmosphere-bearing planet crossing the face of its own parent star. The upcoming transit of Venus will therefore provide a valuable benchmark for interpreting data from extrasolar transits.

The astronomers observing the June transit will have an ace in the hole: They already know the composition of Venus' atmosphere, thanks to spacecraft that have directly measured it. By matching theory with observations, says Seager, "we can test the models for completeness and accuracy," in a way that hasn't before been possible.

Seager calls the June 8 event a "rare chance." In 2012, she notes, Venus' follow-on transit could double the bonanza. After that, the next transit won't occur until 2117.

During the upcoming transit of Venus, says Seager, she would like to tease out the role of two factors that make it tricky to interpret observations of extrasolar transits. One factor is the rotation of a parent star, which broadens the spectrum of the filtered starlight. The other, which can also alter the spectrum, is the tendency of a planetary atmosphere to act as a lens, bending or distorting the starlight passing through it. Observations of these effects during the June 8 transit should help planetary scientists eke out more information from future glimpses of extrasolar transits, Seager notes.

In attempting to treat the transit of Venus as if it were a replica of an extrasolar transit, researchers face an obstacle. As seen from Earth, all the light filtering through the atmosphere of an extrasolar planet appears to emanate from a single source--the unresolved image of the parent star. In contrast, the relatively nearby sun effectively comprises many points. To put the transit of Venus on equal footing with its extrasolar counterparts, astronomers have to devise ways to observe the sunlight as though it were coming from a single point.