The silhouette belongs to the planet Venus, and it will make its solar transit, as the phenomenon is called, for the first time in more than 120 years -- long enough ago that no one now alive witnessed the previous event in 1882.

The transit will occur because the orbit of Venus will put it -- for about 6 1/2 hours -- directly between Earth and the sun.

The event is so rare because the orbital planes of Venus and Earth are not exactly aligned. Even though the planet often travels between Earth and the Sun, it is rare that Venus travels directly in between. So rare, in fact, that for the last transit, John Philip Sousa actually composed the "Venus Transit March" in honor of the occasion.

In the years since the first Venusian transit was recorded -- in 1639 by two Englishmen, Jeremiah Horrocks, who died two years later at age 22, and William Crabtree -- only four more transits have occurred, but much has been learned about the second planet from the sun.

We know, for example, that Venus is 7,521 miles (12,104 kilometers) in diameter, or nearly the size of Earth's 7,926 miles (12,756 km). We know its mass is approximately 80 percent of Earth's. And we also know the planet's orbit is very nearly round, with a minimum distance to the sun of 67 million miles (108 million km) and a maximum of 68 million.

We also know, courtesy of Russia's rugged Venera spacecraft, that the surface temperature of Venus averages 870 degrees Fahrenheit (450 degrees Celsius). Its dense atmosphere represents a cautionary example of what a runaway greenhouse effect can do to a planet.

Still, despite visits by 26 spacecraft from Earth, mysteries remain about Venus, and the June 8 transit could help earthbound scientists unlock some of them.

Planetary expert Timothy Brown, at the National Center for Atmospheric Research in Boulder, Colo., plans to study the chemical composition and winds of the Venusian upper atmosphere, which he said has been poorly observed. Brown intends to take advantage of the intense sunlight streaming through the planet's atmosphere to detect what elements and compounds it contains.

Recent research has suggested the planet's atmosphere contains organic compounds that could support life.

Using a spectroscope at an observatory in the Canary Islands, in the eastern Atlantic Ocean, Brown will attempt to analyze the wavelengths of sunlight that are absorbed as they pass through Venus's atmosphere at altitudes between 40 and 55 miles (about 65 to 85 kilometers).

"This is Venus's first transit in front of the Sun since quantitative astronomical spectroscopy was invented," Brown said, "so it's our first chance to use the technology to observe close up the transit of a planet with an atmosphere."

Several characteristics of the sun are difficult to measure -- including, surprisingly, its brightness.

When Venus moves across the solar disk, it will offer a completely dark surface to contrast against the sun. Normally, the darkest objects that can been seen against the solar surface are sunspots. They actually are quite bright -- and still thousands of degrees hot. The Venus transit will allow astronomers a unique opportunity to measure the brightness of the sun's surface more accurately.

Also under cover of the transit, solar scientists will attempt to study the layers of the solar atmosphere, which also have temperatures that can differ by thousands of degrees.

Another important period will occur during the beginning and end of the transit, when Venus passes between Earth and the sun's edges. There, the solar disk appears to darken in a narrow band because the light is emanating from the atmosphere instead of the surface. This phenomenon is called limb darkening.

The problem for Earth-based telescopes is the nature of optics means masking the sun's bright disk blurs the zone between the surface and the atmosphere. Venus, at about 30 million miles (50 million kilometers) away, offers an opportunity to mask the sunlight with much greater sharpness.

A group of French scientists intends to take advantage of the limb darkening. In a few years, a joint French, Belgian and Swiss space probe called PICARD will attempt to measure the diameter of the sun with great precision to find out how it varies during the solar cycle. The data obtained during next week's transit are considered crucial for that project.

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