Discover.com today posted a guest blog I wrote for them about why the Venus transit matters today. 

“When Venus passes in front of the Sun, a little bit the Sun’s light also passes through the tiny ring of Venus’s atmosphere at the planet’s outer edge. Just 0.001% of the Sun’s light during the Venus transit zips through our neighboring hothouse planet’s atmosphere on its way to Earth.

But isolating and examining that 0.001% of the sun’s light in detail will be very important. When Venus crosses the sun’s face during a transit, that 0.001% of the light carries spectral signatures—absorption lines—as tiny mementos of its passage through Venus’s atmosphere. During the 2004 transit, a team of nine French, Swiss, American, Spanish and German astronomers discovered by examining these absorption lines not only a strong signature of carbon-dioxide in the Venusian atmosphere but even the characteristic windspeeds at various altitudes above the planet’s surface. They spied on Venus’s weather from across the solar system…”