They proved that it cannot, as some had suspected, be an effect of Venus's atmosphere.

ECONOMIST: Transits of Venus

She has calculated the effects that Venus's atmosphere should have on the transit, and observers around the world will be looking for them.

ECONOMIST: Transits of Venus

The reason astronomers want to observe reflected sunlight during the transit of Venus is to test whether they can detect the faint imprint of passage of the sun's light through the atmosphere of Venus.

CNN: A transit of Venus, an age of wondrous science

In 1967, the Soviet Venera 4 spacecraft entered the atmosphere of Venus and transmitted data back to Earth before losing contact about 15 miles above the surface.

CNN: Sunday,

Venus will lose its atmosphere and become a burnt-out planet.

BBC: Date set for desert Earth

Venus Express will make unique studies of this atmosphere and conduct the first radar soundings of the Venusian subsurface.

BBC: Venus mission is on

So it would be nice to test those models by observing a planet whose atmosphere is already well known: Venus.

ECONOMIST: Transits of Venus