The second planet from the Sun, Venus is often called Earth's "sister planet" because it is the closest planet to Earth and because the two planets are nearly identical in size and mass. Venus gets its name from the Roman goddess of love and beauty and, viewed from Earth, it is indeed one of the most brilliant objects in the sky.

The first to see Venus with a telescope was Galileo. He immediately recognized and recorded its different phases, announcing his findings in a letter written in 1610. Followers of Nicholas Copernicus and his heliocentric theory (that the Sun was the center of the solar system) had earlier predicted the phases of Venus and Galileo's discovery gave conclusive evidence in support of the Copernican theory.

Scientists set to work to learn more about Venus. By 1640 there was no longer any doubt that Venus orbited the Sun closer than the...